Researches on the Refraction, Dispersion, and Sensitiveness of Liquids

XIV. Researches on the Refraction, Dispersion, and Sensitiveness of Liquids. By J. H. Gladstone, Ph.D., F.R.S., and the Rev. T. P. Dale, M.A., F.R.A.S. Received February 5,—Read March 5, 1863. In a previous paper "On the Influence of Temperature on the Refraction of Light*," we started some inquiries which have been since pursued, and we now lay before the Royal Society some of the later results. The same apparatus has been employed, with a hollow prism of $61^\circ 0'$ angle, and the method of observation has been essentially the same. But experience has led to some modifications, the most important of which is this: instead of attempting to take the angular measurements at certain foredetermined temperatures, as $10^\circ C.$, $20^\circ C.$, they were taken first at the temperature of the room, whatever that might be, and then at such other temperatures as seemed to offer the most trustworthy results. This involved more calculation, but it still saved time, and secured greater accuracy. The plan of measuring to $10''$ was abandoned as a useless nicety; but, as a rule, two or more observations of each fixed line at each temperature were taken, and if they differed slightly the mean was adopted, but if the discrepancy amounted to $2'$ or $3'$ the observation was repeated. The average of these observations of the lines A, D, and H at different temperatures gave the refractive indices which are placed together in the Table that constitutes Appendix I., and they afford the data for nearly all the comparisons about to be instituted. Appendix II. contains the mean determinations made of the refractive indices of some of these liquids for a larger number of the lines at the temperature of the room. To it have been added some observations on other liquids, and determinations published in our former papers, so as to render it as complete as possible for any who may desire to investigate the irrationality of the spectrum, or the truth of the formulæ of Cauchy. An attempt has been made to determine the amount of probable error, not so much absolutely as with reference to the different purposes for which the observations have been made. The conclusions arrived at are as follows: Where the refraction of different fixed lines at the same temperature is compared, the probable error is very small. The measurements may be easily obtained accurate to $\pm 1'$, corresponding to about $\pm 0.0002$ in the refractive index, and thus the relative refraction of A, D, and H in Appendix I., or of all the lines in Appendix II. for any one substance will rarely differ from the truth by more than that amount. When the refraction of a substance at one temperature is compared with its refraction * Philosophical Transactions, 1858, p. 887. at another temperature, there exists a source of error in the determination of the precise temperature of that part of the liquid through which the solar beam is passing at the time when the measurement is taken. It is difficult to avoid this error, or to estimate its amount. It is, as may be supposed, generally greatest at the temperatures furthest removed from that of the surrounding objects, and in these cases there is reason to fear that it not unfrequently amounts to 1 or 2 degrees Centigrade. Even at the ordinary temperature an error may arise from the heating power of the sunbeam that passes through the liquid, and which may not affect the thermometer equally with the substance whose refraction is measured. In some of our more exact and our later determinations a strong solution of alum in a flat-sided glass was interposed in the path of the ray to reduce its heating power. Where the refraction of one substance is compared with that of another, error may also arise from inaccuracy in obtaining the minimum deviation. Though several adjustments have to be made, the error from this source is practically confined within very narrow limits, and rarely if ever passes beyond the fourth place of decimals even with very dispersive substances. This error was not so well guarded against in the observations recorded in our previous paper; and it may also affect the determination of the sensitiveness of a few substances, namely those where a different adjustment of the prism was made at different temperatures; but these are easily known, as that was only done for low temperatures such as $8^\circ$ C., and they are all marked in Appendix I. with an asterisk. In order to be rigidly correct, the hollow prism ought to have been adjusted afresh for minimum deviation in the case of each line and at each temperature, but the movement of the apparatus necessitated by this would practically have introduced greater errors than resulted from the neglect of it. Yet this has an appreciable effect on the length of the spectrum in highly dispersive substances; and in order to obviate the error as much as possible in the later measurements of such substances, care was taken to fix the minimum deviation not for either of the extremities, but for the middle of the spectrum. It would not have been difficult to make a correction by the usual formula for a small deviation from the minimum angle, but we doubted whether practically anything would be gained, considering the greater complexity of the calculation. If the indices of refraction were to be considered not relatively, but absolutely, other sources of error would have to be taken into account; for instance, inaccuracy in the determination of the prism-angle, faults of workmanship in the apparatus. For these it is more difficult to assign a limit: they may even affect the third place of decimals, whereas the combined errors from all the other sources are probably confined to the fourth place. But the absolute accuracy of an index is of minor importance in the present research. The purity of the liquids experimented on is of course a matter of the utmost consequence. When commercial specimens were employed they were always purified, or their purity ascertained. Many of the liquids were prepared in Dr. Gladstone's laboratory with special reference to this inquiry, and many others were kindly placed at our disposal by those chemists who had paid special attention to them, and we have generally taken their word for the purity of the specimen. In this way we are under obligations to Professor Williamson, Professor Hofmann, Professor Frankland, Dr. Warren De la Rue and Dr. Hugo Müller, Mr. Buckton, Dr. Odling, Mr. A. H. Church, Mr. Greville Williams, and Mr. Piesse, to whom we return our best thanks. The present paper takes up five points. I. The relation between sensitiveness and the change of volume by heat. II. The refraction and dispersion of mixed liquids. III. The refraction, dispersion, and sensitiveness of different members of homologous series. IV. The refraction, dispersion, and sensitiveness of isomeric liquids. V. The effect of chemical substitution on these optical properties. Section I.—The relation between Sensitiveness and the Change of Volume by Heat. Having examined now about ninety different liquids, we have uniformly found that the refraction diminishes as the temperature increases. This property we have already named "sensitiveness." We have uniformly found also that the spectrum diminishes in length as the temperature increases. In a very few instances this diminution is lost within the limits of errors of observation, but we believe it always occurs. This diminution in length is progressive, the different rays being more sensitive in the order of their refrangibility. The following observations on a most dispersive and sensitive substance exhibit this: | Substance | Temp. | Refractive indices. | |----------------------------|-------|---------------------| | | | A. | B. | D. | E. | G. | H. | | Bisulphide of Carbon.......| 11 | 1·6142 | 1·6207 | 1·6333 | 1·6465 | 1·6584 | 1·6836 | 1·7090 | | Bisulphide of Carbon.......| 36·5 | 1·5945 | 1·6004 | 1·6120 | 1·6248 | 1·6362 | 1·6600 | 1·6827 | | Difference .................| ......| 0·0197 | 0·0203 | 0·0213 | 0·0217 | 0·0222 | 0·0236 | 0·0263 | That there is some intimate connexion between the sensitiveness of a liquid and its change of volume by heat was pointed out in our former paper; and our subsequent experiments only confirmed this opinion. It became therefore a matter of interest to determine, if possible, what this relation is. The determinations of the sensitiveness of bisulphide of carbon, water, benzole, alcohol, wood-spirit, fousel-oil, ether, acetone, acetic acid, formic, acetic, and butyric ethers, and the iodides of methyl and ethyl afforded an opportunity of examining the matter, since the alteration of their volume by heat has been very accurately determined by Kopp and others; cumole, xylole, nitrobenzole, hydrate of phenyl, oil of turpentine, rectified oil of Portugal, eugenic acid, bromoform, and salicylate of methyl also answered the same purpose, since we determined the expansibility of the specimens employed for measuring the refractive indices at different temperatures. In the case of every one of these liquids the refractive index of any ray alters less rapidly than the volume; but it was found that the refractive index minus unity, multiplied by the volume, gives nearly a constant. It is otherwise with the contraction of the spectrum itself. In some cases, as bisulphide of carbon, it contracts much more rapidly than the volume increases, and in other cases, as ether, much less rapidly. Here it must be borne in mind that every refractive index contains at least two coefficients. Whatever may be the physical reason, and to whatever extent we may accept such theoretical explanations as those given by Cauchy, Lubbock, Sir William Hamilton, B. Powell, and others, the formula $\mu = A + \frac{B}{\lambda^2} + \frac{C}{\lambda^4} + \ldots$ does certainly give results very near the truth, $\mu$ being the refractive index, $\lambda$ the length of an undulation, and $A$, $B$, $C$ coefficients depending on the nature of the medium. As we must employ $A$, $B$, $C$ for the fixed lines of the spectrum so designated by Fraunhofer, we shall write the above formula for the future $\mu = \nu + \frac{\kappa}{\lambda^2} + \frac{\kappa'}{\lambda^4} + \ldots$ and shall suppose $\kappa'$ and all subsequent coefficients too small to be sensible within the limits of error. Hence we have $\nu$ the coefficient of refraction, and $\kappa$ the coefficient of dispersion; and $\nu$ may evidently be considered the refractive index of any substance freed from the influence of dispersion. As it appears that the function $\mu - 1$ is of peculiar interest in these investigations, we propose giving it a distinct name, that of "refractive energy," this number really representing the influence of the substance itself on the rays of light. $(\mu - 1) \times \text{vol.}$, or, which is the same thing, $\frac{(\mu - 1)}{\text{density}}$, we propose calling the "specific refractive energy." As the value of $\mu$ for any particular luminous ray is affected by the dispersion, it was clearly desirable to calculate $\nu$ in certain cases, and see whether $(\nu - 1) \times \text{vol.}$ would give a constant. Some doubt rests on the position of this theoretical limit; but its value was calculated by the formula given on pages 82 and 132 of Baden Powell's treatise 'On the Undulatory Theory as applied to Dispersion.' It will easily be seen by referring to the example on p. 132, that, in consequence of an accidental relation between the coefficients, $\nu = \mu_H - 3(\mu_F - \mu_B)$ to very considerable exactness. This formula has been used by us, but in all cases given below the results have been verified by the accurate one. Bisulphide of carbon and water were the liquids chosen, being very definite substances and extremely different in their degree of expansibility, water also having the advantage of a very irregular rate of change of volume. The refractive indices of the fixed lines B, F, and H (on which the calculation of $\nu$ depends) were determined at different temperatures with every precaution*. * The determinations for water in the accompanying Table were substituted during the printing for less accurate numbers. The subjoined Table contains the calculations founded on these numbers. Column I. gives the refractive index of the theoretical limit, or \( \nu \). Column II. the specific refractive energy for this limit, or \( (\nu - 1) \) vol. Column III. the specific refractive energy for the line B, or \( (\mu_B - 1) \) vol. Column IV. the same for H, or \( (\mu_H - 1) \) vol. Column V. gives what Newton called the "absolute refractive power" reckoned for the limit, or \( (\nu^2 - 1) \) vol. It thus appears that the specific refractive energy is nearly a constant, whether we take the limit \( \nu \) or the line B as the basis of calculation. The "absolute refractive power" is evidently not a constant. The following Table exhibits the specific refractive energy at various temperatures for some of the other liquids mentioned above, the selection being made not of those which give the most accordant results, but of those which may be considered representative bodies, or of which we happen to possess observations at the longest range of temperature. The columns are numbered as before, the only difference being that in Column III. the line A is taken instead of B. The refractive indices observed will be found in Appendix I., or in our previous paper. | Substance | Temp. | Volume | I | II | III | IV | |-------------------|-------|--------|-------|-------|-------|-------| | Alcohol | 0 | 0·9132 | 1·3598| 0·3286| 0·3340| 0·3480| | Alcohol | 20 | 0·9326 | 1·3518| 0·3280| 0·3337| 0·3478| | Alcohol | 40 | 0·9534 | 1·3435| 0·3275| 0·3332| 0·3473| | Alcohol | 60 | 0·9762 | 1·3347| 0·3268| 0·3326| 0·3473| | Difference | ......| +0·0630| −0·0251| −0·0018| −0·0014| −0·0007| | Formic Ether | 22 | 1·0305 | 1·3476| 0·3582| 0·3650| 0·3807| | Formic Ether | 31 | 1·0436 | 1·3434| 0·3584| 0·3653| 0·3811| | Formic Ether | 40 | 1·0573 | 1·3390| 0·3584| 0·3654| 0·3815| | Difference | ......| +0·0268| −0·0086| +0·0002| +0·0004| +0·0008| | Iodide of Ethyl | 23·5 | 0·9440 | 1·4878| 0·4604| 0·4720| 0·5116| | Iodide of Ethyl | 36 | 0·9583 | 1·4795| 0·4595| 0·4712| 0·5103| | Iodide of Ethyl | 48 | 0·9730 | 1·4718| 0·4590| 0·4710| 0·5108| | Difference | ......| +0·0290| −0·0160| −0·0014| −0·0010| −0·0008| | Acetic Acid | 20·5 | 1·0228 | 1·3656| 0·3739| 0·3794| 0·3969| | Acetic Acid | 28·5 | 1·0305 | 1·3624| 0·3734| 0·3792| 0·3967| | Acetic Acid | 40 | 1·0432 | 1·3579| 0·3733| 0·3791| 0·3964| | Acetic Acid | 47·5 | 1·0517 | 1·3543| 0·3726| 0·3786| 0·3963| | Difference | ......| +0·0289| −0·0113| −0·0013| −0·0008| −0·0006| | Benzole | 10·5 | 1·0125 | 1·4777| 0·4836| 0·4940| 0·5371| | Benzole | 23 | 1·0278 | 1·4704| 0·4834| 0·4939| 0·5370| | Benzole | 39 | 1·0481 | 1·4601| 0·4822| 0·4929| 0·5353| | Difference | ......| +0·0356| −0·0176| −0·0014| −0·0011| −0·0018| | Oil of Turpentine | 24 | 1·1621 | 1·4521| 0·5253| 0·5341| 0·5630| | Oil of Turpentine | 41 | 1·1778 | 1·4449| 0·5240| 0·5323| 0·5611| | Oil of Turpentine | 47 | 1·1831 | 1·4414| 0·5222| 0·5308| 0·5594| | Difference | ......| +0·0210| −0·0107| −0·0031| −0·0033| −0·0036| | Eugenic Acid | 18 | 0·9349 | 1·5159| 0·4818| 0·4942| 0·5403| | Eugenic Acid | 27·5 | 0·9412 | 1·5119| 0·4817| 0·4934| 0·5383| | Difference | ......| +0·0063| −0·0040| −0·0001| −0·0008| −0·0020| These results suffice to show that any refractive index minus unity, multiplied into the volume or divided by the density, gives nearly a constant. Indeed the numbers generally fall within the limits of experimental error. It is worthy of notice, too, that in the majority of cases, as bisulphide of carbon or alcohol, the products show a tendency to diminish as the temperature rises; but there are other cases, as formic ether, where the tendency seems to be to increase. Again, in some cases $(\nu - 1)$ vol. gives the most accordant results; in other cases $(\mu_H - 1)$ vol. Supposing this true of the coefficient of refraction, does the law equally hold good of the coefficient of dispersion? It is evident from the formula $\mu = \nu + \frac{\chi}{\lambda^2}$ that in the difference of any $\mu$ and $\nu$, or of the refractive indices of any two rays, we have a measure of the coefficient of dispersion \( z \). For convenience sake we adopt \( \mu_H - \mu_A \) as this measure; and this is what is headed "Dispersion" in many subsequent tables. It is the same as "Length of Spectrum" in our former paper. This, multiplied by the volume, or \((\mu_H - \mu_A)\) vol., we call "Specific Dispersion." But, as already stated, there is no simple relation holding good for different liquids between the increase of volume and the decrease of dispersion by heat. The phenomena seem independent. We therefore arrive at the empirical law, that the refractive energy of a liquid varies directly with its density under the influence of change of temperature, or, in other words, that the specific refractive energy of a liquid is a constant not affected by temperature. But in concluding thus, we wish it to be borne in mind that there is some influence, arising wholly or partially from dispersion, which we have not been able to take into account, but which gives rise to the slight progression of most of the calculated products, and perhaps to the non-inversion of the sensitiveness of water at 4° C., remarked on already by Jamin and ourselves. **Section II.—The Refraction and Dispersion of Mixtures of Liquids.** This subject engaged the attention of M. Deville as far back as 1842*; and of late years Messrs. Handl and A. and E. Weiss† have published elaborate papers on it, but without arriving at a solution of the question. M. Hoek‡, however, proceeding on the assumption of Fresnel, that the density of the ether enclosed in a medium is \( \mu^2 - 1 \) if the density of the ether in space is 1, found that the formula deduced from it gave numbers closely agreeing with those found experimentally by Deville for mixtures of alcohol and water, or wood-spirit and water. Yet it happens that these results can equally well be explained on the supposition that the specific refractive power of a mixture is the mean of the specific refractive power of its components. And this supposition seemed also warranted by most of the results of Messrs. Weiss, and by several that we ourselves obtained. It was clearly desirable to test these two, or any other suppositions, in a case where the refractive indices of the liquids mixed were very wide apart. Fortunately bisulphide of carbon and ether, substances almost at the opposite limits of the scale, were found to mix, and that without perceptible condensation, not indeed in equal volumes, but in the proportion of three volumes of ether to one of the bisulphide at low temperatures, and in the proportion of two to one at 20° C. Two experiments were made at different seasons on mixtures of commercially pure specimens of these substances. The greatest care was taken to prevent evaporation as far as possible during the progress of the experiments. It will be seen that in a case such as this, where there is no condensation on mixture, the calculation is much simplified, since for the specific refractive powers we may sub- --- * Ann. de Chim. et de Phys. (sér. 3) tome v. p. 129. † Wien. Ber. xxv. xxx. xxxi. and xxxiii. 589–656. ‡ Poggendorff’s Annalen, cxii. stitute the refractive indices themselves, and the supposition will stand thus: the refractive index of a mixture is the mean of the refractive indices of its components. And in such a case Hoek's formula resolves itself into the mean of $\mu^2 - 1$. | Liquid | Temperature | Specific gravity | Refractive index | |-------------------------|-------------|------------------|------------------| | | | | A | D | H | | Bisulphide of Carbon | 8° C. | 1·2790 | 1·6184 | 1·6366 | 1·7093 | | Ether | 8 | 0·7374 | 1·3542 | 1·3575 | 1·3692 | | Mixture of 1 vol. Bisulph. | 8 | 0·8710 | 1·4165 | 1·4235 | 1·4480 | | and 3 vols. Ether | | | | | | | Mean | 8 | | 1·4202 | 1·4272 | 1·4542 | | Hoek's theory | 8 | | 1·4247 | 1·4323 | 1·4619 | | Bisulphide of Carbon | 20 | 1·2685 | 1·6121 | 1·6299 | 1·7008 | | Ether | 20 | 0·7246 | 1·3487 | 1·3525 | 1·3636 | | Mixture of 1 vol. Bisulph. | 20 | 0·9059 | 1·4305 | 1·4390 | 1·4686 | | and 2 vols. Ether | | | | | | | Mean | | | 1·4365 | 1·4450 | 1·4760 | | Hoek's theory | | | 1·4417 | 1·4509 | 1·4845 | These two experiments confirm one another, but they fail to support either hypothesis. The calculation founded on $\mu^2 - 1$ gives numbers which are far too high; and though the mean of the indices is certainly much nearer to the calculated numbers, the discrepancy in each case is beyond the limits of probable error. The calculation for A is certainly nearer than that for H, but evidently not much would be gained by assuming the theoretical limit as the basis of calculation. Similar experiments were made by mixing aniline and alcohol of 90 per cent. together in equal volumes, but in this case a slight condensation ensues. | Liquid | Temperature | Specific gravity | Refractive indices | |-------------------------|-------------|------------------|--------------------| | | | | A | D | H | | Aniline | 23·5° C. | 1·0073 | 1·5642 | 1·5772 | 1·6263 | | Alcohol, 90 per cent | 23·5 | 0·8154 | 1·3576 | 1·3614 | 1·3729 | | Mixture of equal vols., | 23·5 | 0·9167 | 1·4621 | 1·4707 | 1·5018 | | mean of two experiments | | | | | | | Mean deduced from spe- | 23·5 | | 1·4636 | 1·4721 | 1·5025 | | cific refractive powers.| | | | | | | Hoek's theory | | | 1·4668 | 1·4754 | 1·5070 | This shows precisely the same thing as the previous mixture; and, as in that case, the experimental numbers are slightly below those deduced from the mean of the specific refractive powers. This is also the case in other mixtures examined; yet no other simple formula gives numbers so closely approaching those obtained by experiment. The hypothesis that the specific refractive power of a mixture of liquids is the mean of the specific refractive powers of its constituents must therefore stand as the nearest approximation to the truth. In one or two cases, as in the mixtures of sulphuric acid and water examined by Messrs. Weiss, the refraction is not at all in accordance with the above theory. This probably arises from some chemical combination between the two substances, different hydrates being formed. We hope to revert to this subject more fully on some future occasion, when we propose extending our inquiry to solutions of solids. **Section III.—The Refraction, Dispersion, and Sensitiveness of different members of Homologous Series.** In our paper on the influence of temperature we remarked an advance in refraction and dispersion with each increment of \( C_2 H_2 \) in the alcohol series. This has been examined more carefully, and the investigation has been carried much further in the same direction. The new data for the comparisons are given in Appendix I., from which the subsequent Tables are calculated, a reduction of the indices to 20° C. of temperature being always made, and the sensitiveness being calculated for 10 degrees rising from that temperature. The length of the spectrum, or the dispersion, is also reckoned at 20° C. The refractive index for only one line is given, in order to save space; and \( A \) is the line chosen, as it is least affected by dispersion. Where two specimens of the same substance have been examined, the mean of the observations has usually been adopted. ### The Alcohol Series. | Liquid | Formula | Refractive index of \( A \) at 20° C. | Length of spectrum or dispersion | Sensitiveness for 10° C. | Specific refractive energy | Specific dispersion | Specific sensitiveness | |----------------------|-----------|---------------------------------------|---------------------------------|--------------------------|---------------------------|---------------------|------------------------| | Methylic Alcohol | \( C_2 H_4 O_2 \) | 1·3268 | 0·0128 | 0·0036 | 0·4105 | 0·0163 | 0·0045 | | Ethylic Alcohol | \( C_4 H_8 O_2 \) | 1·3578 | 0·0151 | 0·0041 | 0·4482 | 0·0190 | 0·0052 | | Amylic Alcohol | \( C_{10} H_{12} O_2 \) | 1·4005 | 0·0174 | 0·0039 | 0·4895 | 0·0212 | 0·0047 | | Caprylic Alcohol | \( C_{16} H_{18} O_2 \) | 1·4186 | 0·0195 | 0·0042 | 0·5096 | 0·0237 | 0·0051 | From this it is evident that on ascending the series the refraction increases, the dispersion more rapidly still, while the sensitiveness remains nearly the same. It should be borne in mind that on account of the small numbers by which the sensitiveness is expressed, and the serious source of error arising from the difficulty of determining the temperature with accuracy, comparisons of the sensitiveness of different liquids cannot be so satisfactory as comparisons of their refractive indices, or the length of the spectrum. As all the degrees of sensitiveness at 20° C. known to us lie between 0·0007 and 0·0074, we propose in future omitting the zeros, and simply stating that the sensitiveness of methylic alcohol for instance is 36. We shall omit the zeros also in the last three columns. As we have already learnt the importance of comparisons of specific refractive energy, we have added in the three last columns the refractive energy (\( \mu_A - 1 \)), the dispersion (\( \mu_H - \mu_A \)), and the sensitiveness (\( \mu_A \) at 20° C. — \( \mu_A \) at 30° C.), all divided by the density. It will be seen that the progression is maintained. But some might prefer that the different alcohols should be compared, not at the same absolute temperature, but at the same distance from their boiling-points. This is MDCCCLXIII. attempted in the following Table for $82^\circ$ below the boiling-points, but as the observations do not extend nearly to that in the case of methylic or caprylic alcohols, there is more left for calculation than is desirable. | Liquid | Temperature | Refractive index A. | Dispersion | Sensitiveness | Specific refraction | Specific dispersion | Specific sensitiveness | |-------------------|-------------|---------------------|------------|---------------|--------------------|---------------------|-----------------------| | Methylic Alcohol | $-29^\circ$ | 1·3410 | 0·0135 | 33 | 4079 | 161 | 40 | | Ethylic Alcohol | $-4^\circ$ | 1·3674 | 0·0154 | 40 | 4515 | 189 | 49 | | Amylic Alcohol | $+50^\circ$ | 1·3888 | 0·0167 | 40 | 4914 | 211 | 50 | | Caprylic Alcohol | $100^\circ$ | 1·3807 | 0·0180 | 51 | 5123 | 242 | 68 | Here the advance of the refraction and dispersion with each addition of $\text{C}_2\text{H}_2$ appears again (with one exception), and the sensitiveness advances likewise; and this is still more evident when the numbers are divided by the density. It was a matter of interest to compare with these results the refractive indices of other homologous series belonging to the same group. ### Iodide of Methyl Series. | Substance | Formula | Refractive index A. | Dispersion | Sensitiveness | Specific refractive energy | Specific dispersion | Specific sensitiveness | |-------------------|--------------|---------------------|------------|---------------|---------------------------|---------------------|-----------------------| | Iodide of Methyl | C$_9$ H$_8$ I | 1·5171 | 0·0460 | 73 | 2359 | 209 | 33 | | Iodide of Ethyl | C$_4$ H$_7$ I | 1·5026 | 0·0420 | 66 | 2614 | 218 | 34 | | Iodide of Propyl | C$_8$ H$_7$ I | 1·4934 | 0·0408 | 63 | 2882 | 235 | 36 | | Iodide of Amyl | C$_{10}$ H$_{11}$ I | 1·4804 | 0·0335 | 50 | 3213 | 224 | 33 | In this case the refraction, dispersion, and sensitiveness are also progressive, but in the opposite direction, for they all decrease as we ascend the series, instead of increasing, as was the case with the alcohols. This may be attributed to the larger proportion of iodine which the earlier members of the series contain, for iodine has a very great influence on the rays of light. If the numbers be divided by the specific gravity, the progression becomes in the direction of increase as with the alcohols, both in regard to refraction and dispersion, while in regard to sensitiveness the four members give nearly the same number, as was also the case in the series of alcohols. ### Formic Ether Series. | Substance | Formula | Refractive index A. | Dispersion | Sensitiveness | Specific refractive energy | Specific dispersion | Specific sensitiveness | |-------------------|--------------|---------------------|------------|---------------|---------------------------|---------------------|-----------------------| | Formic Ether | C$_4$ H$_5$ O, C$_2$ H O$_3$ | 1·3549 | 0·0154 | 44 | 3905 | 168 | 48 | | Acetic Ether | C$_4$ H$_5$ O, C$_4$ H$_8$ O$_3$ | 1·3659 | 0·0157 | 48 | 4152 | 178 | 55 | | Propionic Ether | C$_4$ H$_5$ O, C$_6$ H$_5$ O$_3$ | 1·3707 | 0·0164 | 44 | 4333 | 191 | 51 | | Butyric Ether | C$_4$ H$_5$ O, C$_8$ H$_7$ O$_3$ | 1·3864 | 0·0168 | 48 | 4402 | 191 | 54 | | Valerianic Ether | C$_4$ H$_5$ O, C$_{10}$ H$_9$ O$_3$ | 1·3908 | 0·0172 | 42 | 4502 | 198 | 48 | Here, as in the case of the alcohols, there is a progressive increase of refraction, dispersion, and specific energy. The numbers representing the sensitiveness appear rather irregular, but it is difficult to say how far this may be due either to impurity of specimens or to errors of observation. During the progress of these experiments we found Professor DELFFS has preceded us in examining the refraction of members of the formic ether series*. He gives as the indices of the red ray— Formic Ether . . . . 1·3570 Acetic Ether . . . . 1·3672 Butyric Ether . . . . 1·3778 Valerianic Ether . . . 1·3904 CEnanthlyc Ether . . 1·4144 Laurostearic Ether . . 1·4240 He does not note the temperature. His conclusion is, that "the indices of refraction of the compound ethers increase with their equivalents." His experiments afforded him no means of drawing a conclusion in regard to the dispersion; and the sensitiveness was a property not fully recognized at that time. ### Acetate of Ethyl Series. | Substance | Formula | Refractive index A. | Dispersion. | Sensitiveness. | Specific refractive energy. | Specific dispersion. | Specific sensitiveness. | |--------------------|-----------------|---------------------|-------------|----------------|----------------------------|----------------------|------------------------| | Acetate of Ethyl | C₄H₅O, C₄H₃O₃ | 1·3659 | 0·0157 | 48 | 4152 | 178 | 55 | | Acetate of Amyl | C₁₀H₁₁O, C₄H₃O₅ | 1·3911 | 0·0172 | 43 | 4506 | 198 | 49 | | Acetate of Capryl | C₁₆H₁₇O, C₄H₃O₃ | 1·4088 | 0·0211 | 58 | | | | This resembles the preceding series, or that of the alcohols, as might be anticipated. Professor DELFFS in his second paper gives Acetate of Methyl . . . . 1·3576 Acetate of Ethyl . . . . 1·3672 Acetate of Amyl . . . . 1·3904 He also gives the following indices, which bear similar witness: Butyrate of Methyl . . . 1·3752 Butyrate of Ethyl . . . 1·3778 Butyrate of Amyl . . . 1·4024 Oxalate of Ethyl . . . 1·3803 Oxalate of Amyl . . . 1·4168 Formiate of Ethyl . . . 1·3570 Formiate of Amyl . . . 1·3928 * Poggendorff's Annalen, lxxxi. 470. Hydride Series. | Liquid | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |-------------------------|-------------|---------------------|------------|-------------|---------------------------|--------------------|---------------------| | Hydride of (Enanthyl | C₁₄H₁₅H | 1·3898 | 0·0172 | 55 | 5499 | 242 | 77 | | Hydride of Capryl | C₁₆H₁₇H | 1·3971 | 0·0170 | 47 | 5522 | 236 | 65 | This also bears similar evidence. Mercuric and Stannic Series. Through the kindness of Mr. Buckton and Dr. Frankland, we have been able to examine some of the combinations of the metals with the compound radicals. Unfortunately the specimens had all suffered a partial decomposition on standing, and thus the results are not so trustworthy as might be desired. | Substance | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |-------------------------|-------------|---------------------|------------|-------------|---------------------------|--------------------|---------------------| | Mercuric Methyl | C₂H₃Hg | 1·5241 | 0·0431 | 43 | 1707 | 140 | 14 | | Mercuric Ethyl | C₄H₅Hg | 1·5162 | 0·0416 | ? | 2112 | 170 | ? | | Stannic Ethyl-methyl | {C₂H₃}Sn | 1·4550 | 0·0313 | 50 | 3727 | 256 | 41 | | Stannic Ethyl | (C₄H₅)₂Sn | 1·4621 | 0·0301 | 50 | 3876 | 268 | 42 | The specific index here, as in every preceding case, increases with the addition of C₂H₂; the great absolute influence of mercury on the rays of light makes itself manifest, as iodine did, in the inversion of the order of progress in regard to actual refraction and dispersion; it should be remembered that mercuric methyl contains close upon 87 per cent. of mercury. It is worthy of notice that in the two series last given there occur the heaviest and about the lightest known liquid in the whole range of organic chemistry; and the light hydride of enanthyl has a very high, and the heavy mercuric methyl a very low specific refractive energy. All these series containing the compound radicals methyl and its congeners, agree in exhibiting a progressive change in refraction and dispersion with the advancing members of the series; but in which direction and to what extent depend on the other substances with which the radical is combined. Yet, if we regard not the actual indices, but these minus unity, divided by the specific gravity, we find an invariable increase as the series advances. The following Tables exhibit this:— DISPERSION, AND SENSITIVENESS OF LIQUIDS. Specific Refractive Energy. | Radical | Formula | Alcohol | Iodide | Ether of acid | Formiate | Acetate | Butyrate | Oxalate | Mercuric compound | Stannic compound | Hydride | |-------------|---------|---------|--------|---------------|----------|---------|----------|---------|------------------|-----------------|--------| | Methyl | C₂H₅ | 4105 | 2359 | 3905 | ... | ... | ... | ... | 1707 | 3727* | ... | | Ethyl | C₄H₉ | 4482 | 2614 | 4152 | 3905 | 4152 | 4402 | 3502 | 2112 | 3876 | ... | | Propyl | C₆H₁₁ | ... | 2882 | 4333 | ... | ... | ... | ... | ... | ... | ... | | Butyl | C₈H₁₇ | ... | 4402 | ... | ... | ... | ... | ... | ... | ... | ... | | Amyl | C₁₀H₁₉ | 4895 | 3213 | 4502 | 4432 | 4506 | 4724 | 4306 | ... | ... | ... | | Õnanthyl | C₁₄H₁₅ | ... | ... | 4750 | ... | ... | ... | ... | ... | ... | 5499 | | Capryl | C₁₆H₁₇ | 5096 | ... | ... | ... | ... | ... | ... | ... | ... | 5522 | | Laurostearyl| C₂₄H₂₅ | ... | 4890 | ... | ... | ... | ... | ... | ... | ... | ... | Specific Dispersion. | Radical | Alcohol | Iodide | Ether of acid | Acetate | Mercuric compound | Stannic compound | Hydride | |-------------|---------|--------|---------------|---------|------------------|-----------------|--------| | Methyl | 163 | 209 | 168 | ... | 140 | 256* | ... | | Ethyl | 190 | 218 | 178 | 178 | 170 | 268 | ... | | Propyl | ... | 235 | 191 | ... | ... | ... | ... | | Butyl | ... | ... | 191 | ... | ... | ... | ... | | Amyl | 212 | 224 | 198 | 198 | ... | ... | ... | | Õnanthyl | ... | ... | ... | ... | ... | ... | 242 | | Capryl | 237 | ... | ... | ... | ... | ... | 236 | Other Homologous Series. It seemed desirable to examine other groups of homologous bodies in order to see whether there existed in them the same progressive change in the optical properties answering to the progressive additions of the increment C₂H₂. Through the kindness of Mr. Church and others we were able so to test the benzole, the pyridine, and the chinoline series. Benzole Group. | Substance | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |-------------|---------|---------------------|------------|-------------|---------------------------|---------------------|----------------------| | Benzole | C₁₂H₅ | 1·4823 | 0·0419 | 60 | 5564 | 483 | 69 | | Toluole | C₁₄H₈ | 1·4835 | 0·0402 | 55 | 5584 | 464 | 63 | | Xylole | C₁₆H₁₀ | 1·4835 | 0·0408 | 58 | 5583 | 472 | 67 | | Cumole | C₁₈H₁₂ | 1·4819 | 0·0377 | 52 | 5547 | 425 | 60 | | Cymole | C₂₀H₁₄ | 1·4696 | 0·0312 | 53 | 5454 | 362 | 61 | The first four members of this series, all of which were derived from coal-tar, bear a close resemblance to one another, instead of showing that progression in refractive and dispersive properties which marks all the series of the preceding group. Cymole gives lower numbers; but the difficulties arising from isomerism, which we shall shortly advert to, render any deduction from this group very doubtful. * This compound contains both methyl and ethyl. Hydrate of Phenyl Series. Allied to benzole and toluole are the two main constituents of creasote. | Substance | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |--------------------|-----------|---------------------|------------|-------------|---------------------------|---------------------|---------------------| | Hydrate of Phenyl | C₁₃H₅O₂HO | 1·5344 | 0·0503 | 46 | 5034 | 475 | 43 | | Hydrate of Cresyl | C₁₄H₇O₂HO | 1·5319 | 0·0467 | 33 | 5122 | 450 | 32 | Pyridine Group. | Substance | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |-------------------|-----------|---------------------|------------|-------------|---------------------------|---------------------|---------------------| | Pyridine | C₁₀H₅N | 1·4948 | 0·0447 | 55 | 5081 | 458 | 56 | | Picoline | C₁₂H₇N | 1·4902 | 0·0427 | 56 | 5132 | 446 | 57 | | Lutidine | C₁₄H₉N | 1·4909 | 0·0416 | ? | 5244 | 448 | ? | | Collidine | C₁₆H₁₁N | 1·4946 | 0·0404 | 51 | 5370 | 444 | 53 | In this series the actual refractive indices are nearly the same, but somewhat irregular; yet the density is progressive, and in such a manner that when the refractive power is divided by it, a series of increasing numbers is obtained. The dispersion decreases regularly and more rapidly than the density does, so that an addition of C₂H₂ yields a lower number in regard to specific dispersion, though a higher one in regard to specific refractive energy. Chinoline Group. | Substance | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |------------------|-----------|---------------------|------------|-------------|---------------------------|---------------------|---------------------| | Chinoline | C₁₈H₇N | 1·5590 | 0·0631 | 55 | 5170 | 583 | 50 | | Lepidine | C₂₀H₉N | 1·6045 | 0·0783 | 58 | 5639 | 730 | 54 | In this case, unlike the pyridine group, which it so closely resembles (chemically speaking), the refraction and dispersion increase rapidly, whether we consider the absolute numbers or these divided by the specific gravity. Lepidine, kindly given by its discoverer Mr. C. Greville Williams, proves to be the most refractive organic liquid known, very nearly equalling bisulphide of carbon. This examination of other homologous groups shows that the influence of each addition of C₂H₂, which was observable throughout the series of the methyl group, does not necessarily hold good when we pass to substances of quite another type. Postscript to Section III., February 26, 1863.—A few days after the above was presented to the Royal Society, we observed, on taking up the last number of Poggendorff’s Annalen (cxvii. 353), a paper by M. Landolt "On the Refractive Indices of Fluid Homologous Compounds." He has examined, evidently with great care, the acids of the \( \text{C}_n\text{H}_n\text{O}_4 \) type, and finds that on ascending the series the refraction and dispersion increase, and the sensitiveness very slightly diminishes, with the exception of formic acid, which appears unconformable. This, however, is clearly due to the high density of that acid; and if we divide the numbers of Landolt by the densities, the anomaly disappears, and we obtain a series of valuations confirmatory in every way of those drawn out in the preceding Tables. Landolt measured, not \( A \) and \( H \), but \( \alpha \) and \( \gamma \) of the hydrogen light, which are nearly coincident with \( C \) and \( G \) of the solar spectrum. | Liquid | Formula | Specific refractive energy \( (\mu_\alpha - 1)/\text{density} \) | Specific dispersion \( (\mu_\gamma - \mu_\alpha)/\text{density} \) | |-------------------|---------------|---------------------------------------------------------------|---------------------------------------------------------------| | Formic Acid | \( \text{C}_2\text{H}_2\text{O}_4 \) | 3024 | 91 | | Acetic Acid | \( \text{C}_4\text{H}_4\text{O}_4 \) | 3517 | 98 | | Propionic Acid | \( \text{C}_6\text{H}_6\text{O}_4 \) | 3860 | 105 | | Butyric Acid | \( \text{C}_8\text{H}_8\text{O}_4 \) | 4115 | 114 | | Valeric Acid | \( \text{C}_{10}\text{H}_{10}\text{O}_4 \) | 4318 | 121 | | Caproic Acid | \( \text{C}_{12}\text{H}_{12}\text{O}_4 \) | 4449 | 125 | | ÕEnanthic Acid | \( \text{C}_{14}\text{H}_{14}\text{O}_4 \) | 4569 | 129 | This also shows, what is apparent both in our Tables given above and in some in Section V., that the amount of optical change is less between the higher than between the lower members of the series. **Section IV.—The Refraction, Dispersion, and Sensitiveness of Isomeric Liquids.** There are some isomeric bodies which we know differ from one another in their chemical constitution, while there are others to which we cannot yet assign any different arrangement of their elements. We have examined instances of both these classes. **Benzole Group.**—This group offers a remarkable number of isomeric bodies differing slightly in their physical and chemical characters: | Substance | Boiling point | Density | Refractive index \( A \) | Dispersion | Sensitiveness | Specific refractive energy | Specific dispersion | Specific sensitiveness | |----------------------------|---------------|---------|--------------------------|------------|---------------|---------------------------|---------------------|-----------------------| | Benzole | 80°-8 | -8667 | 1·4823 | 0·0419 | 60 | 5564 | 483 | 69 | | Parabenzoic | 97·5 | -8469 | 1·4814 | 0·0402 | ... | 5684 | 474 | ... | | Toluole | 103·7 | -8650 | 1·4739 | 0·0377 | 58 | 5478 | 435 | 67 | | Paratoluole | 119·5 | -8333 | 1·4715 | 0·0363 | 59 | 5658 | 435 | 70 | | Toluole (2nd specimen) | 113 | -8658 | 1·4835 | 0·0402? | 55 | 5584 | 464? | 63 | | Cumole (from Cummin Acid) | 148·4 | -8710 | 1·4825 | 0·0372 | 56 | 5547 | 427 | 65 | | Cumole (from Wood-spirit) | 149·5 | -8580 | 1·4631 | 0·0311 | 51 | 5400 | 363 | 59 | | Pseudo-cumole (from Coal-tar) | 140·5 | -8692 | 1·4819 | 0·0370 | 52 | 5544 | 425 | 60 | | Cymole (from Oil of Cumin) | 171 | -8600 | 1·4696 | 0·316 | 53 | 5460 | 367 | 61 | | Cymole (from Camphor) | 170 | -8565 | 1·4693 | 0·317 | 48 | 5478 | 370 | 56 | Here we have a variety of results:—isomeric bodies probably identical in refractive index, specific energy, and dispersion (cumole from cuminic acid, and Dr. H. Müller's pseudocumole, and the two cymoles); isomeric bodies nearly identical in their actual optical properties, but, on account of a difference in their densities, differing in their specific refractive energy (benzole and parabenzoie, toluole and paratoluole); isomeric bodies identical in density, but differing in optical properties (two toluoles); isomeric bodies differing in density, and in each of the optical properties (two cumoles). Essential Oil Group.—There are a large number of essential oils which consist of carbon and hydrogen in the proportion of 5 equivs. of the first to 4 equivs. of the second, and which differ from one another slightly in physical characters. Through Mr. Piesse we obtained pure specimens of the crude oils, from which many of these hydrocarbons were prepared, carefully purified, and examined. They are arranged in the following Table according to their boiling-points. When two or more from different plants appeared to be identical, or nearly so, in all their known physical properties except odour, only one is given: | Substance | Boiling-point | Specific gravity | Refractive index A | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |--------------------|---------------|------------------|--------------------|------------|-------------|---------------------------|---------------------|---------------------| | Hydrocarbon from Turpentine | 160 | -8644 | 1·4612 | 0·0250 | 47 | -5319 | 289 | 55 | | Anise | 160 | -8580 | 1·4608 | 0·0269 | 48 | -5370 | 313 | 56 | | Thyme | 160 | -8635 | 1·4617 | 0·0262 | 48 | -5346 | 326 | 56 | | Carraway | 160 | -8530 | 1·4610 | 0·0261 | 48 | -5391 | 305 | 56 | | Bergamot | 173 | -8467 | 1·4619 | 0·0297 | 49 | -5456 | 350 | 57 | | Bay | 173 | -8510 | 1·4542 | 0·0257 | 47 | -5337 | 302 | 55 | | Cloves | 252 | -9041 | 1·4898 | 0·0284 | 45 | -5417 | 314 | 50 | | Cubebs | 259 | -9260 | 1·4950 | 0·0302 | 41 | -5345 | 326 | 44 | Here, as in the case of parabenzoie and paratoluole, we have five isomeric bodies with sensibly the same refraction 1·461, although there are slight differences in the density and other properties, differences that seem to be real. The dispersion varies considerably; and the difference between turpentine, the least, and bergamot, the most dispersive, is only increased when the difference of density enters into the calculation. The sensitiveness seems the same in each of the five. The hydrocarbon from bay seems slightly lower in refraction; while those from cloves and cubebs, with much higher boiling-points and densities, are much higher in refraction and dispersion, and lower in sensitiveness. The specific refractive energies of the whole group do not differ widely. Sugars.—We do not propose entering now on the subject of solutions, but we may state that solutions of cane-, grape- and honey-sugar, and gum, of the same strength, gave the same, or very nearly the same, amount of refraction and dispersion. Compound Ethers.—It is well known that among the compound ethers pairs exist which have the same ultimate composition and similar density, but which are broken up by alkalies into different acids and alcohols. If it does not matter, as to its action on light, whether the increment C₂H₂ be in the electro-positive or electro-negative element, these pairs will present an identity in refraction, dispersion, and sensitiveness. Such indeed seems to be the case. | Liquid | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |-----------------------|------------------|---------------------|------------|-------------|---------------------------|---------------------|---------------------| | Valerianic Ether | C₄H₅O, C₁₀H₉O₃ | 1·3908 | 0·0173 | 42 | 4502 | 199 | 48 | | Acetate of Amyl | C₁₀H₁₁O, C₄H₃O₃ | 1·3911 | 0·0172 | 43 | 4506 | 198 | 49 | In this also we find that Professor DELFFS has preceded us, as far as refraction is concerned. He gives Formic Ether . . . 1·3570 Acetate of Methyl . . 1·3576 and Valerianic Ether . . . 1·3904 Acetate of Amyl . . . 1·3904 Aniline and Picoline.—These two bodies have each the ultimate composition C₁₂H₇N, but the action of chemical reagents proves that they are constructed very differently. The following comparison will show that they differ widely in refraction, dispersion, and sensitiveness, even when the difference in specific gravity is taken into account. | Liquid | Rational formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |-----------|------------------|---------------------|------------|-------------|---------------------------|---------------------|---------------------| | Aniline | C₁₂H₅N | 1·5650 | 0·0653 | 47 | 550 | 635 | 47 | | Picoline | C₁₂H₇N | 1·4902 | 0·0428 | 56 | 513 | 448 | 59 | It thus appears that isomeric bodies are sometimes widely different in these optical properties; but in many cases, especially when there is close chemical relationship, there is identity also in this respect. Section V.—The Effect of Chemical Substitution. The doctrine of types and substitution is fully recognized, at least by all the students of organic chemistry. It becomes a matter of interest to determine the amount of change in the optical properties which results from a replacement of one element by another, the type remaining the same. By this means we may attain to a knowledge of the influence of the individual elements on the rays of light transmitted by them. The data for such an inquiry ought to be very numerous, but those which we have already collected point to some conclusions. The Substitution of Hydrogen by an Organic Radical. In the following instance the substitution of amyl has not produced much change when the refractive power is divided by the density. | Liquid | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |--------------|------------------|---------------------|------------|-------------|---------------------------|---------------------|----------------------| | Aniline | C₅H₅N | 1·5650 | 0·0653 | 47 | 550 | 635 | 45 | | Amyl-aniline | C₁₀H₁₁N | 1·5130 | 0·0508 | 46 | 559 | 554 | 49 | But a more interesting case is that of ether, alcohol, and water, which, according to Williamson, are of the same type,—a theory which the subjoined numbers favour, as the optical properties of the three are analogous, and those of alcohol are intermediate between those of water and ether when the proper allowance is made for the difference of specific gravity. The comparison is made, not at the same temperature, but at 30° below their respective boiling-points. | Liquid | Williamson's formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |--------------|----------------------|---------------------|------------|-------------|---------------------------|---------------------|----------------------| | Water | H₂O | 1·3203 | 143 | 24 | 0·3272 | 145 | 25 | | Alcohol | C₂H₆O₂ | 1·3460 | 148 | 44 | 0·4499 | 192 | 57 | | Ether | C₂H₆O₂ | 1·3575 | 154 | 52 | 0·4888 | 210 | 70 | The Substitution of Hydrogen by Oxygen. Of the effect of this replacement we have the following instances: | Liquid | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |--------------|------------------|---------------------|------------|-------------|---------------------------|---------------------|----------------------| | Alcohol | C₄H₅O₂ | 1·3578 | 0·0151 | 41 | 0·4482 | 190 | 52 | | Acetic Acid | C₄H₅O₂ | 1·3690 | 0·0172 | 37 | 0·3483 | 162 | 35 | | Ether | C₄H₅O₂ | 1·3487 | 0·0149 | 51 | 0·4868 | 208 | 71 | | Acetic Ether | C₄H₅O₂ | 1·3659 | 0·0157 | 48 | 0·4152 | 178 | 55 | | Carvone | C₂₀H₁₆O₂ | 1·4610 | 0·0261 | 48 | 0·5391 | 305 | 56 | | Carvole | C₂₀H₁₄O₂ | 1·4886 | 0·0345 | 46 | 0·5126 | 362 | 48 | | Eugenic Acid | C₂₀H₁₂O₄ | 1·5277 | 0·0495 | 42 | 0·4945 | 463 | 39 | In all these cases the replacement of hydrogen by oxygen has increased the actual refraction and dispersion, but decreased the specific refractive energy. The two pairs, alcohol and acetic acid, ether and acetic ether, are interesting for comparison, since the chemical change is the same in the two, and it will be observed that the optical change is nearly the same also. The three last bodies occur together, or under similar circumstances in nature, and form a series of which the second (carvole) is precisely intermediate in composition. It is also intermediate in optical properties. **The Substitution of Hydrogen by Peroxide of Nitrogen.** Of this the following instances have been examined: | Substance | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |--------------------|--------------|---------------------|------------|-------------|---------------------------|-------------------|---------------------| | Benzole | C₁₂H₆ | 1·4823 | 0·0323* | 60 | 5671 | 380* | 70 | | Nitrobenzole | C₁₂H₅(NO₄) | 1·5356 | 0·0501* | 50 | 4610 | 423* | 43 | | Dinitrobenzole† | C₁₂H₄(NO₄)₂ | 1·5486 | 0·0570* | 48 | 3880 | 404* | 34 | | Glycerine | C₆H₈O₅ | 1·4659 | 0·0191 | 25 | 3690 | 151 | 19 | | Nitroglycerine | C₆H₅(NO₃)₃O₆| 1·4654 | 0·0264 | 45 | 2909 | 165 | 28 | | Amylic Alcohol | C₁₀H₁₀O₅ | 1·4005 | 0·0174 | 39 | 4895 | 212 | 47 | | Nitrate of Amyl... | C₁₀H₁₁(NO₄)O₂| 1·4065 | 0·0210 | 45 | 4061 | 202 | 44 | Here we observe in the case of the benzole compounds a considerable increase of actual refraction, but in those of glycerine and amylic alcohol little change, while in each case there is a very marked increase of dispersion; yet when the numbers are divided by the density the refraction at least shows a decrease. The sensitiveness is greatly diminished by the substitution in the benzole compounds. **The Substitution of Hydrogen by Chlorine.** | Substance | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |--------------------|--------------|---------------------|------------|-------------|---------------------------|-------------------|---------------------| | Benzole | C₁₂H₆ | 1·4823 | 0·0419 | 60 | 5564 | 483 | 69 | | Chlorobenzole | C₁₂H₅Cl | 1·5135 | 0·0437 | 53 | 4634 | 394 | 48 | | Trichlorobenzole | C₁₉H₃Cl₃ | 1·5563 | 0·0502 | 46 | 3836 | 346 | 31 | In this case there is also an increase both of actual refraction and dispersion, and a decrease of sensitiveness; but when the density of the chlorinated products is taken into account, the result is a great diminution in each of the optical properties. * In these cases \( \mu_a - \mu_A \) is taken as the measure of dispersion, since H was invisible through nitrobenzole. † Calculated on the assumption that the specific refractive energy of a mixture is the mean of the specific refractive energies of its constituents. The Substitution of Chlorine by Bromine. Of this we have the following instances: | Substance | Formula | Refractive index A. | Dispersion | Sensitivity | Specific refractive energy | Specific dispersion | Specific sensitivity | |----------------------------|-----------|---------------------|------------|-------------|---------------------------|---------------------|---------------------| | Terchloride of Phosphorus | PCl₃ | 1·5062 | 394 | 58 | 3489 | 271 | 40 | | Terbromide of Phosphorus | PBr₃ | 1·6730 | 808 | 64 | 2338 | 280 | 22 | | Chloroform | C₂HCl₃ | 1·4400 | 220 | 54 | 2949 | 148 | 37 | | Bromoform | C₂HBr₃ | 1·5554 | 418 | 55 | 2107 | 158 | 21 | | Bichloride of Chlorethylene | C₄H₈Cl₂ | 1·4619 | 228 | 59 | 3259 | 160 | 42 | | Bibromide of Chlorethylene | C₄H₈Cl₂Br₂| 1·5430 | 354 | 56 | 2415 | 157 | 25 | | Bibromide of Bromethylene | C₄H₈Br₂ | 1·5809 | 430 | 50 | 2220 | 164 | 19 | Here in each case the bromine has greatly increased the refraction; but that this is owing to its great weight is evident from the fact that the specific refractive energy is much diminished. The dispersion is increased, but this is very nearly counterbalanced by the increase of weight. The sensitiveness is diminished, at least in the ethylene group. It will be observed that, in each of the five cases mentioned in this section where there are two substitution products, the lower one is intermediate between the original substance and the higher product. These observations put us in a position to consider the question, Does an element retain its special influence on the rays of light with whatever other elements it may be combined? As the specific refractive energy of a mixture, or a feeble combination such as alcohol and water, is approximately the mean of the specific refractive energies of its constituents, we are prepared to find the rule holding good in more distinct chemical compounds. In the only case in which we have been able to try it among liquid elements, namely terbromide of phosphorus, the result was pretty near; but there is no doubt that chemical combination often greatly changes the optical as it does the other properties of elementary bodies. Yet it is quite conceivable that an element may retain a specific influence on the rays of light through many if not all its compounds; and this view certainly finds some support in our experiments. Witness the fact of the great increase both of refraction and dispersion caused by the addition of nitrogen, whether combined with oxygen or not, to compounds of carbon and hydrogen (see Appendix I., Nos. 32, 52–55, 57–62, 75, 76). But when we look more narrowly at the numbers, we find this general permanence of special optical properties subject to much modification. Thus the difference in the optical properties of some isomeric bodies shows that such a generalization cannot be strictly true. Again, we may examine the different cases of replacement of hydrogen by oxygen mentioned above; and as in each case the atomic volume of the substitution- product is the same, or nearly so, as the atomic volume of the primary compound, the comparison is peculiarly legitimate. We infer at once that oxygen in combination is actually more refractive and dispersive than hydrogen, but that, if we take into account its much higher density, its specific refractive energy is less. But when we come to compare the different cases quantitatively, we see that a good deal depends on the peculiar nature of the compound. In the following Table the effect of the replacement of two equivalents of hydrogen by two of oxygen is given both with respect to refraction and dispersion. The specific refractive energy of \( \nu \) is taken as the best exponent of the influence of refraction, and \( \mu_H - \mu_A \), divided by density, is assumed, as before, for the specific dispersion. | Substance | Atomic volume | Specific refractive energy (\( \nu \)) | Effect of substitution | Specific dispersion | Effect of substitution | |-----------------|---------------|--------------------------------------|------------------------|--------------------|------------------------| | Alcohol | 58 | 444 | | 190 | | | Acetic Acid | 57 | 344 | -100 | 162 | -28 | | Carvone | 62 | 530 | | 305 | | | Carvole | 63 | 501 | -29 | 362 | +57 | | Eugenic Acid | 64 | 474 | -27 | 463 | +101 | Hence we find that the substitution of two equivalents of oxygen for two of hydrogen has produced a far greater reduction in specific refractive energy in the case of alcohol than in that of the essential oil; while in specific dispersion it has produced a reduction in the one case, and an augmentation in the other. As the main conclusions have been marked by italics under each head as they were arrived at, they are not recapitulated; but the following may be taken as a larger generalization deduced from them, and approximately if not absolutely true. Every liquid has a specific refractive energy composed of the specific refractive energies of its component elements, modified by the manner of combination, and which is unaffected by change of temperature, and accompanies it when mixed with other liquids. The product of this specific refractive energy, and the density, at any given temperature, is, when added to unity, the refractive index. APPENDIX I. Table of refractive indices of the lines A, D, H at different temperatures. The initials in the column headed “From whose laboratory,” are those of Messrs. G. B. Buckton, A. H. Church, Warren De la Rue, E. Frankland, J. H. Gladstone, A. W. Hofmann, W. Odling, C. Greville Williams, and A. W. Williamson. The sign ? attached to a liquid denotes that the purity of the specimen is doubted. An asterisk * attached to a degree of temperature signifies that the observations at that temperature were made on a different occasion to the observations at other temperatures. Specific gravities not determined from the specimens examined are included in brackets. | No. | Liquid | From whose laboratory | Specific gravity | Temperature of observation | Refractive indices | |-----|-------------------------|-----------------------|------------------|----------------------------|-------------------| | | | | | | A. | D. | H. | | 1. | Methylic Alcohol | E. F. | 0·7972 at 20 °C | {20°} | 1·3264 | 1·3299 | 1·3395 | | | | | | {37°} | 1·3205 | 1·3238 | 1·3330 | | 2. | Ditto from oxalate | J. H. G. | 0·796 at 20 | {20°} | 1·3268 | 1·3297 | 1·3396 | | | | | | {29·5°} | 1·3230 | 1·3262 | 1·3359 | | 3. | Amylic Alcohol | J. H. G. | 0·8179 at 15·5° | {24·5°} | 1·3988 | 1·4030 | 1·4161 | | | | | | {41°} | 1·3924 | 1·3966 | 1·4093 | | 4. | Caprylic Alcohol | C. G. W. | 0·8214 at 15·5° | {27°} | 1·4157 | 1·4202 | 1·4351 | | | | | | {47°} | 1·4073 | 1·4118 | 1·4266 | | 5. | Iodide of Methyl | A. W. W. | 2·1912 at 20 | {23·5°} | 1·5203 | 1·5307 | 1·5670 | | | | | | {29·5°} | 1·5104 | 1·5202 | 1·5549 | | 6. | Iodide of Ethyl | A. W. W. | 1·9228 at 20 | {23·5°} | 1·5003 | 1·5095 | 1·5420 | | | | | | {36°} | 1·4918 | 1·5006 | 1·5326 | | 7. | Iodide of Propyl | A. W. H. | 1·7117 at 20 | {48°} | 1·4841 | 1·4934 | 1·5250 | | | | | | {8·5°} | 1·5001 | 1·5095 | 1·5418 | | 8. | Iodide of Amyl | J. H. G. | 1·4950 at 20 | {20°} | 1·4934 | 1·5024 | 1·5342 | | | | | | {30°} | 1·4871 | 1·4963 | 1·5272 | | 9. | Formic Ether | A. W. W. | 0·9088 at 20 | {17·5°} | 1·4816 | 1·4892 | 1·5149 | | | | | | {37°} | 1·4720 | 1·4797 | 1·5046 | | 10. | Acetic Ether | A. W. W. | 0·8648 at 20 | {22°} | 1·3540 | 1·3582 | 1·3694 | | | | | | {31°} | 1·3500 | 1·3540 | 1·3652 | | 11. | Acetic Ether | J. H. G. | 0·8972 at 20 | {40°} | 1·3456 | 1·3494 | 1·3608 | | | | | | {20°} | 1·3645 | 1·3685 | 1·3798 | | 12. | Propionic Ether | A. W. W. | 0·8555 at 20 | {28°} | 1·3606 | 1·3644 | 1·3755 | | | | | | {23·5°} | 1·3653 | 1·3692 | 1·3809 | | 13. | Butyric Ether | A. W. W. | 0·8778 at 20 | {33°} | 1·3606 | 1·3643 | 1·3757 | | | | | | {41°} | 1·3563 | 1·3602 | 1·3711 | | 14. | Valerianic Ether | J. H. G. | 0·868 at 20 | {22·5°} | 1·3696 | 1·3736 | 1·3860 | | | | | | {32°} | 1·3657 | 1·3698 | 1·3819 | | 15. | Acetate of Amyl | J. H. G. | 0·8680 at 20 | {42°} | 1·3610 | 1·3651 | 1·3771 | | | | | | {23°} | 1·3850 | 1·3888 | 1·4018 | | 16. | Ditto, second specimen | J. H. G. | | {40°} | 1·3768 | 1·3808 | 1·3933 | | | | | | {18°} | 1·3916 | 1·3958 | 1·4089 | | 17. | Acetate of Capryl? | C. G. W. | | {32·5°} | 1·3856 | 1·3898 | 1·4024 | | | | | | {24·5°} | 1·3910 | 1·3950 | 1·4081 | | | | | | {34·5°} | 1·3867 | 1·3905 | 1·4037 | | | | | | {44°} | 1·3817 | 1·3859 | 1·3985 | | | | | | {8·5°} | 1·3944 | 1·3988 | 1·4113 | | | | | | {21·5°} | 1·3886 | 1·3928 | 1·4058 | | | | | | {35°} | 1·3820 | 1·3866 | 1·3990 | | | | | | {27·5°} | 1·4045 | 1·4092 | 1·4255 | | | | | | {40°} | 1·3972 | 1·4020 | 1·4181 | | No. | Liquid | From whose laboratory | Specific gravity | Temperature of observation | Refractive indices | |-----|-------------------------------|-----------------------|-----------------|---------------------------|-------------------| | | | | | | A. | D. | H. | | 18. | Hydride of OEnanthyl | J. H. G. | 0·7090 at 20 | 9·5 | 1·3956 | 1·3996 | 1·4135 | | | | | | 22 | 1·3888 | 1·3931 | 1·4059 | | | | | | 36 | 1·3811 | 1·3854 | 1·3976 | | | | | | 9 | 1·4022 | 1·4065 | 1·4197 | | | | | | 28·5 | 1·3931 | 1·3972 | 1·4097 | | | | | | 41 | 1·3870 | 1·3911 | 1·4032 | | | | | | 8·5* | 1·5274 | 1·5378 | 1·5726 | | | | | | 15* | 1·5262 | 1·5355 | 1·5694 | | | | | | 26·5 | 1·5197 | 1·5296 | 1·5626 | | | | | | 8·5* | 1·5300 | 1·5397 | 1·5729 | | | | | | 24·5 | 1·5124 | 1·5217 | 1·5538 | | | | | | 19 | 1·4555 | 1·4625 | 1·4868 | | | | | | 34·5 | 1·4479 | 1·4747 | 1·4783 | | | | | | 23 | 1·4606 | 1·4673 | 1·4905 | | | | | | 35 | 1·4551 | 1·4621 | 1·4844 | | | | | | 48 | 1·4481 | 1·4549 | 1·4769 | | | | | | 19·5 | 1·4598 | 1·4669 | 1·4919 | | | | | | 26·5 | 1·4588 | 1·4657 | 1·4906 | | | | | | 24 | 1·3674 | 1·3718 | 1·3846 | | | | | | 34·5 | 1·3635 | 1·3680 | 1·3803 | | | | | | 45 | 1·3596 | 1·3634 | 1·3757 | | | | | | 25·5 | 1·3540 | 1·3580 | 1·3706 | | | | | | 40 | 1·3469 | 1·3512 | 1·3631 | | | | | | 23 | 1·3832 | 1·3878 | 1·4028 | | | | | | 35 | 1·3756 | 1·3834 | 1·3982 | | | | | | 22 | 1·3773 | 1·3810 | 1·3936 | | | | | | 31 | 1·3746 | 1·3787 | 1·3898 | | | | | | 40 | 1·3692 | 1·3734 | 1·3846 | | | | | | 22·5 | 1·3664 | 1·3698 | 1·3815 | | | | | | 40·5 | 1·3578 | 1·3604 | 1·3724 | | | | | | 20 | 1·3781 | 1·3821 | 1·3940 | | | | | | 33·5 | 1·3724 | 1·3768 | 1·3881 | | | | | | 21 | 1·5206 | 1·5319 | 1·5810 | | | | | | 37 | 1·5140 | 1·5253 | 1·5735 | | | | | | 10 | 1·4109 | 1·4157 | 1·4320 | | | | | | 22·5 | 1·4053 | 1·4097 | 1·4256 | | | | | | 36·5 | 1·3988 | 1·4035 | 1·4191 | | | | | | 18 | 1·4411 | 1·4463 | 1·4630 | | | | | | 30 | 1·4346 | 1·4397 | 1·4561 | | | | | | 44 | 1·4253 | 1·4308 | 1·4471 | | | | | | 15·5 | 1·5579 | 1·5674 | 1·5998 | | | | | | 29 | 1·5505 | 1·5598 | 1·5921 | | | | | | 39 | 1·5437 | 1·5531 | 1·5846 | | | | | | 21 | 1·4175 | 1·4221 | 1·4371 | | | | | | 38 | 1·4082 | 1·4126 | 1·4276 | | | | | | 18 | 1·5819 | 1·5915 | 1·6249 | | | | | | 39·5 | 1·5701 | 1·5787 | 1·6112 | | | | | | 13 | 1·5477 | 1·5559 | 1·5839 | | | | | | 24 | 1·5413 | 1·5495 | 1·5770 | | | | | | 13 | 1·4661 | 1·4714 | 1·4892 | | | | | | 29·5 | 1·4563 | 1·4619 | 1·4789 | | | | | | 10·5 | 1·4879 | 1·4975 | 1·5305 | | | | | | 23 | 1·4806 | 1·4900 | 1·5225 | | | | | | 39 | 1·4703 | 1·4793 | 1·5108 | | | | | | 20 | 1·4814 | 1·4903 | 1·5216 | | | | | | 25·5 | 1·4709 | 1·4794 | 1·5090 | | | | | | 32·5 | 1·4672 | 1·4755 | 1·5048 | | | | | | 39 | 1·4629 | 1·4710 | 1·5001 | | No. | Liquid | From whose laboratory | Specific gravity | Temperature of observation | Refractive indices | |-----|-------------------------------|-----------------------|-----------------|---------------------------|-------------------| | | | | | | A | D | H | | 42. | Paratoluole | A. H. C. | 0·8333 at 20 | 28° | 1·4667 | 1·4751 | 1·5030 | | | | | | 40 | 1·4590 | 1·4671 | 1·4944 | | | | | | 14 | 1·4869 | 1·4957 | 1·5271 | | 43. | Toluole | W. O. | 0·8658 at 20 | 33 | | 1·4856 | | | | | | | 11 | 1·4888 | 1·4982 | 1·5300 | | | | | | 28 | 1·4788 | 1·4879 | 1·5192 | | | | | | 42 | 1·4716 | 1·4805 | 1·5116 | | 44. | Xylole | W. O. | 0·866 at 20 | 7 | 1·4898 | 1·4983 | 1·5280 | | | | | | 27·5 | 1·4783 | 1·4864 | 1·5148 | | | | | | 8·5 | 1·4687 | 1·4759 | 1·5008 | | 45. | Cumole (from Cuminic Acid) | A. H. C. | 0·871 at 20 | 24 | 1·4608 | 1·4680 | 1·4919 | | | | | | 34 | 1·4555 | 1·4634 | 1·4848 | | | | | | 12·5 | 1·4843 | 1·4932 | 1·5236 | | | | | | 35·5 | 1·4728 | 1·4812 | 1·5093 | | | | | | 8 | 1·4760 | 1·4834 | 1·5076 | | 46. | Cumole (from impure Wood-spirit) | J. H. G. | 0·858 at 20 | 29 | 1·4648 | 1·4717 | 1·4957 | | | | | | 12 | 1·4731 | 1·4803 | 1·5050 | | | | | | 26 | 1·4659 | 1·4729 | 1·4975 | | | | | | 36 | 1·4614 | 1·4684 | 1·4927 | | | | | | 9 | 1·5194 | 1·5290 | 1·5636 | | | | | | 27·5 | 1·5095 | 1·5189 | 1·5528 | | | | | | 20 | 1·5563 | 1·5671 | 1·6065 | | | | | | 37 | 1·5495 | 1·5600 | 1·5983 | | 47. | Pseudocumole | W. D. L. R. | 0·8692 at 20 | 25 | 1·5331 | 1·5465 | 1·5832 G. | | | | | | 38 | 1·5266 | 1·5399 | 1·5766 G. | | | | | | 23·5 | 1·5460 | 1·5600 | 1·5994 G. | | | | | | 35 | 1·5404 | 1·5542 | 1·5932 G. | | | | | | 56 | 1·5296 | 1·5425 | 1·5816 G. | | | | | | 21·5 | 1·5644 | 1·5784 | 1·6297 | | | | | | 37 | 1·5567 | 1·5701 | | | | | | | 42 | 1·5537 | 1·5676 | 1·6183 | | | | | | 47 | 1·5520 | 1·5647 | 1·6145 | | | | | | 23·5 | 1·5114 | 1·5222 | 1·5622 | | | | | | 42 | 1·5035 | 1·5138 | 1·5532 | | | | | | 11·5 | 1·5341 | 1·5454 | 1·5824 | | | | | | 32 | 1·5281 | 1·5377 | 1·5733 | | | | | | 21·5 | 1·4940 | 1·5030 | 1·5387 | | | | | | 36 | 1·4860 | 1·4951 | 1·5301 | | | | | | 72·5 | 1·4888 | 1·4980 | 1·5314 | | | | | | 37·5 | 1·4803 | 1·4890 | 1·5213 | | | | | | 52 | 1·4718 | 1·4807 | 1·5122 | | | | | | 8·5* | 1·4932 | 1·5028 | 1·5353 | | | | | | 22·5 | 1·4894 | 1·4987 | 1·5308 | | | | | | 23·5 | 1·4927 | 1·5013 | 1·5329 | | | | | | 45 | 1·4820 | 1·4907 | 1·5210 | | | | | | 24 | 1·5567 | 1·5687 | 1·6198 | | | | | | 35 | 1·5466 | 1·5587 | 1·6084 | | | | | | 37 | 1·5496 | 1·5617 | 1·6124 | | | | | | 21 | 1·6039 | 1·6189 | 1·6822 | | | | | | 47 | 1·5999 | 1·6054 | 1·6473 G. | | | | | | 11 | 1·4653 | 1·4718 | 1·4921 | | | | | | 30 | | 1·4625 | | | | | | | 10* | 1·4669 | 1·4734 | 1·4934 | | | | | | 24 | 1·4596 | 1·4653 | 1·4845 | | | | | | 47 | 1·4487 | 1·4545 | 1·4730 | | | | | | 14 | 1·4640 | 1·4701 | 1·4901 | | | | | | 37 | 1·4529 | 1·4589 | 1·4783 | --- **Note:** The table continues from previous pages. ### Table (continued). | No. | Liquid | From whose laboratory | Specific gravity | Temperature of observation | Refractive indices | |-----|-------------------------------|-----------------------|-----------------|---------------------------|-------------------| | | | | | | A. | D. | H. | | 66. | Hydrocarbon from Thyme | J. H. G. | 0·8635 at 20 | 25 | 1·4594 | 1·4652 | 1·4856 | | | | | | 35·5 | 1·4545 | 1·4606 | 1·4805 | | 67. | Bay | J. H. G. | 0·851 at 20 | 23 | 1·4545 | 1·4610 | 1·4818 | | | | | | 43 | 1·4468 | 1·4528 | | | 68. | Bergamot | J. H. G. | 0·8467 at 20 | 26·5 | 1·4574 | 1·4640 | 1·4865 | | | | | | 38 | 1·4517 | 1·4578 | 1·4800 | | 69. | Cloves | J. H. G. | 0·9041 at 20 | 17 | 1·4918 | 1·4985 | 1·5209 | | | | | | 28·5 | 1·4870 | 1·4936 | 1·5157 | | | | | | 39 | 1·4828 | 1·4892 | 1·5110 | | 70. | Cubebs | J. H. G. | 0·927 at 20 | 10·5 | 1·4988 | 1·5055 | 1·5294 | | | | | | 20 | 1·4950 | 1·5014 | 1·5252 | | | | | | 31 | 1·4905 | 1·4977 | 1·5209 | | 71. | Carvole | J. H. G. | 0·9530 at 20 | 12·5 | 1·4913 | 1·4992 | 1·5270 | | | | | | 24·5 | 1·4862 | 1·4935 | 1·5196 | | | | | | 34 | 1·4812 | 1·4884 | 1·5145 | | 72. | Eugenie Acid | J. H. G. | 1·064 at 20 | 18 | 1·5285 | 1·5394 | 1·5780 | | | | | | 27·5 | 1·5244 | 1·5347 | 1·5722 | | 73. | Camphor of Peppermint | | 0·8786 at 43 | 30 | 1·4503 | 1·4553 | 1·4703 | | | | | | 43 | 1·4451 | 1·4505 | 1·4653 | | 74. | Glycerine | J. H. G. | 1·261 at 17 | 20 | 1·4659 | 1·4705 | 1·4850 | | | | | | 30 | 1·4634 | 1·4680 | 1·4823 | | | | | | 48 | 1·4586 | 1·4631 | 1·4773 | | 75. | Nitroglycerine? | J. H. G. | (1·60) | 13·5 | 1·4683 | 1·4749 | 1·4947 | | | | | | 32·5 | 1·4596 | 1·4662 | | | 76. | Nicotine | J. H. G. | 1·026 at 18 | 18 | 1·5149 | 1·5234 | 1·5542 | | | | | | 32 | 1·5107 | 1·5194 | 1·5493 | | 77. | Terbromide of Phosphorus | J. H. G. | 2·88 at 20 | 25 | 1·6698 | 1·6866 | 1·7506 | | | | | | 36 | 1·6627 | 1·6792 | 1·7422 | | 78. | Terchloride of Phosphorus | J. H. G. | 1·453 at 20 | 25·5 | 1·5030 | 1·5118 | 1·5418 | | | | | | 38 | 1·4957 | 1·5042 | 1·5334 | | 79. | Oxychloride of Phosphorus | J. H. G. | 1·680 at 20 | 17 | 1·4810 | 1·4882 | 1·5118 | | | | | | 26 | 1·4756 | 1·4832 | 1·5067 | The determinations of iodide of propyl were added, and those of acetic acid and terchloride of phosphorus were altered during the printing of the paper. ### Appendix II.—Table of Refractive Indices. The liquids in this Table are arranged according to their power of refracting the line A at 20° C. | Liquid | Temp. | Refractive indices | |-------------------------------|-------|--------------------| | | | A. | B. | C. | D. | E. | F. | G. | H. | | Phosphorus | 35 | 2·0389 | ...... | ...... | 2·0746 | ...... | 2·1201 | 2·1710 | 2·2967? | | Phosphorus in Bisulphide of Carbon | ? | 1·9209 | 1·9314 | ...... | 1·9527 | 1·9744 | 1·9941 | 2·0361 | 2·0746 | | Terbromide of Phosphorus | 25 | 1·6698 | 1·6752 | ...... | 1·6866 | ...... | 1·7083 | 1·7300 | 1·7506 | | Bisulphide of Carbon | 11 | 1·6142 | 1·6207 | 1·6240 | 1·6333 | 1·6465 | 1·6584 | 1·6836 | 1·7090 | | Lepidine | 21 | 1·6039 | 1·6094 | ...... | 1·6189 | ...... | 1·6403 | 1·6615 | 1·6822 | | Bibromide of Bromethylene | 13 | 1·5819 | 1·5851 | ...... | 1·5915 | ...... | 1·6037 | 1·6149 | 1·6249 | | Rectified Oil of Cassia | 28 | 1·5649 | 1·5699 | 1·5727 | 1·5801 | 1·5909 | 1·6014 | 1·6244 | ...... | | Aniline | 21·5 | 1·5644 | 1·5684 | ...... | 1·5774 | ...... | 1·5951 | 1·6125 | 1·6297 | | Chinoline | 24 | 1·5567 | 1·5617 | ...... | 1·5687 | ...... | 1·5879 | 1·6030 | 1·6198 | | Trichlorobenzole | 20 | 1·5563 | 1·5602 | ...... | 1·5671 | ...... | 1·5809 | 1·5945 | 1·6065 | | Bromoform | 15·5 | 1·5579 | 1·5610 | 1·5628 | 1·5674 | 1·5737 | 1·5790 | 1·5901 | 1·5998 | | Dinitrobenzole in nitrobenzole | 23·5 | 1·5460 | 1·5506 | ...... | 1·5600 | ...... | 1·5791 | 1·5994 | ...... | | Bibromide of Chlorethylene | 12·5 | 1·5472 | 1·5500 | ...... | 1·5554 | ...... | 1·5659 | 1·5748 | 1·5830 | MDCCCLXIII. | Liquid | Temp. | Refractive indices | |------------------------|-------|-------------------| | | | A | B | C | D | E | F | G | H | | Nitrobenzole | 25 | 1·5331 | 1·5374 | 1·5398 | 1·5465 | 1·5554 | 1·5643 | 1·5832 | | | Hydrate of Phenyl | 13 | 1·5377 | 1·5416 | 1·5433 | 1·5488 | 1·5564 | 1·5639 | 1·5763 | 1·5886 | | Hydrate of Cresyl | 11·5 | 1·5341 | 1·5377 | | 1·5445 | | 1·5573 | 1·5699 | 1·5813 | | Eugenic Acid | 18 | 1·5285 | 1·5321 | 1·5341 | 1·5394 | 1·5464 | 1·5528 | | 1·5780 | | Mercuric Methyl | 26·5 | 1·5197 | 1·5232 | | | 1·5296 | | 1·5368 | 1·5526 | | Salicylate of Methyl | 21 | 1·5206 | 1·5241 | 1·5263 | 1·5319 | 1·5402 | 1·5478 | 1·5640 | 1·5810 | | Iodide of Methyl | 16 | 1·5203 | 1·5234 | | 1·5307 | 1·5377 | 1·5440 | 1·5558 | 1·5670 | | Mercuric Ethyl | 8·5 | 1·5300 | 1·5333 | | 1·5397 | | 1·5518 | 1·5634 | 1·5729 | | Nicotine | 18 | 1·5149 | 1·5174 | | 1·5234 | | 1·5346 | 1·5449 | 1·5542 | | Chlorobenzole | 9 | 1·5194 | 1·5223 | | 1·5290 | | 1·5418 | 1·5530 | 1·5636 | | Amyl-aniline | 23·5 | 1·5114 | 1·5150 | 1·5168 | 1·5232 | 1·5292 | 1·5361 | 1·5491 | 1·5622 | | Terchloride of Phosphorus | 23·5 | 1·5052 | 1·5088 | | 1·5148 | | 1·5252 | 1·5357 | 1·5446 | | Iodide of Ethyl | 23·5 | 1·5003 | 1·5034 | | 1·5095 | 1·5156 | 1·5214 | 1·5321 | 1·5420 | | Rectified Oil of Santal-wood | 25·5 | 1·4954 | 1·4977 | | 1·5015 | | 1·5093 | 1·5161 | 1·5223 | | Hydrocarbon from Cubebs | 10·5 | 1·4988 | 1·5012 | | 1·5055 | | 1·5145 | 1·5227 | 1·5294 | | Pyridine | 21·5 | 1·4940 | 1·4967 | | 1·5030 | | 1·5155 | 1·5278 | 1·5387 | | Lutidine | 22·5 | 1·4894 | 1·4924 | | 1·4987 | | 1·5100 | 1·5204 | 1·5308 | | Collidine | 23·5 | 1·4927 | 1·4958 | | 1·5013 | | 1·5127 | 1·5232 | 1·5329 | | Hydrocarbon from Cloves | 17 | 1·4918 | 1·4944 | | 1·4985 | | 1·5064 | 1·5140 | 1·5209 | | Pseudocumole | 12·5 | 1·4843 | 1·4872 | | 1·4932 | | 1·5040 | 1·5146 | 1·5236 | | Iodide of Amyl | 17·5 | 1·4816 | 1·4843 | | 1·4892 | 1·4941 | 1·4987 | 1·5074 | 1·5149 | | Oxychloride of Phosphorus | 17 | 1·4810 | 1·4840 | | 1·4882 | | 1·4967 | 1·5047 | 1·5118 | | Benzole | 10·5 | 1·4879 | 1·4913 | 1·4931 | 1·4975 | 1·5036 | 1·5089 | 1·5202 | 1·5305 | | Toluole | 14 | 1·4869 | 1·4898 | | 1·4957 | | 1·5072 | 1·5174 | 1·5271 | | Cymole | 29 | 1·4648 | 1·4671 | | 1·4717 | 1·4766 | 1·4808 | 1·4866 | 1·4957 | | Nitroglycerine | 13·5 | 1·4683 | 1·4706 | | 1·4749 | | 1·4824 | 1·4899 | 1·4947 | | Hydrocarbon from Portugal | 25 | 1·4617 | 1·4640 | | 1·4684 | | 1·4758 | 1·4826 | 1·4894 | | Cumole (2nd specimen) | 8·5 | 1·4687 | 1·4709 | | 1·4759 | | 1·4853 | 1·4936 | 1·5008 | | Stannic Ethyl | 23 | 1·4606 | 1·4629 | | 1·4673 | | 1·4758 | 1·4838 | 1·4905 | | Bichloride of Chlorethylene | 13 | 1·4661 | 1·4680 | | 1·4714 | | 1·4784 | 1·4841 | 1·4892 | | Hydrocarbon from Turpentine | 24 | 1·4596 | 1·4616 | | 1·4653 | 1·4691 | 1·4724 | 1·4790 | 1·4845 | | Hydrocarbon from Carraway | 24 | 1·4594 | 1·4615 | | 1·4652 | | 1·4724 | 1·4789 | 1·4844 | | Hydrocarbon from Bergamot | 26·5 | 1·4574 | 1·4598 | | 1·4640 | | 1·4721 | 1·4798 | 1·4865 | | Rectified Oil of Citronella | 19 | 1·4598 | 1·4619 | | 1·4655 | | 1·4730 | 1·4795 | 1·4860 | | Hydrocarbon from Bay | 23 | 1·4545 | 1·4567 | | 1·4610 | | 1·4690 | 1·4756 | 1·4818 | | Stannic Ethyl-methyl | 19 | 1·4555 | 1·4578 | 1·4590 | 1·4625 | 1·4674 | 1·4716 | 1·4795 | 1·4868 | | Chloroform | 10 | 1·4438 | 1·4457 | 1·4466 | 1·4490 | 1·4526 | 1·4555 | 1·4614 | 1·4661 | | Caprylic Alcohol | 9·5 | 1·4230 | 1·4246 | 1·4255 | 1·4279 | 1·4309 | 1·4338 | 1·4386 | 1·4429 | | Nitrate of Amyl | 10 | 1·4109 | 1·4127 | | 1·4157 | | 1·4219 | 1·4274 | 1·4320 | | Amylic Alcohol | 25 | 1·3981 | 1·3999 | | 1·4024 | | 1·4078 | 1·4122 | 1·4161 | | Hydride of Capryl | 9 | 1·4022 | 1·4037 | | 1·4065 | | 1·4076 | 1·4141 | 1·4197 | | Hydride of Cénanthyl | 9·5 | 1·3956 | 1·3968 | | 1·3996 | | 1·4045 | 1·4087 | 1·4135 | | Acetate of Amyl | 8·5 | 1·3944 | 1·3958 | | 1·3998 | | 1·4035 | 1·4077 | 1·4113 | | Butyric Ether | 23 | 1·3850 | 1·3864 | | 1·3888 | | 1·3938 | 1·3981 | 1·4018 | | Amylene | 8 | 1·3850 | 1·3866 | | 1·3896 | | 1·3944 | 1·3992 | 1·4033 | | Carbonic Ether | 22 | 1·3773 | 1·3785 | | 1·3810 | | 1·3856 | 1·3896 | 1·3936 | | Propionic Ether | 22·5 | 1·3696 | 1·3713 | | 1·3736 | | 1·3785 | 1·3827 | 1·3860 | | Boracic Ether | 22·5 | 1·3654 | | | 1·3698 | | 1·3742 | 1·3785 | 1·3815 | | Acetic Ether | 20 | 1·3645 | 1·3658 | | 1·3685 | | 1·3728 | 1·3766 | 1·3798 | | Alcohol | 15 | 1·3600 | 1·3612 | 1·3621 | 1·3638 | 1·3661 | 1·3683 | 1·3720 | 1·3751 | | Acetone | 25·5 | 1·3540 | 1·3554 | | 1·3582 | | 1·3629 | 1·3670 | 1·3706 | | Formic Ether | 22 | 1·3540 | 1·3553 | | 1·3582 | | 1·3627 | 1·3666 | 1·3694 | | Ether | 15 | 1·3529 | 1·3545 | 1·3554 | 1·3566 | 1·3590 | 1·3606 | 1·3646 | 1·3683 | | Water | 15 | 1·3284 | 1·3300 | 1·3307 | 1·3324 | 1·3347 | 1·3366 | 1·3402 | 1·3431 | | Methylic Alcohol | 20 | 1·3264 | 1·3277 | | 1·3299 | | 1·3330 | 1·3369 | 1·3395 | P.S. [Received May 28.]—It was not till after this paper was read that we became aware of the existence of an elaborate treatise by Dr. Schrauf, "On the Dependence of the Velocity of Light on the Density of Bodies," in Poggendorff's Annalen, cxvi. 193, in which he investigates the question mathematically, taking as the basis of his calculations our former experiments, and those of Deville, Weiss, and others. Our own line of thought has many points of analogy with that pursued by him, but there is this difference in the conclusion: he believes that $\frac{A^2 - 1}{D}$ and $\frac{B}{D^2}$ (or in our notation $\frac{\nu^2 - 1}{D}$ and $\frac{\chi}{D^2}$, $D$ being the density) are the constants at all temperatures, and are the functions on which depend the optical properties of mixtures; while we are led by our new experiments to accord that quality rather to $\frac{\nu - 1}{D}$, and to doubt any such simple formula as $\frac{\chi}{D^2}$ for the changes of dispersion. To this point we propose to recur at some future period if we have the opportunity. There is one point in reference to our method of observation which seems to call for a remark. Schrauf thinks that there is a slight change in the refringent angle of our prism on its being heated. Now our hollow prism has glass ends as well as glass sides; but supposing such a change actually occurs, it is evident it will produce a uniform error running through all our observations in Section I. This may be the reason why at high temperatures the observed is almost always less than the calculated index; but as bisulphide of carbon and water agree so closely with either his or our theory, this source of error must be extremely minute. We await with curiosity the publication of the experiments referred to in Dr. Schrauf's short note, "On the Velocity of Light and Chemical Composition," in the April Number of Poggendorff's Annalen.