Extract of a Letter from M. Geoffroy, Chemist, Member of the Royal Academy of Sciences at Paris, and F. R. S. to Sir Hans Sloane, Bart. President of the Royal Society; Concerning Mr. Seignette's Sal Polychrestus Rupellensis, and Some Other Chemical Salts. By T. S. M. D.

VII. Extract of a Letter from M. Geoffroy, Chemist, Member of the Royal Academy of Sciences at Paris, and F. R. S. to Sir Hans Sloane, Bart. President of the Royal Society; concerning Mr. Seignette's Sal Polychrestus Rupellensis, and some other Chemical Salts. By T. S. M. D. Paris, May 4, 1732. SIR, We have nothing new in Chemistry here, but the Discovery we have made of the Make of Messr. Seignette's Sal polychrestus Rupellensis. It is a Tartarum solubile, composed of Cream, or Crystals of Tartar; and the fix'd Salt of the Kali of Alicante well depurated. This Salt is very singular; for tho' it be a fix'd Alkaline Salt, it has the peculiar Property of crystalizing; nor does it easily dissolve in the open Air, as other fix'd Salts do; but on the contrary, it calcines therein, like Vitriols or Glauber's Salt. Another peculiar Property, which I have observ'd to belong to it, is, that if it be satiated with a vitriolick Acid, and the Liquor be evaporated, there results a Salt that has the Form of Glauber's Salt, and all the Properties requisite to make Messr. Seignette's Salt. In order to which, Take of the Salt of Kali well purified, &c. dissolve it in Water, add thereto of Crystals of Tartar about 1½ lb; boil the whole in order to dissolve the Crystals of Tartar. But the exact Proportion of Crystals of Tartar can be no more determined in this Operation, than in making the Tartarum Solubile; either because the Salt of Kali has retain'd more or less Humidity in its Crystallization, or because the Tartar has more or less Impurities in it. But if there be too much Tartar in the alcaline Liquor, after the Fermentation is over, filtrate the Liquor, and as it cools, the superfluous Tartar will fall to the Bottom. After the Separation of the Tartar from the Liquor, evaporate the Lixivium by a gentle Heat, set it in a cool Place to crystallize, and you will have very fine Crystals. If the Liquor be evaporated a little too much, there will be no Crystals of Salt formed, but the Liquor will be converted into a hard transparent Mass, not unlike Glue. But if you dissolve this Mass again, you may make it crystallize, as upon dissolving Seignette's Salt. This Salt purges very well, from one to two Ounces dissolved in a Quart of Water. Such is the Discovery of this Salt, which has hitherto passed for an Arcanum. We have likewise his crystallized alcaline Salt, which is the Salt of Kali, that dissolves not in the Air. I am actually at work in perfecting this Salt, in examining that of Kali, and comparing it with Borax. From this last I extract Glauber's Salt, Salt, by mixing it with Oil of Vitriol. The Mixture of Borax 3iv with O.l of Vitriol, 3j 3j upon Sublimation gives me the Sal Sedativum described by M. Homberg; and the Residue exposed to a strong Fire affords Glauber's Salt. I have found out a Method to shorten this Operation; for instead of subliming this Salt, I get it by Crystallization in light foliated Laminae. This Salt, whether sublimated or crystallized, has the Property of dissolving in Spirit of Wine; and if you set this Spirit of Wine on Fire, its Flame is green. Spirit of Wine has no Effect on Borax; the Oil of Vitriol, digested with Spirit of Wine, communicates no Greenness to its Flame: therefore it is requisite that the Borax should be united to an Acid, in order to produce this green Flame. POSTSCRIPT: I send you a Specimen of Salt made of Crystals of Tartar and Lime Water, which Messrs. Grosse and Duhamel, two Members of our Academy, have prepared; to which I join Crystals of Seignette's Salt, that M. Bolduc and I have made separately. You will also find some Sal Sedativum made by Crystallization, which crystallizes in a peculiar Manner. This Operation is perform'd with 3iv of Borax, and 3i 3i of concentrated Oil of Vitriol, the most fixt and weighty that can be had. The Borax is put into a Glais Retort, the Oil of Vitriol is poured on it, and then half an Ounce of common Water. This Mixture being exposed to a Fire gradually encreased, after the Phlegm has passed off, and even while it is passing, there rises Flowers, or a volatile Salt in very beautiful foliated Lamina; some of which melt by the Heat of the Fire. After the Operation, the finest of these Flowers, which are round the Neck of the Retort, are gathered; and those that are grey, are thrown upon the remaining Mass: which Mass is dissolved in Water, filtrated, and evaporated slowly. Sometimes, even without Evaporation, the shining Talcous Lamina are to be seen in the Liquor. In 24 Hours the Liquor is poured off these Lamina: they are washed in fair Water, set to drain, and then to dry in a Stove. If these Crystals do not calcine in the Stove; or in the Sun, 'tis a Sign there is nothing crystallized but the Sal neutrum: if they do calcine, 'tis a Sign that there is some Glauber's Salt mixed. And then this Salt must be dissolved again in hot Water, and recrystallized. No body before me has thought of extracting this Salt by Crystallization: It was always sublimed hitherto. Printed for W. Innys and R. Manby, Printers to the Royal Society, at the West-End of St. Pauls. MDCCXXXVI.