Experiments on Ignited Bodies. By John Roebuck, M. D. F. R. S.

XXXI. Experiments on ignited Bodies. By John Roebuck, M.D. F.R.S. Extract of a Letter from Dr. ROEBUCK to Dr. BROCKLESBY. R. Feb. 15, 1776. Mr. BUFFON asserts, that he found a ball of iron, which weighed 49 lbs. 9 oz. when cold, to weigh, when heated to a white heat, 49 lbs. 11 oz. which is an augmentation of weight of $19\frac{1}{5}$ grains to the pound. This extraordinary fact, circumstantially narrated by the very eminent and ingenious M. BUFFON, being contrary to the opinions of those philosophers who have most enlarged our natural Knowledge by their candid and cautious inquiry into the qualities of bodies, made me very solicitous to make similar experiments. Some time ago, when I was at Birmingham, I had very luckily an opportunity, by the aid of two accurate balances of my friend Mr. BOLTON's; one of which would, without straining the beam, weigh a pound and turn with one-tenth of a grain; and the other weigh half an ounce, and turn with the hundredth part of a grain. X x x 2 I heated I heated a piece of iron, of nearly one pound weight, to a white heat, or what the smiths call a welding heat, and found, by the most accurate experiments which I could make, and which I again and again repeated, that the iron, when left several hours in the balance to cool, weighed nearly one grain less when cold than when hot; and that a piece of iron, of about five pennyweights, which was tried by the smaller and more accurate balance, weighed, as appeared by an index which moved opposite to a quadrant, somewhat more when cold than when hot. I tried the same experiment on copper; but, to my great surprize, I found a piece of copper, of nearly one pound weight, four grains lighter after it had been left some hours to cool in the balance than when it was put in. I repeated the experiment, and found the event the same; but suspecting this might possibly arise from the copper casting scales, I placed a sheet of white paper under the balance, and collected as many scales as made up nearly the deficiency of the weight. TO SIR JOHN PRINGLE, BART. P. R. S. SIR, R. May 9, 1776. I BEG leave to inform you of the following experiments, which were made to ascertain the variation of the weight of bodies when hot and cold. London, May 9, 1776. April April 29, 1776, I heated a cylinder of wrought iron, which weighed fifty-five pounds, to a white heat, and exactly balanced the same, when hot, in the presence of the hon. Henry Cavendish, Nath. Piggot, Matthew Raper, Andrew Crosby, Edw. Delaval, Hamilton, David Hartley, William Russel, Esquires; Doctors Hunter, Brocklesby, Morton, William Fordyce, George Fordyce, Russel, Watson junior, Musgrave; Mess. John Hunter, Ben. Wilson, James Russel, Ramsden, Whitehouse, Magellan. After the cylinder had been two hours cooling in the scale, I weighed it again, and found that it had increased in weight three pennyweights and a few grains. Five hours after cooling, Mr. Magellan weighed it, and found it had increased in weight three pennyweights seventeen grains. Six hours after cooling, when the cylinder was blood-warm, I weighed it again, in the presence of Dr. Hunter, Dr. Brocklesby, Mr. Gray, and Mr. Neisbit, and found it to have increased in weight four pennyweights. The day following, about twenty-two hours after cooling in the presence of Matthew Raper and Andrew Crosby, esquires, I again weighed it, and found that it had increased in weight six pennyweights seventeen grains. Mr. Abram Whitehouse, who was very solicitous to have the above experiment made accurately, procured from Mr. Samuel Read a very exact beam, which readily turned, to the conviction of all the above gentlemen, with less than a pennyweight, though loaded with the above iron cylinder; but Matthew Raper, Andrew Crosby, Esquires, and myself self examined the beam leisurely and accurately, and found it turned very distinctly with four grains, though loaded as above. In order to discover the cause of this increase of weight of the cylinder when cold, I heated two ounces eight pennyweights of the scales or calx of wrought iron, and found the same to increase in weight five grains when cold. I heated two pieces of pure silver which weighed two pounds, ten ounces, five pennyweights; and when the silver was cold it increased in weight five grains, though it produces no calx from being heated red-hot. The above experiments are conformable to those which I formerly made with some very exact scales at Birmingham; an account of which I transmitted to Dr. Brocklesby. I have the honour to be, &c.