Sir Isaac then heated a sufficiently thick piece of iron till it was red-hot; and having fixed it in a cold place, where the wind blew uniformly, he put upon it small pieces of different metals and other fusible bodies, and noted the times of cooling, till all the particles, having lost their fluidity, grew cold, and the heat of the iron was equal to that of the human body. Then, by assuming that the excesses of the heats of the iron and of the solidified particles of metal above the heat of the atmosphere, were in geometrical progression when the times were in arithmetical progression, all the heats were obtained. The iron was placed in a current of air, in order that the air heated by the iron might always be carried away by the wind, and that cold air might replace it with a uniform motion; for thus equal parts of the air were heated in equal times, and received a heat proportional to that of the iron. But the heats thus found had the same ratio to one another with the heats found by the thermometer; and hence he was right in assuming that the rarefactions of the oil were proportional to its heats. Another short chymical paper by Sir Isaac Newton has been published by Dr. Horsley. It is entitled _De Natura Acidorum_, but is principally occupied with a number of brief opinions on chymical subjects. This paper was written later than 1687, as it bears a reference to the Principia; and the most important facts which it contains seem to have been more distinctly reproduced in the queries at the end of the Optics.