sides of a cylinder, is to the velocity with which it is given off by
the blackened sides, as 5,654 is to 10,000 very nearly, the velocities
being as the times of cooling inversely.
Before he proceeds further in his investigation, the author finds it necessary to describe an additional instrument which he contrived for measuring, or rather for discovering, those very small changes of temperature in bodies which are occasioned by the radiations of other neighbouring bodies that happen to be at a higher or a lower tempera- ture. This instrument, which he calls a Thermoscope, consists of two glass balls joined with and opening into the two ends of a glass tube, which is bent in two places at right angles, so that the balls, when the instrument is erected, are at the same horizontal height. A small quantity (about one drop) of coloured spirits of wine was introduced into this tube before it was finally closed, which, when the tempera- ture of the air in the whole tube and tle two balls is equal, keeps its place nearly at the middle of the lower or horizontal part of the tube. No sooner, however, does this perfect equilibrium cease, than the drop will move towards the side that is least heated. A scale is here ap- plied, which indicates the difference of the temperature of the air in the two sides of the tube, and in the respective balls. A vertical ścreen between the two balls prevents thıe radiance of a heated body approached to one of them from affecting the other. This instrument was found of so delicate a sensibility, that the naked hand presented to one of the balls at the distance of several inches, would put the spirit of wine in motion, and the approach of a person at some feet from it would immediately affect it.
A conjecture is now proposed, which this instrument was intended to elucidate and probably confirm. There being great reason to con- clude, that all the heat which a hot body loses when exposed to the air, is not given off to the air which comes into contact with it, but that a large proportion of it escapes in rays which do not heat the transparent medium through which they pass, but, like tlhe rays of light, generate heat only tlhen and there where they are intercepted and absorbed; it may hence be concluded, that in general, as has been in particular observed in the foregoing experiments, the cooling of the instruments is in fact promoted by the coverings applied to their surfaces; those coverings, considered as substances on which the rays impinge, being the means which in some way or other acce- lerate, or at least facilitate, the emission of calorific rays from the hot surfaces.
The first experiment, which has thrown some light upon this sub- ject, was made with two brass cylinders equally heated, but in one of which one of the flat surfaces had been blackened, while the whole of the other cylinder was left in its polished state. The black sur- face of the one, and one of the bright surfaces of the other, were presented to tlie two opposite balls of the thermoscope, each to each, and at equal distances. Here the little column of spirit of wine in the tube beneatlı was instantly driven out of its place by the superior action of the blackened surface, and did not return to its former
