14-2 



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 the 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 

 screen between the two balls prevents the radiance of a heated body 

 approached to one of them from affecting the other. This instrument 

 \vas 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 the rays of 

 light, generate heat only then 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 the two opposite balls of the thermoscope, each to each, 

 and at equal distances. Here the little column of spirit of wine in 

 the tube beneath was instantly driven out of its place by the superior 

 action of the blackened surface, and did not return to its former 



