Vapour in Experiments on the Absorption of Heat. 421 



air; but then also, almost every time, a precipitate of water upon 

 it could be observed. 



The only other vapour I have hitherto investigated has been 

 that of alcohol : as the vapours escaped from the tube into the 

 working-room, almost all others are excluded in this process. 

 But those of alcohol exhibit the phenomena of aqueous vapour 

 in a heightened degree. The cooling which ensued on forcing 

 air containing alcohol-vapour into one of the brass tubes was 

 so great that it could not be observed in the ordinary manner, 

 but the shunt had to be used. In the blackened as well as in 

 the velvet tube the action was much stronger than on for- 

 cing in air charged with aqueous vapour, though in this case the 

 shunt was not needed. In the pasteboard tube also, in which 

 there was little or no perceptible action on forcing in aqueous 

 vapour, cooling occurred when alcohol- vapours were forced in. 



In the double tube also the action of the vapour of alcohol was 

 much stronger than that of aqueous vapour. When the tempe- 

 rature of this tube was 20 o, 25 C, while that of the air saturated 

 with vapour of alcohol was only 13°5 C, the cooling was so 

 strong that the galvanometer went beyond the scale, and the 

 shunt had to be used for the observation ; the same was the case 

 when the temperature of the tube was 37° C. (that is, 23°*5 C. 

 above that at which the air was saturated with aqueous vapour). 



This great action of alcohol-vapours pointed to a great adhe- 

 sion of vapour ; and this was observed even when it was forced 

 into the tubes without the application of a source of heat ; for 

 there was heating both in the brass tube and in the blackened 

 and the velvet one, and in all much stronger than by aqueous 

 vapour ; this was especially the case in the brass tube, in which 

 aqueous vapour had produced no effect at all. But besides that, 

 alcohol- vapours absorb rays of heat, as is evident from the fol- 

 lowing experiments : — 



Four^ narrow brass tubes, 8 millims. in diameter and 66 cen- 

 tims. in length, lay horizontally at distances of 12 millims. from 

 each other. Each of them contained forty fine holes in a row, 

 at a distance of 12 millims. from each other. One end of these 

 tubes was closed, all four were joined at the other to a cross 

 tube which was connected with a bellows. As the fine holes of 

 this system were all turned upwards, on forcing air through, a 

 number of very fine upward currents of air were obtained. This 

 system of tubes was substituted for one of the brass tubes be- 

 tween the pile and the cube, in such a manner that the heat- 

 rays could only reach the pile after passing through those cur- 

 rents of air. 



When the air forced in was saturated with alcohol-vapour, a 

 considerable cooling ensued. When it was free from vapour, the 



