Vapour in 'Experiments on the Absorption of Heat. 419 



Although, as I believe, the manner in which aqueous vapours 

 act has been sufficiently settled by the above-mentioned experi- 

 ments, it appeared to me desirable to prove this action in some 

 other way. With this view I arranged a tube in such a manner 

 that it could be conveniently kept at various temperatures. A 

 tinplate tube of the dimensions in question w T as concentrically 

 surrounded by a wider metal tube. The intervening space, 18 

 millims. in width, could be filled with water of various tempera- 

 tures. To prevent cooling as much as possible, the outside tube 

 was covered with several layers of a thick porous woollen cloth. 

 When the temperature of this double tube, which was quite 

 bright on the inside, was exactly the same as that of the air forced 

 in, the effects observed were just those described above. 



But when the temperature of the inner side was only a few de- 

 grees lower than that o£ the air forced in, the temperature of the 

 pile diminished to such an extent that the indications of the gal- 

 vanometer exceeded the limits of the scale. A longer time elapsed 

 before this decrease of temperature attained its extreme limit 

 than in the case in which the temperature of the tube was the 

 same as that of the air forced in. The tube was obviously filled 

 in both cases within the same time ; but the great quantity of 

 vapour deposited on the cooler tube required a greater quantity 

 of moist air, the entrance of which required a longer time. When 

 the maximum cooling had been attained and dry air was forced 

 in, evaporation of water again ensued : in consequence of this 

 the decrease of temperature was somewhat more rapid than 

 the decrease had previously been ; but it required much longer 

 time than if the tube had had the same temperature as the air 

 forced in, because again a great quantity of dry air was needed 

 to take up the water formed. 



When the temperature of the double tube was higher than 

 that of the air forced in, an action ensued of the kind mentioned 

 by Prof. Wild, but it was less the higher the temperature of the 

 double tube. Even when this was 38°, and dry and moist air 

 at the temperature of the room from 16° C. to 17° C. were 

 alternately forced in, a cooling ensued on the entrance of moist, 

 and heating when dry air entered. As this result was very un- 

 expected, I convinced myself by another method that there was in 

 fact a condensation of vapours on the inside of the tube, even when 

 the moisture forced in was still far from its point of saturation. 



For this purpose a thermopile was introduced into a space 

 which had a constant temperature of 38° C. One side of this 

 pile was closed by a tightly fitting cap. Hence a somewhat 

 considerable time elapsed before it had acquired on both sides 

 the temperature of the space and the galvanometer had come to 

 rest. Air which had the same temperature as the pile was blown 



2B2 



