422 Prof. Magnus on the Influence of the Adhesion of 



pile assumed its previous temperature. There was in this case 

 no reflecting body, the cooling could only have resulted from an 

 absorption of heat by the alcohol-vapour. 



The case was quite different with aqueous vapours ; for when 

 saturated and perfectly dry air were alternately forced through 

 this system of tubes, there was no alteration in the heating of 

 the pile. This experiment has been frequently repeated, and 

 always with the same result. 



When carbonic acid was blown through instead of moist air, 

 there was a cooling each time. 



By this my former statement is, I think, entirely confirmed, 

 that between the absorptive power of moist and that of dry air 

 no difference can with certainty be established. 



If, as Messrs. Tyndall and Wild maintain, the great cooling 

 which they obtained when moist air was forced through the po- 

 lished metal tubes they used depended on a corresponding ab- 

 sorption of heat by aqueous vapours, it must take place to the 

 same extent in all tubes, whatever be the condition of the inner 

 surface. The path which the rays traverse on the internally 

 polished tubes is greater indeed than in the rest ; but a consi- 

 derable increase of absorption could not be occasioned thereby, 

 and, as compared with the total heat issuing from the tube, the 

 absorbed part must be very nearly the same. In no case could 

 there be an increase instead of a decrease of temperature on 

 blowing in moist air, as was seen in the blackened and in the 

 velvet tube, and even in one of the pasteboard ones. 



The process taking place in the tubes is, as has been already 

 mentioned, tolerably complicated. For, first of all, by the con- 

 densation of vapours, or the adhesion of vapours, heat becomes 

 free on the inside of the tube ; this cannot indeed be perceived in 

 the polished metal tubes, on account of their conductivity, but 

 can in the badly conducting velvet tubes, or the strongly black- 

 ened metal ones ; the deposited aqueous particles, moreover, di- 

 minish the reflexion of heat ; and, in addition, air reaches the 

 pile and produces cooling or heating, according as it is dry or 

 moist. 



Only in the case of special dimensions of the tubes, as those 

 which Messrs. Tyndall and Wild used, and which were also 

 used in this investigation, does no air reach the pile — and even 

 with these dimensions, only under certain conditions. 



. In the second series of his experiments M. Wild states that 

 he always passed the air into the tube through the lateral aper- 

 ture which was more distant from the pile, because the disturb- 

 ing influence of air- currents against the pile was thereby avoided. 

 But even if it is passed through the more distant aperture, the 

 air still reaches the pile, if the tube is a little narrower at the 



