as deduced by himself and Dr. Suerman. 34<7 



derived from the former source necessarily augments with the 

 rapidity of the gaseous current, and in a somewhat quicker 

 ratio, owing to the increased depression, while the heat de- 

 rived from radiation is in its amount uninfluenced by the 

 speed of the blast save in as far as such speed affects the dif- 

 ference between the stationary temperatures of the wet and dry 

 thermometers. The relative amount therefore of the caloric 

 obtained from the former source will obviously rise with the 

 velocity of the current of air or gas ; and if this velocity be ren- 

 dered very great, it is easy to conceive that the caloric of radia- 

 tion may be a negligible quantity, and that the latent heat of 

 the vapour formed may be considered without sensible error 

 as exclusively derived from the aeriform fluid which is the sub- 

 ject of experiment. In this latter case the depression is a maxi- 

 mum, and with a quick blast it is greater than with a slow one, 

 because in such case, the caloric of the vapour being derived 

 in greater relative quantity from the gas, this latter must be 

 cooled through a greater number of degrees in order that the 

 heat which it evolves should be larger in amount. So far the 

 matter is sufficiently plain, and indeed presents no difficulty 

 whatever. Doubts, however, may be entertained, whether, 

 with any velocity of blast which can be conveniently employed, 

 the radiation of the sides of the tube can be prevented from 

 exerting an appreciable influence ; and whether, with a given 

 velocity of blast, the reduction of the depression due to radia- 

 tion may not be different in the case of different gases. 



In reference to the first of these two points we are without 

 any precise information. It is certain, as we have seen, that 

 radiation must always tend to diminish the value of ^— ^', but 

 the degree of its influence remains to be ascertained. In re- 

 lation to the second point M. Suerman has hazarded some 

 opinions, in the correctness of which I certainly cannot con- 

 cur. It is well known that gases differ materially as to the 

 mobility of their particles, and that this mobility follows some 

 ratio reciprocal to that of the specific gravity, so that if the 

 same heat be applied to equal volumes of hydrogen and car- 

 bonic acid, it is propagated through the former with much 

 greater velocity than through the latter. This, as is well 

 known, is one of the causes which renders the method of in- 

 vestigating the specific heats of gases by the velocity of heating 

 incapable of yielding accurate results. Now M. Suerman ex- 

 presses a strong opinion that this difference of mobility between 

 the integrant molecules of different gases has an influence also 

 upon the cold of evaporation. " Vix autem dubitandum vi- 

 detur quin et frigus evaporationis aliquatenus hinc pendeat." 

 This is to me quite unintelligible. How can the difference 



