424 Experimental Researches on the Depressions [July, 



circumstances that can occur. The rule for its use will be given in 

 the proper place. 



§ 5.- — Depression of wet -bulb in other gaseous media. 



It has been seen that the theory of the wet- bulb thermometer is 

 entirely based on the relation of the specific heats, or capacities, of 

 water, of vapour, and of air. It may be made therefore to furnish an 

 unexceptionable and easy method of solving the much- contested ques- 

 tion of the relative capacity of different gaseous fluids, by substituting 

 any of the latter for common air in the experimental determination f 

 the depression. 



By Gay Lussac's formula we perceive that the depression varies 

 precisely in the inverse ratio of the air's capacity, c (see p. 405.) 

 Apjohn's formula is based on the same datum ; thus the specific heat of 

 vapour at 50° being 1 129 (= 967 + 212 — 50) ; that of water being 

 1 ; and that of air c = 0.267 ; " one part of air in cooling through d 

 degrees will raise the temperature of 0.267 part water through the 

 same number, and will consequently be adequate to vaporize a quan- 



267 d " 



tity of water represented by '— TT^r~ ' Now, as *267 d (= c d) is a 

 constant quantity, any change in the value of c must affect d in an 



opposite or inverse sense, that is c' = — , d' being the depression ob- 

 served in other medium than common air. 



As most likely to exhibit any difference of specific heat, and without 

 reference to any prior determination of the question, I selected two 

 gases, hydrogen and carbonic acid, as far at variance in essential points 

 as could be wished, and proceeded with them exactly as had been 

 done with ordinary air. On account of the mode of preparing the 

 two gases by distillation through a water-trough, they entered the 

 gasometer surcharged with moisture : and, as noticed below, even 

 after being well dried by the acid in the chamber, they took up mois- 

 ture from the discharge-pipe on their passage to the wet-bulb. I 

 could only approximatively remedy this evil by immediately filling in 

 common air, and finding how much moisture the latter also absorbed 

 in its passage. The error was of course less, if at all, perceptible at 

 the high temperatures, and in a fresh series of experiments it was 

 obviated by the introduction of my tell-tale hair hygrometer. 



Wishing to save the gas, it was made to pass into another gasome- 

 ter instead of into the open air ; on which account the current both of 

 hydrogen and of carbonic acid passed more slowly through the steam- 

 heated tube than the air had done, and their temperature only rose 

 to 160 and 170, in lieu of 180 and even 190 as at first. Here follow 



