investigating the Spectre Heats of the Gases. 7 



events, assuming such to be the case, I shall, without further preface, proceed to 

 explain the particulars of a new method which I have adopted for comparing the 

 capacities for caloric, of the different aeriform fluids. 



In a paper which I had the honour of reading in November last, before the 

 Royal Irish Academy, I showed that the formula 



480^ p^ 

 ^ —-^ e ^30 



expresses the relation between the indications of the wet bulb hygrometer and 

 dew-point ; e being the caloric of elasticity of vapour, at the temperature t' of the 

 hygrometer; a, the specificheat of air; dzzt — t', the difference between the tem- 

 perature of air and hygrometer; andy and/"", the elastic forces of the vapour of 

 water, at the temperatures of the hygrometer and dew-point. If, therefore, the 

 dew-point and temperature of hygrometer be taken in each of the gases, their 

 specific heats become known, for in each case, as may be easily deduced from the 

 expression given above, 



Sucha method, though theoretically exact, is beset with such difficulties, that it 

 may, I think, be considered as practically impossible. The artificial gases, as 

 usually collected, are saturated with moisture — a state in which they are quite 

 unsuited for the necessary experiments ; and even though this difficulty were 

 overcome, it would, I conceive, be nearly impossible to determine their dew- 

 point by direct observation. 



By recurring, however, to the former equation, 



J —J e ^30' 



and modifying it, so as to suit a particular case, a much simpler method of investi- 

 gation is suggested. If the air be supposed perfectly dry, y"=o, and 



_ ef 30 

 "-48^ ^7' 



an expression involving no unknown quantity but d, and which will therefore 



* a, is here assumed to represent the specific beat under a given volume of the gas, which is the 

 subject of experiment. 



