1821.] Causes of Calorific Capacity, Latent Heat, fyc. 377 



equal to that due to the tension of vapour at the same tempera- 

 ture. Let us denote the higher temperature, or temperature of 

 the atmosphere by t, the corresponding tension of the vapour 

 by t, the megethmerin of the vapour in the atmosphere by M, 

 and of the vapour at its proper tension by m ; and suppose E the 

 elasticity, the vapour in the atmosphere would have occupying 

 at the same temperature the same space in vacuo. Also let t' 

 be the proper tension of the vapour at the lower temperature t' t 

 or at that temperature at which the vapour of the atmosphere 

 begins to be sensibly deposited, and m' its megethmerin. Then 

 m' being the megethmerin of the vapour in the atmosphere at the 

 temperature t' ', by the laws of gases, and supposing the atmo- 

 spheric pressure the same, 



m' : M :: t 3 : t-' 



But t' : E :: m' t-' : M t°- 



by the laws of gases in my former paper ; consequently t' = E ; 

 that is, the elasticity of the vapour in the atmosphere, had it the 

 same temperature and megethmerin in vacuo, would be equal to 

 the tension of the vapour at the temperature at which it sensibly 

 begins to be deposited or to form dew. Therefore r : E or m : 

 M :: t : r, and consequently the humidity of the atmosphere is 



equal to -, perfect humidity being, at the temperature t, sup- 

 posed to be when the megethmerin is the same as that of the 

 vapour at its proper tension. 



xMr. Dalton, and after him M. Biot, in the celebrated Traite de 

 Physique, has given a different method of determining the humi- 

 dity of the atmosphere ; but these philosophers not having the 

 advantage of a knowledge of the true nature and laws of aeriform 

 bodies could scarcely avoid falling into some inconsistencies, 

 where they had not experiment to direct them. Mr. Dalton, 

 says M. Biot, " determine le degre precis du thermometre, oil 

 l'humidite de l'air commence a se deposer en rosee sur les parois 

 exterieures du vase. Quand il connait cette temperature, il 

 calcule la force elastique de la vapeur qui y correspond, et cette 

 force, ramenee a la temperature exterieure par les lois ordinaires 

 de la dilatation des gaz, est precisement celle de la vapeur aqueuse 

 qui se trouve actuellement dans l'air." Now I do not know of 

 any reason why this should be the case. On the contrary, expe- 

 rience proves that in a given space the quantity of vapour over 

 its generating fluid is not augmented nor diminished by the pre- 

 sence or absence of any quantity of atmosphere ; consequently, 

 if at the temperature where sensible deposition begins to appear 

 all the atmosphere could be taken from the vapour, and this left 

 to occupy the same space, its elasticity and megethmerin would 

 be the same as the tension and megethmerin of vapour over its 

 fluid in vacuo, which precisely accord with our theory. But the 

 pressure of the atmosphere being the same on the compound 



