Gay-Lussac on the Evaporation of Liquids. 295 



by the surrounding bodies, would be more rapid than the cool- 

 ing due to the evaporation ; and thus, the cold could not reach 

 its limit. 



In the second place, the liquid, evaporating only by means of 

 the air, which impels against its surface, cannot evidently cool 

 as much as in vacuo ; and for a given initial temperature, the 

 cold produced is at its maximum, when the caloric, absorbed by 

 the vapour, is equal to that which the air loses, to put itself in 

 an equilibrium of temperature and pressure with it, plus the 

 caloric poured into the evapornting surface by the surrounding 

 bodies ; but the quantity of the latter, when the cold produced 

 is only a few degrees, is small in comparison of the other, and 

 may be neglected. From the latent heat of the vapour of the 

 evaporable liquid, the law of its elastic force relative to the tem- 

 perature and its density, on one hand ; and, on the other, the 

 capacity of the air for heat, its temperature, its density, and its 

 pressure, M. Gay-Lussac has constructed a formula, for calcu- 

 lating the degree of cold, which should be produced by evapo- 

 ration. In order to compare his theory with experiment, he 

 determined directly the depression of temperature produced by 

 a current of dry air on a mercurial thermometer, surrounded with 

 moistened cambric. The air issuing from a gasometer, under a 

 constant pressure, passed first through a tube filled with chloride 

 of calcium ; from this tube it entered another, where it met a 

 thermometer destined to show its temperature ; then five cen- 

 timetres further on, (two inches E.,) another thermometer with a 

 moistened surface, which it enveloped on every side. Thence, it 

 diffused itself freely in the atmosphere, without suffering fur- 

 ther change of pressure. The calculated and experimental 

 results coincide very nearly. We shall content ourselves with 

 giving the latter. 



