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Art. XI. PROGRESS OF FOREIGN SCIENCE. 



1 . On the Cold ■produced by the Evaporation of Liquids. By 

 M. Gay-Lussac. 



This memoir was read to the Academy of Sciences so long 

 ago as 1815, but its publication was deferred, in the view of 

 rendering it more complete ; an intention which its author has 

 not possessed leisure to realize. 



The evaporation of a liquid may take place in a vacuum or in a 

 gas. The depression of temperature, which results, differs in these 

 two circumstances. In a vacuum, supposing the vapour to be 

 absorbed as soon as it is produced, the greatest cold takes place 

 for a determinate temperature of the ambient medium, when the 

 caloric absorbed for the transformation of the liquid into vapour 

 is equal to that entering the liquid from the sides of the re- 

 ceiver *. For it is evident that, since the latter augments with 

 the difference of temperature between the liquid and the sur- 

 rounding medium ; and as, on the contrary, the elaS'tic force of 

 the vapour goes on continually to diminish, as well as its velo- 

 city, (of formation,) there must necessarily be a period, at which 

 the caloric absorbed by the vapour shall be equal to the caloric 

 poured in by the surrounding walls. But if we lower the tem- 

 perature of the ambient medium, the limit of the cold will re- 

 trocede, and it may do so, even indefinitely, whilst the vapour 

 of the liquid shall preserve an appreciable tension. Thus M. 

 Gay-Lussac has frozen mercury with ease, by surrounding with 

 a frigoritic mixture of ice and salt, the vessels in which the 

 aqueous vapour was produced and absorbed by the apparatus 

 of professor Leslie ; and he does not doubt, that, with analogous 

 means, and very evaporable liquids, we may arrive at a degree 

 of cold much more considerable than by the mixtures. 



Suppose, now, that the evaporation takes place in a gas, per- 

 fectly dry, of a determinate temperature. Here new causes 

 come to influence the production of the phenomenon, which it is 

 necessary to appreciate. 



In the first place, the evaporation is retarded by the gas, 

 which presses on the liquid. It would amount to nothing, in a 

 gas perfectly at rest, whose density, under the same pressure, 

 would be equal to that of the vapour ; and the temperature 

 being supposed constant, it would augment nearly in proportion 

 to the velocity of the gas, until this velocity was equal to that 

 which the vapour would assume in vacuo. The cold produced 

 by the vapour, depends on it, up to a certain point ; for if it 

 were very little, it would be possible, that the heating produced 



* It is here supposed, that the evaporation takes phice over the whole 

 surface of the liquid, as with a thermometer with a moistened bulb. This 

 is the most favourable case for obtaining the maximmn of cold. 



