64 Imperial Institute of France. 



emanates, the more it is accelerated : on the other hand, we 

 know that atmospherical pressure retards evaporation, and 

 that this change of state takes place in vacuo the more 

 speedily, the more perfect the vacuum is. 



Mr. Leslie, fellow of the Royal Society of London, ha« 

 thought to increase still more this effect, by placing under 

 the recipient of the air pump bodies which are greedy of 

 moisture, and which, seizing the vapour as fast as it is 

 formed, multiplies indefinitely its production ; and he ob- 

 tained HI this way a cold so rapid and violent, that water 

 froze in a few minutes. This is a method of always having 

 ice at conniiand, almost without any other expense than the 

 fire necessary for once more drying the body greedy of 

 humidity which had been employed. 



Highlv concentrated vitriolic acid and the muriate of 

 lime are the most convenient absorbents for this purpose. 



']'wo young chemists, Messrs, Clement and Desormes, 

 have been occupied with determining the lin)its of this 

 process, and the degree of ceconomy to which it can be 

 carried; both from a calculation of the quantity of caloric 

 contained in the vapour of the water, and from the quantity 

 of charcoal necessary to produce a given quantity of vapour: 

 they ascertained that it requires but little more than one 

 part of charcoal to restore to its pristme state the absorbL-nl 

 which served to freeze 100 parts of water. Thus 100 

 pounds of ice will only require a pound and a few ounces 

 of charcoal. 



We may increase the effect, by preventing any caloric 

 frotn getting access externally ; and it is sufficient for this 

 purpose to render the recipient less of a conductor of heat, 

 by making it for instance of two plates of polished metal 

 separated by a stratum of air. 



We derive also from this acceleration of the evaporation 

 in vacuo, increased by the presence of absorbents, a more 

 evident advantage, wiien it is required merely to dry humid 

 substances, because we then avoid making them undergo 

 the action of the fire, which always alters them more or less. 



Our associate the late M. Montgolfier had already con- 

 ceived the idea of dryiiig contpletely the sap of plants, and 

 i)articularly the juice of raisins, by the pneumatic pump; and 

 le ascertained that, by diluting the latter juice in water, atter 

 it had been dried, it might still be fermented, and good wine 

 obtained from it ; but it cost him too much labour. 



It is nevertheless necessary to prevent their juices from 

 freezing, an inconvenience which would not be less trouble 

 sonie than that which results from the effects of fire.— - 



Messrs. 



