EVAPORATION. 171 



water under it to freeze. We shall thus have the singular exhibition of two 

 liquids, one resting upon the other, the one boiling and the other freezing at 

 the same moment ; and, after the lapse of a few minutes, one altogether disap- 

 pearing in the form of vapor, while the other solidifies in the form of ice. 



A beautiful experiment was contrived by Leslie, in which water is frozen on 

 this principle. A shallow vessel containing water is placed under the receiver 

 of an air-pump. Under the same receiver is placed a large flat dish, contain- 

 ing strong sulphuric acid. The receiver is now exhausted as rapidly as pos- 

 sible by the pump, and immediately the evaporation of the water takes place. 

 If the sulphuric acid were not present, the space within the receiver would be 

 saturated almost instantaneously with the vapor of the water, and all further 

 evaporation would be stopped ; but the sulphuric acid not being itself subject 

 to sensible evaporation, has besides a strong affinity for water, by virtue of 

 which it attracts the aqueous vapor, and causes it to be condensed on its sur- 

 face. As fast, therefore, as the water evaporates, its vapor is seized upon by 

 the sulphuric acid in the large dish, and the space within the receiver is still 

 maintained a vacuum ; so that the evaporation of the water continues as rap- 

 idly as in the first instance. Now the heat necessary to give the vaporous form 

 to the water can only be received from the water itself which remains in the 

 dish, and therefore it must undergo a rapid depression of temperature. It will 

 speedily fall to the temperature of 32, and in a few minutes will be frozen. 

 By this process, conducted under favorable circumstances, Leslie was not only 

 able to freeze water, but to congeal mercury ; and it is said that he even pro- 

 duced a cold of 120. The property on which this beautiful experiment is 

 founded is not recommended alone by the surprise and pleasure which its re- 

 sult always produces ; it is susceptible of useful application in chemistry when 

 it is necessary to separate water from liquids which heat would decompose ; 

 and to dry animal and vegetable substances without exposing them to disorgan- 

 ization. 



By the same method, the fact that ice itself, at all temperatures, is subject 

 to evaporation, may be made manifest. If a few ounces of ice be placed un- 

 der the receiver of an air-pump over a similar dish containing concentrated 

 sulphuric acid, and the receiver be exhausted, the ice will altogether disappear 

 in about twenty-four hours. During the whole of this time the temperature 

 will be considerably below 32. After the ice has disappeared the sulphuric 

 acid will be found to be combined with water and to have increased its weight 

 by the exact weight of the ice. 



In climates where the temperature of the air never falls so low as the freez- 

 ing point, and, therefore, where no natural ice ever exists, ice is obtained arti- 

 ficially by a cold produced by evaporation. In India it is obtained by making 

 extensive shallow excavations in large open plains. In these water is exposed 

 to evaporation in small earthern pots, unglazed, so as to be porous, and pene- 

 trable by water. Soft water, previously boiled, is placed in these vessels in 

 the evening, in the months of December, January, and February. A part of it 

 is usually frozen in the morning, when the ice is collected and deposited in 

 pits, surrounded by straw and other bodies which exclude heat. Radiation, 

 also, has a part in producing this effect as will be explained hereafter. 



Evaporation being extensively used in the arts and manufactures, it has be- 

 come a matter of considerable importance to conduct it with as much economy 

 and expedition as possible. The circumstances which principally promote it 

 being increase of temperature and a constant change in the air which is imme- 

 diately above the evaporating surface, these two objects have received special 

 attention. In factories where evaporation is used, the vessels containing the 

 liquid to be evaporated, are usually placed where they shall be exposed to a 



