54 



HEAT. 



condensed by permitting it to rush into a 

 refrigerator immersed in cold water. 

 The stop- cock in the pipe leading from 

 the boiler is then opened, and a large 

 proportion of its air rushes into the ex- 

 hausted sphere. This process is re- 

 peated, until the vacuum in the boiler 

 is sufficiently perfect. Heat is applied 

 to the boiler by a water-bath. The pe- 

 culiarities of this plan are the produc- 

 tion of a vacuum without the aid of an 

 air-pump, and the particular methods 

 used for condensation. 



By combining the exhaustion of an 

 air --pump, and the absorbent power of 

 sulphuric acid, Mr. Leslie succeeded in 

 freezing water. The experiment was 

 first made in June 1810. A watch-glass, 

 filled with water, was placed over a 

 shallow vessel, filled with sulphuric 

 acid; the air-pump being partly ex- 

 hausted, vapour was raised abundantly 

 from the water, and absorbed by the 

 acid, which occasioned such a degree 

 of cold as to freeze the water in a very 

 short time. Fig. 19, represents an 



Fig. 19. 



arrangement used for this purpose : a is 

 the glass receiver ; b a brass plate fur- 

 nished with a stop-cock and stand, and ca- 

 pable of being attached to the air-pump ; 

 c, the flat glass dish for holding the sul- 

 phuric acid ; d, the vessel containing the 

 water to be frozen, supported upon a 

 stand ; e is a cover attached to a sliding- 

 rod. By this process ice may be readily 

 obtained, in small quantities, at any sea- 

 son of the year. When the water to be 

 frozen has been previously deprived of 

 air by boiling, the process of congela- 

 tion goes on more slowly, but the ice 

 formed acquires a greater degree of 

 solidity. The process may be stopped 

 by lowering the cover e, upon the vessel 

 containing the water; and it proceeds as 

 oelore, the moment the cover is raised 



up again. A saucer of porous earthen- 

 ware is best adapted for holding the 

 water; and other absorbents, such as 

 parched oatmeal, the powder of mould- 

 wing whinstone, porous ignited pieces 

 of muriate of lime, and even the dry 

 powder of pipeclay may be used instead 

 of sulphuric acid. A hemispherical 

 porous vessel of earthenware, containing 

 one pound and a quarter of water was 

 placed by Mr. Leslie over a body of 

 parched oatmeal, one foot in diameter, 

 and one inch deep: by working the 

 pump for some time, the whole of the 

 water was frozen. 



The natives of India dexterously 

 avail themselves of the combined ef- 

 fects of radiation and evaporation, for 

 the purpose of obtaining a supply of 

 ice, when the temperature of the air is 

 much higher than the freezing point. 

 Excavations, about thirty feet square 

 and two feet deep, are made in the 

 large open plains near Calcutta ; these 

 excavations are covered at the bottom, 

 to the depth of about a foot, with the 

 dried stalks of Indian corn or sugar- 

 canes. Rows of small unglazed earthen 

 vessels, about an inch and a quarter 

 deep, are placed upon this bed. Soft 

 water which has been boiled, and 

 suffered to cool, is poured into these 

 vessels ; in the dusk of the evening, 

 in December, January, and February, 

 part of the water is usually frozen when 

 the weather is clear. The ice is col- 

 lected at sunrise, and thrown into a 

 deep pit, which is lined with straw and 

 coarse blanketing ; the mouth of the 

 pit is closed up with straw, and sheltered 

 by a thatched roof. 



Quicksilver may be frozen by the 

 united influence of evaporation, rare- 

 faction and absorption. A portion of 

 the metal being put into a hollow pear- 

 shaped piece of ice, it must be sus- 

 pended over a large surface of sul- 

 phuric acid ; when the exhaustion of the 

 receiver is made as complete as possi- 

 ble, the quicksilver will be frozen, and 

 may be kept in a solid state for several 

 hours. Other modes of effecting the 

 congelation of quicksilver have been 

 adopted with success. 



Dr. Ure, in his Lectures at Glasgow, 

 recommended several plans for effect- 

 ing the congelation of water in con- 

 siderable quantities. A series of cast- 

 iron plates, ground so as to fit the 

 receivers accurately which were placed 

 upon them, were attached to an air- 

 pump, by screws and stop-cocks, so. 



