VAPORIZATION. 63 



rupted manner. Various bodies, which have a powerful attrac- 

 tion for watery vapour, may he used as absorbents, such as 

 parched oatmeal, the powder of mouldering whinstone, and even 

 dry sole-leather, by means of any one of which a small quantity 

 of water may be frozen, during summer, in the exhausted receiver 

 of an air-pump. No substance, however, is superior, in this res- 

 pect to concentrated sulphuric acid. When this liquid becomes 

 too dilute to act powerfully as an absorbent, it may be rendered 

 again fit for use, by boiling it and driving off the water. Ice 

 might be procured in quantity, in a warm climate, by this pro- 

 cess. The necessary vacuum would be most easily commanded, 

 on the large scale, by allowing the receivers to communicate with 

 a strong drum, filled with steam, which could be condensed. 



In the Cryophorus of Dr. Wollaston, water is also frozen by 



its own evaporation. This instrument consists of two glass 



e \ bulbs, connected by a 



6s*\ tube, and containing a 

 ^r portion of water, as re- 

 presented in the figure. The air is first entirely expelled from 

 the instrument by boiling the water, in both bulbs, at the same 

 time, and allowing the steam to escape by a small opening at 

 the extremity of the little projecting tube e. While the in- 

 strument is entirely filled with steam, the point of e is fused 

 by the blow-pipe flame, and the opening hermetically closed. 

 In experimenting with this instrument, the water is all poured 

 into one bulb, and the other or empty bulb placed in a basin 

 containing a freezing mixture of ice and salt. The vapour in 

 the cooled bulb is condensed, but its place is supplied by vapour 

 from the water in the other bulb. A rapid evaporation takes 

 place in the water bulb, and condensation in the empty bulb, 

 till the water in the former bulb is cooled so low as to freeze. 

 The instrument derives its name of the cryophorus^ or frost- 

 bearer, from this transference of the cold of the bulb in the 

 freezing mixture to the bulb at a distance from it. 



It is by the evaporation of liquefied carbonic acid, that Thilo- 

 rier produces the extreme depression of temperature, 135, 

 which he has attained and measured. He allows a small stream 

 of liquid carbonic acid to escape, from a magazine of the liquid, 

 into a cylindrical box of wood, like a round snuff box in shape. 

 The stream of liquid, which immediately becomes in part gas, 

 is made, as it enters the box, to strike against a plain surface at 



