THE CRYOPHORUS. 835 



0. The density and pressure of the rarefied air must thus amount 



4'6 

 to considerably less than - - = T ^ of its original density and 



7oO 



pressure. 



The shallow basin for the sulphuric acid is cracked off with 

 pastille from a bottle, of which the upper portion may be broken. 

 The little cork vessel is cut with a knife. The three legs are 

 formed of small pieces of glass tubing, of which the lower ends are 

 closed by the blowpipe, otherwise the sulphuric acid rises in them 

 by capillary attraction and destroys the cork vessel. The blackening 

 of the inner side of the cork vessel (over a burning strip of wood 

 dipped in turpentine or paraffine oil) has the advantage that very 

 little heat can be transmitted by the cork to the water, because 

 lampblack is not moistened by water, and there is hence only very 

 slight contact between the water and the vessel in which it is 

 contained. 



The evaporation of the water proceeds sometimes without the 

 production of bubbles. Sometimes it boils briskly, and in some 

 instances the surprising phenomenon may be observed that a layer 

 of ice is lifted by boiling water at 0. 



Ice itself evaporates in a vacuum, and when the pump and 

 receiver hold a vacuum well, the ice formed may be seen to diminish 

 and disappear altogether in a few hours without previously melt- 

 ing. 



Water may be frozen by its evaporation without the 

 help of the air pump, by means of a contrivance called 

 the cryophorus. It consists of a bent glass tube provided 

 with a bulb at each end. The apparatus is prepared 

 by introducing a small quantity of water, which is then 

 boiled until all air is expelled by the steam. It is then 

 hermetically sealed, so that on cooling it contains nothing 

 but water and the vapour of water. If now the water is 

 allowed to collect in one bulb while the other is im- 

 mersed in a freezing mixture, the vapour in the tube is 

 condensed, and as the water in the other bulb commences 

 to yield rapidly more vapour, this rapid evaporation, 

 which requires a large amount of heat, lowers the 

 temperature of the water so considerably that it freezes. 



3 H 2 



