836 WATER FROZEN BY ITS OWN EVAPORATION. 



The water-hammer previously used may be employed 

 like a cryophorus. The water is allowed to collect in 

 the bulb, and the apparatus is suspended, as in fig. 

 404, over the edge of a vessel filled with a mixture of 



FIG. 404 (\ real size). 



pounded ice and salt. The vapour in the apparatus 

 being condensed, rapid evaporation of the water com- 

 mences, the temperature sinks gradually to 0, and the 

 water in the bulb freezes. 



In this experiment also the solidification is frequently retarded, 

 and the apparatus must be slightly shaken before the water will 

 freeze. This should be done by gently tapping the sides of the 

 bulb, so that the water may not be too much agitated ; for at first 

 only the upper layers sink to 0, while for some time afterwards the 

 remainder has still a temperature of 4, and a somewhat stronger 

 agitation might cause the warmer water to mix with the colder, the 

 temperature of which would thus rise again above the freezing 

 point. 



The apparatus should be taken out of the freezing mixture when 

 a small quantity of ice appears on the top of the water in the bulb. 

 If the experiment is continued until a compact layer of ice is formed, 

 the apparatus is apt to be broken in consequence of the expansion 

 of the ice. 



