788 RETARDATION OF SOLIDIFICATION. 



night to several degrees below ; but the least agita- 

 tion, as in attempting to pour the water out, converts 

 the whole instantly into a mass of ice crystals. 



This retardation of solidification is also observable in 

 water deprived as much as possible of the air which it 

 ordinarily holds in solution ; hence water kept per- 

 fectly at rest in the vacuum of a receiver will not freeze 

 unless cooled several degrees below 0. A glass vessel 

 which is partly filled with water, while the remaining 

 space is free from air, a so-called i water-hammer ' (see 

 art. 56), is especially suitable for showing the pheno- 



menon of continued liqui- 



_) dity. One form of the 



water-hammer, which is 

 very serviceable for a 

 variety of experiments, is 

 represented in fig. 389. 

 FIG. 389 (| real size). Jf a water-hammer of 



this kind be placed on a moderately cold winter's day 

 in the open air, we may be pretty sure that the water 

 will not freeze as long as the instrument is kept at rest, 

 even if the temperature should fall to 10 or 15 below 

 the freezing point of water ; but as soon as it is agi- 

 tated, the water will immediately congeal. 



The experiment with the water-hammer may be made in a room 

 and in a comparatively short time, by placing it in a freezing mix- 

 ture (see art. 58) made of common salt and ice. If ice at is 

 mixed in a suitable proportion with salt, the temperature of the 

 mixture falls to from 18 to 21. This is too low a temperature for 

 the experiment, and the water in the vessel simply freezes ; but the 

 experiment will mostly succeed if made in the following way. A 

 basin or wide pot, of about 1 litre capacity, is filled with ice pounded 

 into small fragments, and water is poured on to the ice until it is 

 nearly covered with it ; a good handful of salt is now sprinkled 





