72 Chemistry. [Lecture 26. 



imagined that if he extracted the air from water, 

 he could produce an ice heavier than water. He 

 extracted, therefore, the air from small glass 

 globes filled with water, and after exposing them 

 to an intense cold, they were a long time in 

 freezing, though cooled greatly below the freez- 

 ing point ; but upon breaking them to examine 

 them, the air rushed in, which, from the sudden 

 shock, occasioned the water to freeze in a mo- 

 ment. 



He afterwards found that simple agitation had 

 the same effect. If water is set at rest, it may be 

 cooled several degrees, at seven, eight, nine, or 

 ten, below the freezing point, without being 

 congealed ; but if touched with a bit of ice, or 

 the end of a wire, or if the vessel is agitated, the 

 congelation pervades it like a flash of lightning. 

 This is confirmed by Mairan in his treatise of 

 frost. He exposed small drinking-glasses full 

 of water, which might be cooled below the 

 freezing point, and if left undisturbed, might 

 remain so, but upon being agitated they froze 

 immediately ; and if a thermometer was put to 

 the water during its freezing, the moment it was 

 frozen it rose up to the congealing point; there- 

 fore a quantity of latent caloric seems to have 

 quitted the water : only part of the water was 

 congealed into a spongy substance, which con- 

 tained water in its pores ; the water was eight 

 or nine degrees below frost, and suddenly be- 

 came warmer. Here we see a quantity of latent 



