386 



THE APPLICATIONS OF PHYSICAL FORCES, [BOOK iv. 



more or less considerable quantity of heat. The set of substances 

 thus* mixed to produce cold is called a freezing mixture. The 



FIG. 266. Ice-pail. 



FIG. 267. Goubaud's ice-mr. chine. 



following are some of the freezing mixtures most commonly em- 

 ployed : 



Two parts of snow or pounded ice with one part of salt produce a 



FIG, 268. Rocking ice-machine. 



cold which may reach 21 C. below zero. Five of ammonia chloro- 

 hydrate, five of porassic nitrate, eight of sodium sulphate, and sixteen 

 of water, produce a cold of 1 5. 



