ICE, WATER, AND STEAM 



123 



the liquid ammonia expands are usually surrounded with 

 brine, and this is cooled by the evaporating ammonia several 

 degrees below the freezing point of water. The brine, in 

 turn, cools the water which has been placed in the freezing 

 chambers. The gaseous ammonia passes on to the pump and 

 is used again. 



In cold-storage plants the machinery is similar to that used 

 in making ice, but it is used to cool rooms instead of to cool 

 vessels of water. Of course any de- 

 gree of cold from the freezing point 

 upward can be had, and thus it is 

 possible to store each kind of food in 

 the particular temperature that is best 

 suited to it. Cold storage has made 



^ingpipes 



Cold-uxite 



FIG. 62. An artificial-ice machine 



possible a supply of fresh foods at seasons when they could 

 not otherwise be obtained. In meat and fish markets brine 

 pipes are often carried along the back of the counters where 

 perishable food is displayed. The cool air, being heavy, does 

 not mix with the air of the store by convection currents. The 

 brine pipes are often white with frost. Why ? 



141. Perspiration. The bodies of men and of some animals 

 are cooled by evaporation. In the skin there are multitudes of 

 small pores through which water is given off from the body. 

 Usually there is so little water given off that it evaporates as 

 rapidly as it appears, and we do not notice either the water 



