830 LOWERING OF TEMPERATURE BY LIQUEFACTION. 



and the liquid is greater than the cohesion between the 

 molecules of the solid. If a body dissolves, it passes 

 from the solid into the liquid state, and liquefaction by 

 solution is accompanied by a disappearance of heat, 

 just as if the body had been heated in order to liquefy 

 it. Every solution of a body, therefore, produces a 

 lowering of temperature. In those cases, where an 

 apparent exception to this law is observed as, for 

 example, in the solution of caustic potash (see page 

 679) in which a considerable quantity of heat is disen- 

 gaged the solution is accompanied by chemical com- 

 bination which produces in this and other similar cases 

 much more heat than that which becomes latent by the 

 liquefaction. 



Nitrate of ammonia is a very soluble salt, and its solution pro- 

 duces a considerable lowering of temperature. If it is dissolved in 

 about an equal weight of water, and the mixture is briskly stirred, 

 its temperature falls from the ordinary temperature of the air to 

 about 14. 



A mixture of equal weights of nitrate of potash and sal-ammoniac 

 lowers the temperature, not so much as nitrate of ammonia, but still 

 produces a considerable diminution of temperature. If 10Qe r of 

 powdered nitrate of potash (saltpetre) and 100s r of powdered sal- 

 ammoniac are dissolved in 200 CC of fresh water in a large tumbler, 

 and the mixture briskly stirred for about ten minutes with a long 

 thin test-tube which has been half filled with water, a considerable 

 portion of the water in the test tube will be found to be frozen. 



If it is only intended to show the lowering of the temperature by 

 the thermometer, without formation of ice, the quantities used may 

 be 25& r of each salt and 50 CC of water, which are stirred in a small 

 glass with the thermometer. 



If the salt, instead of being dissolved in water, be 

 mixed with ice, the diminution of temperature is still 

 more considerable, because not only the salt but the ice 

 also becomes liquefied, and more heat disappears. 



