58 THE CULTURE OF FARM CROPS. 



may be shown by a simple experiment. A shallow vessel 

 containing sulphuric acid is placed in another containing 

 water and both are placed under the receiver of an air 

 pump. When the air is exhausted the vapor of the water 

 is so rapidly absorbed by the acid that the water is frozen. 

 By using liquid sulphurous acid which evaporates with in- 

 tense force, and pouring it into a red hot vessel, and then 

 adding water, the water is suddenly frozen into ice under 

 the intense cold produced by the rapid evaporation of the 

 acid. 



The lowest degree of cold ever produced; 220 degrees 

 below zero; was by means of the vaporization of liquid pro- 

 toxide of nitrogen mixed with bisulphide of carbon in a va- 

 cuum. These examples however are not of practical interest 

 to the farmer further than to exemplify the vast and varied 

 changes produced in matter by heat and cold. 



The same kind of result may be produced by the sudden 

 liquefaction of solids. Thus a mixture of salt and ice 

 causes the rapid melting of the ice and a sufficient reduction 

 of temperature to freeze water. In this case both the solids 

 are liquefied and the effect is intensified. The cold thus 

 produced is 40 below 7 zero. Four ounces of sal ammoniac 

 and the same quantity of saltpetre, finely powdered and dis- 

 solved in 8 ounces of water, will cause a reduction of 40 

 degrees of temperature; and powdered Glauber's salts, 

 drenched w T ith hydrochloric acid, will sink the temperature 

 from 50 degrees to zero. These mixtures are in common 

 use as the so called freezing mixtures. The newly intro- 

 duced ice machines by which ice is produced at a cost of 

 one dollar per ton, are operated by the vaporization of am- 

 monia in the gaseous form from its solution in water. 



A very useful practical application of the liberation of 

 heat by freezing is that often used to evade the freezing of 

 the contents cf cellars in very cold weather, by placing a 

 pail full of water in the cellar. The w T ater freezes more eas- 

 ily than any other liquid or solid containing liquid; as fresh 

 vegetables and fruits; and in the act cf freezing gives out 

 the latent heat of the water which actually warms the eel- 



