486 



FREEZING 



When a salt is dissolved in water cold is produced. 



If a liquid vaporises, the- vapour is produced at tho expense of tho heat of some 

 neighbouring body. 



When spirits of wine, or ether, is thrown on tho body, a sensation of coldness is 

 produced from tho vaporising liquids robbing the body of heat. 



By placing water in a porous vessel, and exposing it to tho sun, it becomes very 

 cold. Tho solar heat-rays occasion a rapid evaporation of the water which has filtered 

 through tho pores of tho vessel, and some heat is taken by tho process from tho fluid 

 in tho interior. 



If air is allowed suddenly to expand, it takes heat from tho surrounding bodies, or 

 produces cold. 



Sir Jt)hn Herschel some years since recommended tho following method for obtain- 

 ing at moderate cost largo quantities of ice : 



A steam-engine boiler was to be sunk into- tho earth and the quantity of water 

 which it was desired to freeze placed in it. 33y moans of a condensing pump, several 

 atmospheres of air were forced into the boiler, and then everything was allows I to 

 remain for a night, or until tho whole had acquired tho temperature of the surround- 

 ing earth. Then, by opening a stop-cock, tho air expanding escaped with much violence, 

 and tho water being robbed of its heat to supply tho expanding air, tho temperature 

 of the whole was so reduced, that a mass of ice was tho result. 



The most familiar method of producing intense cold is by means of freezing mixtures. 

 A great number of those were invented by Mr. Walker ; tho principal results are con- 

 tained in the following tables : 



I. Table, consisting of Frigorific Mixtures, having tho power of generating or 

 creating cold without the aid of ico sufficient for all useful and philosophical purposes, 

 jn any part of the world at any season. 



Frigorific Mixtures without Ice. 



