-1859] WRITINGS OF JOSEPH HENRY. 203 



acid and copper, then the heat of the chemical combination 

 of the two substances far exceeds the cold due to the lique- 

 faction of the solid, and a high temperature in the mixture 

 is the result. 



On the same general principle is explained the melting 

 of ice by sprinkling the surface of it with salt, — a process 

 sometimes resorted to for clearing the sidewalks after an in- 

 tense cold has succeeded rain. The union of salt and ice pro- 

 duces a liquid the freezing point of which is many degrees 

 below that of water; and hence on their contact in a solid 

 state, liquefaction necessarily ensues; and this in accordance 

 with the general law must be attended with a great reduction 

 of temperature in the surrounding bodies; on which fact de- 

 pends the application of salt and snow to artificial freezing, 

 as in the manufacturing of ice-cream. In places where ice 

 is scarce the same principle may be applied to produce a 

 much greater reduction of temperature from a smaller quan- 

 tity of this substance. Three parts of ice and one of salt 

 mixed together in a thin vessel will reduce the temperature 

 of a large quantity of water; and since the same salt may 

 again be obtained in a solid form by exposing the solution 

 to the sun we think such a freezer might in some cases be 

 economically employed. 



The artificial production of ice in hot countries on a scale 

 sufficient for domestic use, has of late it is said been success- 

 fully accomplished. An attempt of this kind was made a few 

 years ago at New Orleans, by means of the rapid evaporation 

 of water, but the cold produced in this way being small the 

 process was not sufficiently economical to enable the manu- 

 factured article to compete in price, in that city, with the 

 abundant supply of ice imported from New England. 



Another process, which is said to be more efiectual, is that 

 of a Mr. Harrison, of England, and consists in the evapora- 

 tion, liquefaction, and re-evaporation of ether. If the bulb 

 of a thermometer covered with cotton and wet with ether be 

 exposed to the atmosphere, the cold produced by evapora- 

 tion will cause the mercury to descend many degrees below 

 the freezing point; and if the evaporation be made to take 



