198 The Artificial Production [Api'ilj 



civilized man and savages, and most conducive in various ways to 

 the spread of ci\alization. 



In most of the industrial arts the application of heat is involved 

 in some one or other of the operations to be performed. Through 

 the medium of vaporized water it is the principal means of locomo- 

 tion on land and by sea, as well as the chief source of motive- 

 power ; while in the extraction and working of metals and ui most 

 productive arts, involving chemical alteration of raw materials, it is 

 essential for bringing about the various changes to be effected. 

 But though chemical alteration is in many cases the object to be 

 attained in manufacturing operations, and then the application of 

 heat is the means by which that change is facilitated, there are 

 some cases in which it is desirable to hinder or prevent the chemical 

 changes to which certain materials are liable even within the ordi- 

 nary range of atmospheric temperatures. Thus, for mstance, all 

 articles of food, and especially those of animal origin, when kept for 

 any length of time at the ordinary temperature, undergo a chemical 

 change which renders them unfit for use. They pass into a state 

 of putrefaction or decay, and are gradually resolved, under the 

 influence of atmosj)heric oxygen, into the simplest forms of com- 

 bination their elements are capable of assuming. For this change 

 a certain temperature is essential, and though it probably cannot 

 be prevented altogether, it is very considerably hindered by a reduc- 

 tion of temperature. Thus, for instance, in Siberia, the carcasses 

 of mammoths have been found in a very perfect state of preserva- 

 tion, notwithstanding the vast lapse of time they have been buried 

 in the frozen soil. In like manner meat, and game, or fish may be 

 preserved for a long time quite fresh by keeping it at or below the 

 temj)erature of freezing water. 



For this and other purposes which require a low temperature, 

 and where it is desirable to abstract heat instead of applying it to 

 any substance, ice has been largely employed, and the importation 

 of ice into this country from higher latitudes has become a very 

 large trade since the year 1840. 



Ice acts as a cooling agent in virtue of the physical fact that, in 

 common with all solid substances, it requires an expenditure of heat 

 for its conversion into the liquid state. The heat thus apphed does 

 not produce any elevation of temperature, but as the ice melts it 

 disappears, so far as the indications of the thermometer will show, 

 and there remains a quantity of water of the same temperature as 

 the ice itself. Thus when ice or, still better, snow is mixed with 

 three-fourths its weight of boiling water, the water remaining after 

 the ice has melted, has a temperature of 32^ Fahr., the same as the 

 ice itself ; the quantity of heat in the boiling water, corresponding 

 to the interval of temperature between 32 and 212^ Fahr., having 

 been rendered latent, or expended in effecting the liquefaction of the 

 ice. It is in this way that ice cools water, air, or any other substance 



