CALORIC. 31 



tensity of the heat is ascertained, for the clay of 

 which these pieces are prepared, has the property 

 of contracting regularly, according to the degree 

 of heat. 



This is an exception to the general law of bodies 

 expanding by heat ; the expansion of melted metal 

 in the act of cooling is another, as likewise the ex- 

 pansion of water in the act of freezing. 



The greatest degrees of heat which can be raised 

 have been produced by concentrating the solar 

 rays with a mirror or lens, or by supplying a blow- 

 pipe with oxygen gas ; or, what is still more power- 

 ful, by a mixture of oxygen and hydrogen. This 

 last method has been but lately employed, and pro- 

 duces a far greater degree of heat than any other. 

 The mixture is itself of an explosive nature, and, 

 therefore, without proper precaution, exceedingly 

 dangerous. The greatest degree of cold known to 

 have been produced has been obtained by mixing 

 snow with certain salts. The best salt for this 

 purpose is muriat of lime. If this be mixed with 

 dry light snow, and stirred well together, the cold 

 produced will be so intense as to freeze mercury in 

 a few minutes. Salt and snow also produce a great 

 degree of cold. 



Evaporation, likewise, produces cold. The me- 

 thod of making ice artificially in the East Indies 

 depends upon this principle. The ice-makers at 

 Benares dig pits in large open plains, the bottom 

 of which they strew with sugar-canes, or dried 

 stems of maize, or Indian corn. Upon this bed 

 they place a number of unglazed pans, made of so 

 porous an earth that the water oozes through their 

 substance. These pans are filled towards evening, 

 in the winter season, with water which has been 

 boiled, and are left in that situation till morning, 



