CALORIC. 



29 



ces.* Ill all these cases, by chennical mixture, the 

 capacity for caloric is diminished, and of course it 

 escapes. We see the same thing proved by the 

 well-known fact that different substances require 

 different degrees of heat to bring them to the same 

 temperature. Thus a pound of water requires 20 

 times as much heat to bring it to the same tempera- 

 ture as a pound of quicksilver. 



We have stated that bodies differ in their power 

 of transmitting caloric. Metals, for example, are 

 better conductors than substances of less density, 

 as wood, wool, feathers, paper, &c. For this rea- 

 son metallic tools, which are to be used when heat- 

 ed, are furnished with wooden handles, or covered 

 with cloth or paper. If we take into the hand a piece 

 of lead and a piece of cork which have been immer- 

 sed for a few minutes in boiling water, we shall find 

 the cork but slightly warm, while the metal will be 

 so hot that it cannot be held. A geological fact 

 connected with this subject was discovered in 1829, 

 so marvellous in its nature that it seems almost in- 

 credible. A mass of ice was discovered under a 

 thick cover of lava on Mount ^Etna, which had re- 

 mained there ever since the lava had flowed out in 

 its melted state ! On investigating the cause, it was 

 found that the ice had been covered with volcanic 

 sand, which is one of the best non-conductors of 

 caloric, and so had been protected from the heat of 

 the lava. The shepherds on Mount ^Etna preserve 

 snow for the refreshment of their flocks during the 

 summer by covering it with the same kind of sand. 



* When air is violently compressed, it becomes so hot as to 

 ignite cotton and other substances. An ingenious instrument 

 for producing a light for domestic uses has been constructed, 

 cojjsisting of a small cylinder in which a solid piston moves air- 

 tight ; a Tittle tinder or dry sponge is attached to the bottom ot 

 the piston, which is then violently forced into the cylinder; the 

 air between the bottom of the cylinder and the piston becomes 

 intensely compressed, and evolves so much heat as to light tho 

 tinder. , 



