CALORIC. 
2^. 
ces.* In all these cases, by chemical 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 boihng 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 ^tna, 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 ^tna 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, 
consisting of a small cylinder in which a solid piston moves air- 
tight ; a little tinder or dry sponge is attached to the bottom of 
the piston, which is then violently forced into the cylinder; the 
air between the bottom of the cyUnder and the piston becomes 
intensely compressed, and evolves so much heat as to light the 
tinder. 
C 3 
