264 &. Mallet-—Temperature attainable by Rock-crushing. 
sand or other pulverulent matter forced to move over itself in 
opposite directions at opposite sides of the plane; that is to say. 
we may suppose the sand forced along such a plane much in the 
same way that a mass of sandstone or of granite would be forced 
along such a shearing plane as had been produced in it previously 
by mechanical pressure. If this reasoning be admitted, we must 
suppose that heat would be developed along such a plane and 
at short distances from it in a way more or less analogous to 
that produced by forcing one rough surface of stone over another. 
at the coefficient of friction in this case would be can only 
be determined by experiment; but we may justifiably conclude 
that the amount of friction per unit of surface would increase 
proportionately to the pressure applied externally to the entire 
mass—exposed to more or less of which, motion at any such 
surface of friction must take place. Coulomb, Morin, and 
no limit while the circumstances continue the same, except that 
of the distance that one surface is forced over the other. And 
great as is this evolution of heat under such enormous pressures, 
1t woul urther increased in the event of the fragmentary 
particles being heated so as to present incipient viscosity of 
surface and more or less of mutual agglutination. ; 
Temperature with respect to any given solid material is de- 
pendent upon the units of heat present in a unit of mass or of 
volume of the substance. If for the same total heat we dimin- 
ish the mass or volume, the temperature is proportionately 1n- 
creased. When the material is surrounded by matter capable 
of carrying off heat by conduction, or evection, or radiation, 
and the heat is evolved within the mass by work done upon it. 
then another condition, that of time, has to be taken into ac- 
count: for the shorter the time within which a given amount of 
