10 Mr. R. Mallet on the Temperature attainable 



spark struck from the steel of a gun-lock by the flint. In the 

 case of small masses of rock, such as the 1^-inch cubes of the 

 author's experiments, crushed between two opposite surfaces of 

 steel, the actual temperature of the crushed particles can never 

 be found to reach that due to the work of crushing ; for the heat 

 of the relatively small mass of the crushed cube in close contact 

 with far larger masses of cold steel of high conductivity is car- 

 ried off almost as fast as it is evolved; and as the total amount 

 of heat evolved from the crushing of such a cube of the hardest 

 rock experimented upon by the author (namely number 12, 

 Table I. Phil. Trans, part 1, 1873, p. 186) could only raise its 

 own mass through 217°, if the temperature of fusion of the rock 

 may be taken at 2000°, it is obvious that such a cube could 

 not be fused by the work of crushing alone, even though all the 

 heat due to the crushing work remained in the cube, none being 

 dissipated to surrounding objects. 



In the case of a cube such as this losing heat by dissipation, 

 the temperature of the crushed mass depends upon the time in 

 which the work of crushing is done. In the author's experi- 

 ments the crushing of each cube in column 12 occupied a mean 

 time somewhat greater than that in which a heavy body could 

 fall freely through a space of 0*09 foot (No. 12, Table I. /. c, col. 

 19) — that is, 0*075 of a second \ for more rapidly than that the 

 crushing surfaces could not approach each other. If, however, 



the conditions had been such that but -th the above time were 



n 



expended in the crushing, then a proportionately less quantity 

 of the heat evolved would have been dissipated ; and this, we shall 

 see further on, must be the case in nature. When two rock- 

 surfaces are urged against each other in the shell of our globe 

 by the gradual withdrawal of support by contraction of the nu- 

 cleus, the rocky masses for great distances from the opposed 

 surfaces are brought into a state of elastic compression, gradually 

 increasing up to the crushing-point somewhere, when a greater 

 or less portion of rock suddenly gives way by crushing and is 

 more or less removed by detrusion in some lateral direction. 

 The material of the rock for a greater or less distance from the 

 crushing-point is therefore in the condition of a compressed 

 spring which is suddenly released. When so released, the velo- 

 city of resilience depends principally upon the modulus of elas- 

 ticity of the rock; and the velocity of release of the spring, 

 which is that with which the crushing is performed, is extremely 

 great in the case of hard granite or generally similar homoge- 

 neous rock, in which it probably exceeds 10,000 feet per second, 

 though in very much less-elastic rocks falling considerably short 



