E. MALLET ON HIS THEOEY OF VOLCANIC ENEEGY. 513 



the cube ; and the work necessary to crush the cube by its finally 

 giving way, in whatever direction this encastrement by pressure may 

 be least, will be increased over that which would crush it in free air 

 nearly in the same ratio in which the imaginary cube is exposed to 

 external pressure greater than that in air — and this, neglecting the 

 cohesive resistance of the surrounding rock itself. 



Thus, if the cube of Guernsey granite, no. 12 in the author's 

 Table No. 1, were crushed at a depth of twenty miles beneath like 

 rock, it would have required to crush it a pressure of 4*28 times as 

 much as was found necessary to crush it in air ; and if we assume 

 the displacement of the crushed particles after crushing to be the 

 same as in the case of the cube crushed in air, then the work and 

 the heat due to its transformation will also be 4*28 times as great ; 

 and as in the case of the cube crushed in air the heat developed was 

 sufficient to fuse (at 2000° F.) 0*108 of its own volume, or in 

 other words that the crushing of ten cubic feet of the rock would be 

 required to raise to that point one cubic foot, so in the case of the 

 imaginary cube situated at the depth of twenty miles, enough heat 

 would be evolved by the work of crushing each cubic foot to fuse 

 0*462 cubic foot of the same rock, or nearly half the volume crushed — 

 and this, assuming that the initial temperature of the rock at twenty 

 miles depth was only 53° F., as in the author's experiments, 

 instead of from 500° to 1000° or more, as it may be at twenty miles 

 depth. Therefore, under the pressure due to a depth of twenty 

 miles and an initial temperature of 1000° F., the heat developed 

 by the work of crushing each cubic foot of rock will be sufficient to 

 fuse its own volume. Thus also if we assume the fusing-point of 

 the rocks not to be 2000° F., as indicated by the author's experi- 

 ments on the cooling of slags, but considerably higher, say 2500° or 

 more, we have still a sufficient supply of heat to bring 0*8 of the 

 whole to fusion. These considerations, apart from all others yet to 

 be adverted to, appear fully sufficient to refute the Rev. O. Eisner's 

 first objection above quoted. Indeed the statement that if under any 

 circumstances and in the rock-masses of nature " crushing can 

 induce fusion, then the cubes experimented upon ought to have been 

 fused in the crushing " wholly ignores the differences of condition 

 in the two cases, and seems as unsustainable as it would be to affirm 

 that no heat is developed by the slow oxidation (eremacausis) into 

 water and carbonic acid of a pound of wood, whieh, when burned, 

 develops a well-known amount of heat. 



The writer now proceeds to reply to the second objection of the 

 Rev. O. Fisher as above quoted, which appears to him based 

 entirely on a misconception of the physical conditions involved. 

 Let us consider what will happen in the case of a prism or column 

 of rock crushed against the face of an unyielding mass. If the 

 prismatic mass be not homogeneous throughout, crushing will com- 

 mence at the weakest place; if it be perfectly homogeneous, crushing 

 will commence and continue where the prism is in contact with a 

 fixed mass, and that whether the prism be crushed at one or both 

 ends, because it is at such surfaces of contact that the compression 



2 m 2 



