Theory of Volcanic Energy. 305 



diminished, because the four lateral faces of the cube cannot 

 have the same liberty of distention as in the experiments. We 

 cannot, therefore, feel assured that the heat developed will increase 

 in proportion to the superincumbent pressure. If the cube in 

 the experiment had been just crushed in a close-fitting box, and 

 the heat developed measured thermometrically, the proportionality 

 might have been more reliable. It is I think to be regretted 

 that Mr. Mallet has not given the thermometric results of his 

 experiments in § 99, although even in them the box in which 

 the rock was enclosed did not touch it round the periphery, and 

 so was far from representing the case of nature. 



But in truth the principal arguments in the paper under con- 

 sideration have regard to Professor Hilgard's suo-o-estions as to 

 the "combined influence of friction and rock- crushing" And it 

 is really most important to examine the question under this 

 extension; for that Mr. Mallet's sagacity has pointed out a 

 subterraneous source of heat which had been overlooked is 

 undeniable, while at the same time the question whether it 

 will account for volcanic action is, I presume, not yet settled. 

 He now suggests the possibility of the rock, after being crushed, 

 being pushed aside, leaving part of its heat behind it, and the 

 rest of the column being crushed down upon the same spot of 

 a hard rock wall, and taking up some of the heat left from the 

 antecedent portion in addition to what is proper to its own 

 crushing. 



To judge of what must happen under the circumstances, we 

 must consider what the supposed column is. It is in truth a 

 horizontal element of a vertical slice of the solid crust. Mr. 

 Mallet argues thus respecting it : — " The first cubic foot of the 

 column that is crushed has its temperature raised, let us suppose, 

 by the minimum 217°. The crushed fragments at this tempera- 

 ture are pushed aside by the advancing column, whose extremity 

 is thus surrounded by crushed material at a temperature of 217°, 

 and the second foot in length of the column becomes crushed. 

 But the material of this second cubic foot is at a higher tem- 

 perature before it is crushed than was the first cubic foot ; so 

 that the heat due to the transforming work of crushing of each 

 successive cubic foot of rock raises its temperature to a higher 

 point than that of the preceding one, because each successive 

 cubic foot at the instant before crushing is at a temperature 

 already higher than the preceding ones, resulting from the heat 

 taken up by the uncrushed column from the hotter portions of 

 material surrounding it that have already been heated by the 

 crushing/' 



-Vow it seems that the above reasoning might possibly be 

 applicable to an isolated column, crushed down upon a hard 



Phil. Mag. S. 4. Vol. 50. No. 331. Oct. 1875. X 



