474 0. FISHEK ON ME. MALLEUS 



But the great question is, will such crushing, if it occurs, produce 

 volcanic phenomena ? 



On entering upon this part of Mr. Mallet's paper I cannot but 

 express my admiration for the amount of laborious, and apparently 

 accurate, experimental investigations which he has gone through. 

 The records of these alone render his memoir of lasting value. 



The bearing of his experiments of crushing rocks upon estimating 

 the quantity of heat obtainable by that means appears thus : — The 

 tangential pressure within the earth's crust is practically unlimited. 

 If, therefore, any obstacle was interposed to keep it apart along a 

 certain vertical section, and held there until the pressure had attained 

 its full value, and the obstacle then suddenly removed, the pressure 

 being enormous, the energy with which the two faces of the section 

 would rush together would be enormous ; and the work convertible 

 into heat, being the product of the pressure into the distance between 

 the faces of the chasm, would be sufficient to educe a proportionately 

 large quantity of heat. But in the case of nature the supposed 

 obstacle is no other than the rock itself, and the amount to which 

 the pressure can accumulate is just that which the rock will bear 

 without becoming disintegrated ; and were this event to take place, 

 as Mr. Mallet supposes, the space through which the pressure acted 

 would be that which could be gained out of the closer compacting of 

 its particles. 



No doubt the conditions would be largely affected by the state the 

 rock might be in at great depths, and by the compression to which 

 it would be subject from the weight of the overlying strata. But 

 these rather tend to lessen the space through which the pressure 

 could cause motion, and so to lessen one factor in the expression for the 

 amount of heat obtainable. From the different circumstances of 

 the cubes experimented upon by Mr. Mallet, free on three sides, and 

 consequently allowing a greater descent of the plunger, I think it 

 may be assumed that the quantity of heat he calculates upon is 

 probably a fair estimate even under the circumstances of the increased 

 pressure found in the case of nature. 



If I have rightly understood the description of the experiments, 

 they may be shortly thus explained : — Cubes of rock, of 1| inch on 

 the edge, were crushed under a cylindrical piston or plunger of 3^ 

 inches diameter. 



The pressure upon each square inch of the face of the cube was 

 calculated from the known pressure laid upon the plunger ; and the 

 vertical descent of the plunger while the crushing was going on 

 was also obtained. These, multiplied together, gave the work of 

 crushing; whence, with the help of known constants, the heat 

 evolved was calculated, it being supposed that all the work was con- 

 verted into heat. 



Let us now consider the summary of the series of experiments 

 in crushing the cubes of rock as given in Column 19 of Table I. 

 The numbers there represent the vertical range through which the 

 pressure acted in crushing the cubes. of rock. Turning to fig. 9 and 

 § 116, it seems to me that th& length of these ranges must have 



