516 B. MALLEI ON HIS THEOEY OE VOLCANIC ENEEGY. 



proportionately under the enormous pressures to which a discon- 

 tinuous mass at several miles depth may be subjected, we can 

 readily see that the transformed heat of friction produced by internal 

 movements taking place in such materials after crushing has oc- 

 curred, must be the source of a large amount of heat over and above 

 that originally due to the crushing itself. Thus, for example, if we 

 assume a surface of one square foot of such disintegrated material 

 sliding over a similar surface, or over a rough surface of coherent 

 rock, and under the pressure of ten miles of rock of the specific 

 gravity of granite, at the rate of one foot per second, and we 

 take the coefficient of friction as low as 0*5, we have 4,326,600 foot 

 pounds of frictional work per second, which, divided by J (772), 

 gives 5604 units of heat evolved per second from each square foot of 

 surface ; and to this development there is no limit while the circum- 

 stances continue the same and the motion is continued ; and great 

 as is this evolution of heat under such enormous pressures, it would 

 be further 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, in respect of any given solid material, is dependent 

 upon the units of heat present in a unit of mass or of volume of the 

 substance. If for the same total heat we diminish the mass or 

 volume, the temperature is proportionately increased. 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 account ; for the shorter the time within which 

 a given amount of heat due to transformation of work is evolved 

 within the unit of mass, the less of that total is dissipated by con- 

 duction &c. 



In the case of the l|-inch cubes in the author's experiments, the 

 work of crushing could not take place more rapidly than a heavy 

 body could fall freely through a space of rather less than 1*5 inch. 

 In nature, however, the opposed rock-surfaces are brought into a 

 state of elastic compression, the release of which by crushing takes 

 place with a velocity proportional to the elastic modulus of the 

 rock, probably not less than 10,000 feet per second. Had the rock 

 specimen crushed in the author's experiments been a cubic foot in 

 place of an inch-and-half cube, the time of crushing due to gravity 

 must have been rather more than *249 second, or nearly a velocity 

 of 2 feet per second, which is less than -g-oVir °f the velocity with 

 which the same might have been crushed if circumstanced as in the 

 shell of our globe. And if we extend our view from the crushing of 

 a cubic foot or two to that of a cubic mile or more, we see that 

 there would be very little of the total heat evolved lost by dissipa- 

 pation, there being scarcely any time in which that could occur. 



Another and further source of heat arises after crushing and 

 detrusion of the fragmentary matter, and after the latter has arrived 

 at a temperature at which the fragments have become more or less 

 viscous and adherent by reason of the further work expended in the 



