E. MALLET ON HIS THEORY OP VOLCANIC ENEEGY. 515 



n and m being the denominators of the fractions of the heat in the 

 debris successively transmitted to the second and third cubic feet of 

 the uncrushed part of the column. 



No limit arises to this continual augmentation of temperature 

 while the rock retains its rigidity ; after that has been seriously 

 impaired or lost, any further exaltation of temperature becomes 

 dependent upon the deformation and detrusion of a more or less 

 plastic mass. It is well ascertained, however, by observation on a 

 great scale, that granite remains rigid at a temperature approaching 

 that of the softening-point of cast iron, so that a large range of 

 rigidity must exist for the exaltation of its temperature in the way 

 above suggested ; and in the state of aggregation in which we are 

 warranted in supposing rocky masses to exist at considerable depths, 

 it is probable that this range of rigidity would be even further 

 extended than in the case of granite found at or near the present 

 surface of our globe. Fire-brick also retains its rigidity far above 

 bright redness and to within a short range of its melting-point. 



Besides the heat transformed from the work of compression and 

 crushing, a large amount of heat must also be generally produced by 

 transformation of the work expended in friction and detrusion. No 

 experiments have as yet, to the author's knowledge, been made upon 

 the amount of heat developable in pulverulent masses such as sand 

 and fragments of various sizes, by the forcible transposition of more 

 or less of the particles ; nor do we know with certainty the conditions 

 under which external mechanical pressure is transmitted through 

 like discontinuous matter. As in rigid solids exposed to unequal 

 mechanical pressures there exist planes or surfaces within the mass ' 

 such as, in solids, have been denominated by Moseley " planes of 

 easiest shearing," so in masses of fragmentary matter, whatever be 

 the shape or size of the particles, provided these be small in relation 

 to the whole mass, and their mutual adhesion, if any, small also, 

 such shearing-planes must by unequal mechanical pressure be 

 brought into existence. Along any such plane we may imagine the 

 sand or other pulverulent matter forced to move over itself in 

 opposite directions at opposite sides of the plane ; that is to say, we 

 may suppose the sand forced along such a plane much in the same 

 manner that a mass of sandstone or of granite would be forced 

 along such a shearing-plane as had been produced in it previously 

 by mechanical pressure. If this reasoning be admitted, we must 

 suppose that heat would be developed along such a plane and at 

 short distances from it in a way more or less analogous to that 

 produced by the forcing of one rough surface of stone over another. 

 What the coefficient of friction in this case would be, can only be 

 determined by experiment ; but we may justifiably conclude that the 

 amount of friction per unit of surface would increase proportionately 

 to the pressure applied externally to the entire mass, and exposed 

 to more or less of which, motion at any such surface of friction must 

 take place. Coulomb, Morin, and others have found the friction of 

 some sorts of rough stone upon other rough stone to reach as much 

 as three fourths the pressure ; and should this coefficient increase 



