G. Po^dett-Scrope — Source of Volcanic Heat. 345 



which the recent dispute among physicists, to which Mr. Mallet has 

 been a party, whether mineral matter when consolidating (and 

 especially when crystallizing), from a state of fusion, expands or 

 contracts, shows to be yet doubtful. That such shrinking, if it 

 really occurs, takes effect, not in the production of internal rents, 

 but in '• the more or less free descent of the shell by gravitation," is 

 again but a conjecture. And that "the compression or crushing of 

 portions of that shell " thereby occasioned produces an amount of 

 heat, " locally within the solid shell," sufficient to effect its partial 

 fusion, is a further conjecture which, from the probable slowness of 

 the crushing process, if it does take place at all, and the many other 

 modes in which the energy so set in motion may be supposed to be 

 dissipated without the development of any extraordinary amount of 

 heat (as I suggested in the Geological Magazine for May, 1874, 

 p. 237), can hardly be considered, I think, more than possible. 

 Lastly, if all these hypotheses be granted, what proof or probability 

 is there that the heat so generated by an action which must be 

 supposed general and uniform throughout the entire subsiding shell,'^ 

 does not merge in the general body of heat transmitted through it 

 by conduction from the heated interior of the globe, but is ex- 

 clusively directed towards the volcanic vents or fissures, "few and 

 far between," which penetrate its surface? The extremely small 

 fractional proportion of the total amount of heat that escapes into 

 outer space through volcanic activity, which Mr. Mallet him- 

 self estimates at orxlj 1 in 1500,^ — instead of being, as Mr. 

 Mallet seems to suppose it, an argument in favour of his theory, 

 tells, in fact, strongly against it, since it is reasonable to pre- 

 sume that the smaller fractional portion of heat observed to 

 escape from the surface of the globe proceeds chiefly, if not wholly, 

 from the same source as the larger, namely, that enormous store of 

 heat which the interior of the globe on Mr. Mallet's own view 

 contains, and of which such a vast amount is continually trans- 

 mitted by conduction to its surface. It is strange that Mr. Mallet 

 should not have perceived the true inference from his figures. He 

 argues, in fact, that because the amount of heat lost from the globe 

 through volcanic vents is infinitesimally small as compared with 

 that lost by superficial radiation into space, therefore the former must 

 proceed from a different source from that which produces the latter. 

 It might just as well be argued that because the heat received by 

 our globe from the sun is but a minute fraction of the total amount 

 of solar heat radiated through space, therefore the former must have 

 a different origin in the composition of the sun from the latter. If 

 any further argument were needed to show the fallacy of Mr. 

 Mallet's exclusive reference of volcanic heat to the crushing of sub- 



^ In Mr. Mallet's own words, " not a mere local phenomenon, but a great cosmical 

 condition, pervading every part of the thick and solid crust of the globe," p. 130 of 

 Reply, Geol. Mag. for March, 1874. 



- " I have proved that the total amount of heat annually carried off from our globe 

 by existing volcanic action cannot by any possibility exceed the ~^ part of the total 

 heat annually dissipated from our globe." — Mallet, loc, cit. p. 131. 



