R. Mallet — Reply to Mr. Poulett-Scrope. 129 



change of signs in the directions of the play of forces was no doubt gradual, 

 and whether it took place at one or at another geological period is immaterial to 

 the truth of my theory of existing volcanic action, which might remain true, 

 were we to suppose that such a change of action never took place at all. 



Mr. Scrope founds an objection to my theory, "that it seeks and purports 

 to find a second source of heat where one exists fully sufficient for the purpose." 

 Were I to admit with him his hypothesis of the existence of a fused and liquid 

 nucleus within a few miles of the surface, his remark would be in so far true ; 

 but as I neither postulate nor even admit the existence of such a source of 

 volcanic heat, there is no superabundant hypothesis involved in my view. The 

 transfer of heat into space from our cooling globe is the primiim mobile of all 

 existing volcanic action. The heat lost gives rise to mechanical work at a certain 

 stage in the complex train of phenomena called into play by its loss, and part of 

 that work is transformed into heat, which is that of volcanic action. 



It is mere verbiage to talk of this transformation of work into heat as involving 

 any second source of heat, it being derived primarily from the only source con- 

 ceivable, namely, the hotter interior of our planet. When a "billet" of iron is 

 taken from the furnace and passed between the rollers, its temperature rapidly and 

 visibly augments by reason of the violent distortion to which the heated mass is 

 subjected. Would it be any philosophical objection to the true explanation of the 

 phenomena to say that the increased heat of the bar was derived from a second 

 source, and not from the furnace ? If the interior be not in liquid fusion, of which 

 we have no proof whatever, or being assumed in fusion if it be covered by a solid 

 crust of even two or three hundred miles in thickness, then I affirm that there is 

 no other conceivable way in which we can find a source of volcanic heat and 

 energy, except that which I have pointed out. 



An objection is made by Mr. Scrope, that of the three ways in which heat is lost 

 by our globe, — viz. by conduction, by which I presume he means radiation into 

 space, by hot springs, and by volcanic vents, — I ascribe the two former to direct 

 cooling, and the last to an entirely distinct and independent origin. There is here 

 surely a strange confusion of ideas. The heat — which is the transformed work of 

 contraction and source of volcanic action — has no distinct and independent origin ; 

 it is merely one form of the actions brought into play by the loss of heat due to 

 radiation into space. Again, as a matter of fact, Mr. Scrope is quite in error in 

 saying that I ascribe the heat of thermal waters exclusively to hypogeal heat 

 directly brought up by them. The great mass of thermal waters issue at far below 

 212° Fahr. , and their feeble heat in the vast majority of instances is no doubt due 

 to surface-waters penetrating the earth to a small depth, and returning more or 

 less heated by it ; but I have nowhere denied, even suggestively, that waters are 

 also evolved at the surface, heated by direct connexion with or proximity to 

 volcanic vents. However heated, they are but one form of the complex phenomena 

 of our cooling globe, of which the volcano is another. Mr. Scrope seems to have 

 failed to remark that my estimate of the annual heat lost by all the t?iermal waters 

 in the world proves that their total refrigerative action is insignificant ; we may 

 dismiss them from consideration as affecting my theory. 



Mr. Scrope says (p. 29), "There is no difficulty in understanding how the great 

 fissures in the solid crust of the globe, which are marked outwardly by active, or 

 once active, volcanoes, may penetrate so far into the interior of the heated 

 nucleus as to give vent to an amount of heat sufficient to fuse the rocks through 

 which they pass, and to some of the already fused or viscid underlying matter. " 

 To me there is every difficulty. Assuming a liquid nucleus, of which there is no 

 proof whatever, covered by a solid crust of some hundreds of miles in thickness, it is, 

 I believe, demonstrable that no open fissures could penetrate through such a crust ; 

 and if they did, the liquid matter could not reach the surface through them. Can I 

 rightly understand Mr. Scrope to say that his open fissures would pass up heat from 

 the nucleus better than the solid rock forming their walls ? If so, this seems to repeat 

 the error as to the heat-wave of some of the earlier writers on earthquakes, who 

 imagined that the wave of shock passed more readily through cavernous fissures in 

 the earth than through its solid mass. If this be the insecure basis upon which Mr. 

 Scrope has founded the solution — to which he refers — of the great inequalities in 

 the increments of hypogeal heat nearly everywhere observable, —viz. its lateral 

 transfer when stopped by badly conducting strata to such open fissures ; — or even if 



DECADE II. VOL. I. NO. III. 9 



