30 Poulett-Scrope — On Mallet'' s Theory of Yolcanic Energy. 



We are not tlien compelled to seek for any otlier source of volcanic 

 heat than that which even Mr. Mallet is obliged to employ as the 

 " primum mobile " of his theory, namely, the heated nucleus itself. 

 Mr. Mallet does not deny that it is to this source the downward 

 increasing temperature of the surface of the globe is owing, — nay, 

 as has been seen, he attributes to it likewise the high temperature 

 of hot springs : why then stop there, and look for another totally 

 different origin in the case of the heat of lava and the steam evolved 

 from it? This second source of heat is plainly not wanted to 

 explain the latter class of phenomena, so closely allied, as I have 

 said above, with the former, 



I suspect Mr. Mallet has but an imperfect acquaintance with the 

 phenomena of volcanoes in activity, or he would not speak as he 

 does (§ 190) of the expenditure of heat in the explosions of steam 

 from a volcano in eruption as " not resembling that which takes place 

 from a steam engine, but rather that of powder exploded in a cannon, 

 . , . the loss from which is shown by experiment to be much 

 smaller." The contrary is really the case ; the explosions from a 

 volcano in activity resembling precisely in character (and apparently 

 in cause) those of a Perkins steam-cannon fed by a continuous escape 

 of steam from a boiler. Explosions of this character continue in 

 some cases unintermittently for months, and the amount of heat so 

 lost in space must be immense. 



Again, Mr. Mallet says (§ 187), "Observations made everywhere 

 on volcanic cones indicate that but a very small proportion of their 

 total mass has been fused : the rest having been merely heated. It 

 is probably below the truth to assume that there are twenty volumes 

 of such heated matter (dust, lapilli, scorige, etc.) to one of fused 

 lava." This also is the very reverse of the truth. Of the frag- 

 mentary matter ejected by an eruption, at least nineteen-twentieths 

 have been fused, being mere scorise, that is, the hardened scum of 

 the boiling lava thrown up by explosions from its surface, and by 

 their repeated fall into the crater and re-ejection, triturated by col- 

 lision in the air into the " lapilli and dust " of which volcanic ash 

 and conglomerates are mostly composed. Moreover, in any attempt 

 to calculate the heat expended by volcanic eruptions, we ought to 

 take into account not only the mass of the cones themselves (to 

 which Mr. Mallet confines himself), but likewise that probably far 

 larger volume of once fused matter which has been carried away in 

 the form of dust by the winds (or waves) and spread over thousands 

 of square miles of the surrounding areas. 



This mistaken view of the origin of " volcanic ash " is the source 

 of a fundamental error in Mr. Mallet's classification of volcanic 

 action into two kinds, which he distinguishes as the "hydrostatic" 

 and the " explosive or existing volcanic action," and attributes to 

 them entirely distinct dates in the globe's history. " The com- 

 mencement," he says (§ 79), "of existing volcanic action .... 

 probably does not go back much beyond the end of the Secondary 

 period, if so far. Prior to that, Vulcanicity seems to have been de- 

 veloped in the welling-up of huge volumes of liquid rock between 



