Forbes — Chemistry of the Primeval Earth. 435 



mathematicians, to have been " supported to a great extent by 

 fallacious reasonina:." 



That such observers, however, could, as Dr. Hunt imagines, have 

 been led astray by the exceptional case of the crystallization of 

 water, is sufficiently refuted by the intimate knowledge of the 

 behaviour of melted lavas, slags, metals, etc., everywhere displayed 

 in their researches; so that other explanations of the "fallacious 

 reasoning " alluded to must be sought. Dr. Hunt therefore informs 

 us that the solidification could not have commenced at the surface, 

 because the congealed crust is heavier than the molten fluid from 

 which it had cooled — i.e., ought in other words to sink into the 

 same. 



Most of the readers of these remarks, have doubtless seen large 

 open castings produced at foundries, and the question may therefore 

 be put to them, whether they ever have observed in such cases, no 

 matter how large the casting might be, that the pellicle or crust, 

 formed upon the surface of the molten iron, sank down into the metal 

 below it (unless purposely broken up by force) ; yet the cast iron, 

 when solid, is well known to be heavier than when in the fluid state. 

 On the contrary, it supports itself in the same manner as the 

 external surface of an igneous sphere would do, if exposed to a 

 cooling action, operating simultaneously upon the whole of its 

 external area. 



But, admitting even that it would sink, and supposing with Dr. 

 Hunt that the mean density of the earth is 5 "3, and of the solid 

 exterior crust at one-half this, or 2*65, and also that the specific 

 gravity of this latter, when in a fluid state, would be considerably 

 lighter, say 2*3 : then surely Dr. Hunt will not expect it to be believed 

 that this solid crust of sp. gr. 2-65, can sink deep down into the 

 fluid mass of a globe possessing a mean density of 5.3.^ 



If broken up by force, the fragments of such a crust might be 

 imagined to sink a short way, say a few miles, through the upper or 

 lighter stratum ; which at the surface was of a density of 2-3 (be- 

 coming rapidly denser in descending, as the pressure increased by 

 the heightened superposed column of liquid matter) until it came to 

 rest in a liquid stratum of its own density, where it would float 

 (in a solid state, if Dr. Hunt is to be believed in asserting that 

 under the extra pressure at this depth, its fusing point would be 

 elevated, and so prevent its absorption into the main mass). 



Such an action would, on the contrary, tend to bring about the 

 very formation of a superficial crust, like that which Dr. Hunt 

 denies, for this action once commenced, would go on solidifying at 

 the exterior, sinking to a certain depth, then resting there, super- 

 posed on that which had previously congealed and descended, and so 

 on, until a solid crust was eventually formed, extending from this 

 depth to the surface. 



1 If we suppose the mean density of the earth to be 5-3, and that of the surface 

 crust to he 2-65, and further iraagine the earth to be composed of three consecutive 

 layers of equal thickness, and of density increasing in arithmetical progression, we 

 should have 2-65 for the density of the outermost zone, nearly 10 7 for that of the 

 middle one, and about 18-8 for the centre. 



