50 Dr. T. Sterry Hunt — On Chemical Geology. 



planet, if not actually solid to the centre, has a crust far thicker than can be 

 accounted for by the theory of a liquid globe covered only with a crust resulting 

 from superficial cooling. This latter view, which was deduced from the increase 

 of temperature observed in descending into the earth, is in conflict with the 

 various mathematical and physical considerations above noticed, and it becomes 

 necessary to revise the older notions of the conditions of a cooling globe. 



The investigations of Charles Deville, and of Delesse, as well as the earlier 

 ones of Bischof, show that the density of fused rocks is very much less than that 

 of the crystalline minerals of which they are composed. From this we may 

 naturally conclude that the crystalline compounds which would separate by slow 

 cooling from a bath of molten rock would gravitate towards the centre, as 

 Saemann has already justly observed (Bull, Soc. Geol. de Fr., Feb. 4th, 1861). 

 In opposition to this view, Mr. Forbes appeals to the results seen in a small 

 scale in the cooling of melted metals, etc. — where a crust forms over the surface. 

 It must, however, be considered that the conditions presented by a small vessel 

 full of a hquid congealing in an atmosphere greatly below its own temperature, 

 and having a crust growing out from and supported by the sides of the vessel, are 

 widely different from those of a liquid globe slowly cooling beneath a very dense 

 and intensely heated atmosphere. In such a case, with a bath of materials 

 similar to those forming our present rock-crust, the crystalline minerals which 

 have been shown by Deville to be from ^ to ^ heavier than the liquid mass ; 

 these, as they separated, would sink as naturally as the crystals which form at 

 the surface of an evaporating basin of brine. The analogy holds good, since 

 the denser crystals formed at the surface, whether by evaporation or by cooling, 

 obey the inevitable laws of gravity. 



Mr. Forbes next proceeds to some considerations drawn from the mean density 

 of the earth, which, being about 5.3, is twice that of the average specific gravity 

 of the solid materials known at the surface. Admitting that a solid crust of 

 specific gravity 2.65 were to form at the surface of a liquid of density 2.3, and in 

 obedience to natural laws, to sink therein, our critic conceives that, in its descent, 

 it would meet with a denser liquid stratum. He supposes a liquid globe "be- 

 coming rapidly denser in descending, as the pressure increased by the superin- 

 cumbent column of liquid matter ; " and he tells us, in a note, that we may admit 

 a density of "nearly 10.7 for the middle zone, and about 18.8 for the centre" 

 (p. 435). Two pages further on he has completely changed his mind, for he tells 

 us that " experimental research tends to show that a limit is soon reached beyond 

 which the compression, or increase of density, becomes less and less in relation to 

 the force employed ; " and concludes that there are strong reasons for believing 

 that the central parts of the earth " must consist of much denser bodies, such as 

 metals and their metallic compounds," — which he further on explains may mean 

 "dense sulphids." 



To which of these two views does Mr. Forbes mean to hold, that of a rapidly 

 and constantly increasing density from pressure, or that in which, limiting the 

 condensing effect of pressure, he seeks to explain the density of the earth by a 

 nucleus of heavy metallic compounds ? The latter is seemingly an after-thought 

 of the critic, suggested by some notion of the principle involved in the augment- 

 ation by pressure of the fusing point of bodies which expand in melting. As 

 was shown by James Thomson, the effect of pressure upon ice (and naturally 

 upon such metals and metallic alloys as, like it, contract in melting) would be to 

 reduce its melting point, a fact which has been experimentally established for ice. 

 Reasoning from the same principle, Sir Wm. Thomson deduced the conclusion 

 that a reverse effect should result from pressure for all such solids as expand 

 in melting ; that is to say, that their points of fusion would be raised, a conclusion 

 verified by the experiments of Bunsen, and by those of Fairbairn and Hopkins. 

 From some apparent irregularities in these results, and from the fact that certain 

 of the substances submitted to experiment were bodies of the carbon series, 

 which Mr. Forbes calls "organic," he argues against the conclusions which 

 depend upon a well-defined physical law. In the case of the fusible alloys tried 

 by Mr. Hopkins, it is to be remarked that most of these bodies, like ice, expand 

 in cooling, and consequently should not have theirmeltmg points raised by pressure. 

 For the memoirs of James and William Thomson, see Trans. Royal Soc. Edin., 

 xvi. part 5, and L. E. D. Philos. Mag., [3] xxxvii. 125. A simple and popular 



