TIIEOUIE.S OF THE EAilTII'S SOLIDIFICATION. 5 



no clew, unless it be that very possibly sonic of the rarer elements now found 

 mixed with the iron may occur there. 



It was claimed by Sir William Thomson* that the earth must have 

 solidified from the centre outward in accordance with the " thermo-dynamic 

 law " of his brother Professor James Thomson, which may be stated in tlie.se 

 words : All materials ivhich contract on congelation have their melting point rimed ftg 

 pressure J while bodies tvhich expand on freezing hive their melting point lover ed It;/ 

 pressure. Thomson, in common with nearly all physicists, held that the ex- 

 pansion of water and that of bismuth on freezing, were exceptional cases ; 

 but that contraction was the rule, and that pressure would therefore over- 

 come the increment of heat as the centre was a])proached. The mistake 

 made here was, that the cold solid was compared with the hot liquid ; the 

 fact being overlooked that this law applies to the point of passage from the 

 liquid to the solid, and not to the relative density of the two taken at tem- 

 peratures differing hundreds and even thousands of degrees. 



Experiments t by Mallet, Centner, Millar, Whitley, Hannay, Anderson, 

 Nies, Winkelmann, and Wrightson show that in the case of steel, iron, tin. 

 copper, zinc, bismuth, antimony, etc., compared, at a temperature just below 

 the melting point, with the melted material at about its freezing point, the 

 solid is the lighter ; but that these metals contract so on cooling that when 

 cold all, except bismuth, are then heavier. It would seem that if solids 

 and liquids are compared together at about the temperature of their con- 

 gelation the solid is the lighter ; and that therefore the pressure at the 

 earth's interior would cause these metals to remain liquid at a lower tem- 

 perature than they would on the earth's surface. 



The same law holds good for slag, and seems to do so for lava ; in fact 

 this is probably true of almost all rocks, although the evidence is far from 

 being conclusive. 



Experiments $ made by Hopkins, Bunsen, Mousson and others indicate 

 that to change the fusion point a few degrees an enormous j)ressure is 

 required, and that the law of Thomson is really capricious and variable, if 

 always true. Hence, so far as our knowledge extends in regard to the 



* Trans. Gcol. Soc. Glasgow, 1S78, vi. 38-19. 



f Pi-oc. Roy. Soc, 1874, xxii. 3G6-308 ; 1875, xxiii. 209-234; Nature, 1S74, x. lofi, 157; 1S77. xv. 

 529, 530; xvi. 23, 21; 1878, xviii. 397, 398, '161; Proc Roy. Soc. Edia., 1879, x. 359-362; Sitz. Akad. 

 Muiiclieii, 1881, pp. 03-112; Phil. Mag., ISSl, (5), xi. 295-299. 



X Report Brit. Assoc, 1854, xxiv. (sect.) 57, 58 ; Ann. Piiysik Cliemie, 1850, Ixxxi. 5C3-567 ; 1S5S, 

 cv. 161-174; Everett's Dcschanel's Nat. Phil., 1872, pp. 312, 313; 1883, pp. 331, 332. 



