THE INTERIOR OF THE EARTH. 459 



increase in the flow of the lava twice daily at intervals of about twelve 

 hours' duration, and with a little less than an hour's retardation from 

 one day to the next, as observed in the ocean-tides. The eruption com- 

 menced on the 1st of May, and the periodic swelling of the lava-stream 

 was observed from the 5th to the 19th. Such observations could easily 

 be made in the island of Hawaii, on the borders of the lava-lake of 

 Kilauea. 



It must not be forgotten that these subterranean tides do not by 

 any means demonstrate the liquidity of the nucleus, but simply the 

 existence of a liquid mass of a certain depth. It will be shown how 

 astronomic phenomena furnish data for the solution of this question, 

 but we will first glance at some purely physical considerations that 

 have been advanced in elucidation of the matter. 



Mr. James Thompson was the first to point out that compression 

 would have the effect of lowering the point of fusion, and consequent- 

 ly of retarding the congelation of those liquids that expand in solidify- 

 ing. This has been shown to be the case with water, and it would 

 probably be found to be the same with iron. On the other hand, in 

 the case of the far more numerous substances that contract in solidify- 

 ing, compression facilitates congelation by cooling the mass. It ought, 

 therefore, to raise the point of fusion, and this is known to occur with 

 many bodies. Thus the melting-point of sulphur, which shrinks ma- 

 terially in solidifying, is raised from 107 to 140 under a pressure of 

 eight hundred atmospheres. Now, according to the experiments of 

 Bischof, the greater part of the rocks expand by fusion and contract 

 in solidifying. Granite, the schists, and trachyte shrink about one 

 fifth in solidifying. This tends to confirm the supposition, says Sir 

 W. Thomson, that the earth's nucleus has long been solidified. 



Let us conceive the earth as primarily wholly liquid. There would 

 be established in the mass an equilibrium of temperature corresponding 

 to a given pressure. As the mass cooled, solidification would com- 

 mence, either on the surface or at the center. The question is a very 

 complex one and can not be fully solved without a better knowledge 

 than we have of the properties of the liquid under consideration. But 

 assuming that solidification commences at the surface, a thin skin will 

 first be formed, and this being by the hypothesis heavier than the 

 liquid it covers since its volume shrinks in solidifying it follows 

 that it must be broken up and the fragments be carried to the bottom 

 or center, forming there a solid nucleus. Thus, in any event, solidifi- 

 cation will occur at the center, and, when the entire mass has acquired 

 a temperature near the point of solidification, a solid carapace will 

 gradually cover the surface, beneath which here and there masses of 

 liquid will still exist. 



This argument, however, is open to question in several respects. 

 M. Mallet's experiments, for instance, with the scoriae of smelting- 

 furnaces, show that certain silicates contract only six per cent. The 



