CHEMISTRY OF THE EARTH. 187 



asmnch as it is denser ia tlie liquid than in the solid form. HeJice, ice 

 floats on water, and freezing water becomes covered with a hiyer of 

 ice, which protects the liquid below. Some metals and alloys resemble 

 water in this respect. With regard to most other substances, and 

 notably the various minerals and earthy compounds like those which 

 may be supposed to have made up the mass of the molten globe, the 

 case is entirely different. The numerous and detailed experiments of 

 Charles Deville, and those of Delesse, besides the earlier ones of Bi- 

 schof, unite in showing that the density of fused rocks is much less than 

 that of the crystalline products resulting from their slow cooling, these 

 being, according to Deville, from one-seventh to one-sixteenth heavier 

 than the fused mass, so that if formed at its surface they would, in 

 obedience to the laws of gravity, tend to sink as soon as formed. 



§ 11. The stony materials of the earth's crust then, unlike ice and cer- 

 tain metals, expand in melting and contract in passing into the solid 

 state. The melting of ice is a process of condensation, and hence pres- 

 sure favors its liquefaction, causing it to melt at a lower temperature 

 than it would otherwise do; but for bodies with which melting is a pro- 

 cess of expansion, pressure produces an opposite effect, namely, that of 

 augmenting the fusing point. These conclusions of James Thompson 

 and William Thompson have been experimentally veritied by Buusen, 

 Fairbairn, and Hopkins. It results from this physical law that the 

 effect of pressure upon materials like molten rocks would be such that 

 solidification at a depth from the surface would take jilace at a temper- 

 ature much higher than that required for their solidification at t!ie sur- 

 face. Hence, in opposition to the notion of a congealed layer, like a 

 sheet of ice, resting upon the surface of a molten globe, Hopkins, and 

 with him Scrope, supposes solidification to have commenced at the cen- 

 ter of the liquid mass and to have advanced toward the circumference. 

 Mr. Hopkins concludes that the i^ressure existing at great depths must 

 have induced congelation of the molten mass at temperatures at which, 

 under a less pressure, it would have remained liquid. Before the last 

 portions became solidified, there was produced, it is conceived by Mr. 

 Hopkins, a condition of imperfect liqiddity, preventing tlie sinking of 

 the cooled and heavier particles, and giving rise to a superficial crust, 

 from which solidification would proceed downward. There would thus 

 be inclosed between the inner and outer solid parts, a portion of 

 uncongealed matter, which, according to him, may be supposed still 

 to retain its liquid condition, and to be the seat of volcanic action, 

 whether existing in isolated reservoirs or subterranean lakes, or whether, 

 as suggested by Scrope, forming a continuous sheet surrounding the 

 solid nucleus. 



§ 12. This view of the constitution of the globe, or one analogous to 

 it, has found favor with many theorists. Professor N. S. Shaler, of 

 Harvard, who, in a recent essay on the formation of mountain chains, 

 in the proceedings of the Boston Society of Natural History, has 

 adopted it, concisely states it as follows : " The earth consists of an 

 immense solid nucleus, a hardened outer crust, and an intermediate 

 region of comparatively slight depth, in an imperfect state of igneous 

 fusion." In this connection it is curious to remark that, as lately pointed 

 out by Mr. J. Clifton Ward, Halley was led, from the study of terres- 

 trial magnetism, to a similar hypothesis. He supposed the existence of 

 two magnetic poles situated in the earth's outer crust, and two others 

 in an interior mass, separated from the solid envelope b}" a fluid 

 medium, and revolving, by a very small degree, slower than the 



