172 ANNUAL OF SCIENTIFIC DISCOVERY. 



alics indicating the operation of local causes. If a sphere of very large 

 dimensions, like the earth, were heated in any degree and in any manner, 

 and were left to cool in surrounding space, it is shown by accurate investi- 

 gation, that after a sufficient and very great length of time, the law accord- 

 ing to which the temperature would increase in descending beneath the 

 earth's surface, within depths small compared with the cai'th's radius, would 

 be that the increase of temperature would be proportional to the increase 

 of depth. This coincides with the observed law, if we neglect the anoma- 

 lous irregular variations which are found to exist more or less in each lo- 

 cality. Xow, according to this law, the temperature at the depth of GO or 

 70 miles would probably be sufficient to reduce to a state of fusion nearly 

 all the materials which constitute the earth's external solid envelope ; and 

 hence it has been concluded that the earth probably consists of a central 

 molten mass, as a fluid nucleus, and an external solid shell, of not more 

 than 60 or 70 miles in thickness; and some geologists, desirous of rendering 

 the conclusion the foundation of certain theories, have considered the thick- 

 ness even less than that now mentioned. This conclusion, however, rests 

 on reasoning in which an important element is wanting. It involves the 

 hypothesis that the conductive power of the rocks which constitute the lower 

 portions of the earth's crust is the same as that of the rocks Avhich form its 

 upper portion. This conductive power of any substance measures the facility 

 with which heat is transmitted through it, and it is easily proved, by accu- 

 rate investigation, that when the same quantity of heat passes through 

 superimposed strata of different conductive powers, the increase of depth 

 corresponding to a given increase of temperature (as one degree) is in any 

 stratum proportional to the conductive power. Consequently, if the con- 

 ductive power of the lower portion of the earth's solid crust be greater than 

 that of the thin upper portion of it through which man has been able to 

 penetrate, the depth to which we must proceed to arrive at a certain temper- 

 ature (as that of fusion for the lower rocks) will be proportionally greater. 

 The precise nature of the rocks situated at a great depth can only be judged 

 of by analogy Avith those which are accessible to us ; but those geologists 

 Avho adopt the conclusion of the extreme thinness of the earth's crust, Avill 

 doubtless admit that its inferior part must be of igneous origin, and must 

 therefore be allowed to bear a certain resemblance to igneous rocks on the 

 surface of the earth. Mr. Hopkins had recently made a great number of 

 experiments on the conducting powers of various rocks. That of the softer 

 sedimentary rocks, which are great absorbents of water, is very much in- 

 creased by the quantity of moisture they contain ; but taking chalk, one of 

 the best absorbents, its conductive power, even when saturated, is not half 

 so great as that of some of the igneous rocks on which Mr. Hopkins had 

 experimented. Calcareous, argillaceous, and siliceous substances, reduced 

 to fine powder, stand, with reference to their conductive powers, in the order 

 in which they are now mentioned, the conductivity of the first being the 

 least; and when in a compact state, all that contributes to give hard and 

 crystalline character to the substance, and continuity to the mass through 

 which the heat is conducted, increases the conductive power. These consid- 

 erations lead to the conclusion that the conductivity of the inferior portion 

 of the earth's solid crust must be much greater, and may be very much 

 greater than that of the less consolidated and more superficial sedimentary 

 beds. Moreover, the temperature of fusion of certain substances, as Mr. 

 Hopkins had shown by experiment, is much increased by great pressure; 



