140 Mr. Hopkins on the Earth's Temperature, [May 13, 



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 investigation, that after a sufficient and 

 very great length of time, the law according to which the temperature 

 would increase in descending beneath the earth's surface, within depths 

 small compared with the earth's radius, would be — that the increase of 

 temperature would be proportional to the increase of deptli. This 

 coincides with the observed law, if we neglect the anomalous irregular 

 variations which are found to exist more or less in each locality. Now 

 according to this law, the temperature at the depth of 60 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 ren- 

 dering the conclusion the foundation of certain theories, have considered 

 the thickness 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 which 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 1 °), is in 

 any stratum proportional to the conductive power. Consequently, if 

 the conductive power of the lower portions 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 temperature (as that of fusion for the lower rocks) will be pro- 

 portionally greater. The precise nature of the rocks situated at a great 

 depth can only be judged of by analogy with those which are accessible to 

 us ; but those geologists who adopt the conclusion of the extreme thin- 

 ness of the earth's crust, will 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 conductive 

 powers of various rocks. That of the softer sedimentary rocks, which 

 are great absorbents of water, is very much increased 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 a 

 hard and crystalline character to the subslance, and continuity to the 

 mass through which the heat is conducted, increases the conductive 



