86 



IGNEOUS AGENCIES. 



density toward the center, we must seek this temperature at a greater 

 depth. 



If A B (Fig. 76), representing depth from the surface S S, be taken 

 as absciss, aud heat be represented by ordinates, then, in a homogene- 

 ous earth, C D would represent uniform 

 increase of heat, and the heat ordinate 

 of 3,000°, m m, would be reached at the 

 depth of A m = thirty miles. But in an 

 earth increasing in density, and, there- 

 fore, in conductivity, the rate would not 

 be uniform, but gradually decreasing. 

 This would be represented, not by a 

 straight line, C D, but by a curved line, 

 C E ; and the ordinate of 3,000° would 

 not be reached at thirty miles, but at a 

 much greater depth — say at m\ of fifty 

 miles. 



2. Fusing-Point not the same for all 

 Depths. — Nearly all substances expand 

 in the act of melting, and contract in 

 the act of solidifying. Only in a few substances, like ice, is the re- 

 verse true. Now, the fusing point of all substances which expand in 

 the act of fusing must be raised by pressure, since the expanding force 

 of heat, in this case, must overcome not only the cohesion, but also the 

 pressure. That this is true, has been proved experimentally for many 

 substances by Hopkins.* But granite and other rocks have been 

 proved to expand in fusing; therefore the fusing-point of rocks is 

 raised by pressure, and must be greatly raised by the inconceivable 

 pressure to which they are subjected in the interior of the earth. For 

 this reason, therefore, we must again go deeper to find the interior fluid. 

 In the figure, m! is the point where we last found the fusing-point of 

 3,000°. But this is the fusing-point on the surface, or under atmos- 

 pheric pressure. The pressure of fifty miles of rock would certainly 

 greatly raise the fusing-point. Suppose it is thus raised to 3,500° : 

 to find this we must go still deeper, to m", perhaps seventy-five miles 

 in depth. But the increased pressure would again raise the fusing- 

 point ; and thus, in this chase of the increasing heat after the flying 

 fusing-point, where the former would overtake the latter, or whether 

 it would overtake it at all, science is yet unable to answer. 



From this line of reasoning, therefore, we conclude that the solid 

 crust of the earth must be much thicker than is usually supposed, and 

 there may be even no interior liquid at all. 



* American Journal of Science and Art. II, vol. xxxii, p. 367 



