47 



with carbonate of magnesia, and sulphate of magnesia and car- 

 bonate of hme resuUed. 



Mariiinac found at 200° Cent., that cliloride of mao-nesium and 



o 



carbonate of lime reacted on each other, and that double carbo- 

 nate of maicnesia and lime resulted. Senarmont made a similar 

 experiment. Favre estimates that an ocean pressure of from 

 500 to 600 feet is adequate to effect these changes when the 

 water is heated. 



Referring to the increase of temperature at great depths as a 

 means of determining the thickness of the solid crust or shell of 

 the globe, Prof. W. B. Rogers remarked, that much uncertainty 

 must attend such calculations until all the necessary data have 

 been ascertained. It is not merely requisite, to know the law 

 according to which the temperature augments as we descend, 

 and the ordinary melting point of the different rocky materials 

 forming the crust, but we must ascertain how and in what degree 

 the melting point in each case is influenced by the pressure to 

 which the heated mass is subjected. 



According to the experiments of Bunsen, Hopkins, and others, 

 spermaceti, wax, and paraffine, when heated under powerful press- 

 ure, require a higher temperature for their liquefaction than is suffi- 

 cient to melt them under ordinary circumstances, where the press- 

 ing force is only that of a single atmosphere. If, with Hopkins, 

 we assume that the melting point of rocks is in like manner 

 raised by the pressure under which they are placed beneath the 

 surface, we must agree with him in the conclusion that the mate- 

 rials of the earth's crust may retain their solid condition to a 

 much greater depth than has been usually supposed. 



We have, however, no warrant for assuming that all, or even 

 the great mass of rocky materials, obey the same law in regard to 

 their liquefaction as wax and the other similar substances above 

 named. It should be remembered that these latter belong to the 

 class of substances which contract as they pass from the liquid to 

 the solid form, while there is another class typified by ice, in 

 which the act of congelation is accompanied by more or less ex- 

 pansion. Now it has been proved experimentally by Thompson, 

 that pressure, instead of raising, actually lowers the melting point 

 of ice ; and there is reason for regarding it as a general law, that 



