INTERNAL CONDITION OF THE EARTH. 307 



melting points of some fusible substances, though 

 he was not able to extend them so far as to test the 

 effect of pressure in altering the melting point of 

 rock materials. 



Without making direct experiments, however, as 

 to the influence of pressure on the melting point of 

 rocks, we are able to infer by thermo-dynamic 

 reasoning what must be the effect of pressure on the 

 temperature of fusion, if we know whether rocks 

 expand or contract in the act of solidifying. My 

 brother, Professor James Thomson, has pointed out 

 that it is a necessary consequence of thermo-dynamic 

 laws that the temperature of the melting-points of 

 substances which expand in solidifying should be 

 raised by the application of pressure, while the 

 opposite would be the case with substances which 

 contract in becoming solid. Water, for instance, 

 expands when it freezes : and he calculated the 

 amount by which the freezing-point of water 

 ought to be lowered by the application of an 

 additional atmosphere of pressure. He found 

 it to be about -^ of a degree Fahrenheit ; and his 

 theoretical result was afterwards verified by experi- 



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