Remarks on Rock Fusion. 141 



# 025 to 0*021 ; but this would not materially alter the dia- 

 grams, remembering that the other constants of the equations 

 are found independently. 



As to the latent heat of fusion, it is a question whether 

 there is such a datum strictly speaking. What I did was to 

 idealize the case, by slightly prolonging the rate at which 

 liquid thermal capacity changes with temperature in the direc- 

 tion of lower temperatures and the rate at which solid thermal 

 capacity changes with temperature in the direction of higher 

 temperature. The admissibility of such a process is a ques- 

 tion of experiment; and experiment sustains it because the 

 two loci very nearly overlap. Thus one can state that if dia- 

 base fused at such a temperature its latent heat would be so 

 and so. The temperature to be taken is clearly the melting 

 point obtained in the volume work. 



Now, as Mr. Fisher will acknowledge, mere statements of 

 melting point and latent heat for dolerite are inadequate, so 

 long as the volume changes at fusion are missing. He might 

 have made a much more startling statement by comparing 

 diabase and naphthalene, for instance ; the latter has a latent 

 heat say twice as great as the former, in spite of the fact that 

 diabase melts at a temperature enormously high as compared 

 with the other body. In all this there is nothing implied 

 either with reference to the law of fusion in question or the 

 isometrics. 



In the case of diabase, granting merely that the volume 

 changes experimentally observed are correct, the following 

 mean values* hold good : 



Melting point 1200° C, dT/dp = '020 



1100 C, dT/dp -027 



" " 1000 C, dT/dp -049 



To go below this it would be necessary to actually work out 

 the relation of thermal capacity to temperature for liquid 

 diabase. Nevertheless the data given are sufficient to show, 

 that the lower melting point for which Mr. Fisher is angling 

 would only accentuate our cardinal result, viz : that the rate 

 at which melting point increases with pressure is about the 

 same in silicates and in organic compounds. My conviction of 

 the essential correctness of Mr. King's constructions is there- 

 fore unshaken. 



* T being the absolute melting point at the pressure p in atmosphere. 



