520 N. L. BOWEN 



with liquid magma. In 'that connection, however, the heat of 

 mixing of liquids is not without significance, for the heat of solution 

 of a solid is to be regarded as the resultant of two heats, the heat of 

 melting of the solid and the heat of mixing of the liquids. Now the 

 heats of mixing that we have calculated above are really very insig- 

 nificant as compared with the heats of melting of the solids, and the 

 heats of solution of the solids are, therefore, very nearly equal to 

 the latent heats of melting. For the solution of solid anorthite in 

 diopside a little less than the latent heat is required, for the solution 

 of solid albite in diopside a little more than the latent heat is re- 

 quired. The differences are noteworthy in connection with any 

 theory that postulates an evolution of heat when an acid rock is 

 immersed in basic magma, but for the purposes of the present 

 inquiry which seeks to find merely the order of magnitude of the 

 heat effect when solid rock is added to magma, we may state that 

 the heat of solution of solid anorthite, albite, or diopside in any 

 liquid mixture of them is substantially equal to the latent heat of 

 melting. Moreover it is probably true of silicates in general that the 

 solution of the solid is attended by a large absorption of heat, 

 though not many determinations lend themselves to interpretation 

 in the same way as the above. Thus when one attempts similar 

 calculations from the equilibrium of anorthite with nephelite and of 

 anorthite with silica it is found necessary to assume molecular associ- 

 ation in nephelite and silica, and, while this is not surprising since 

 both occur in more than one crystal form, it nevertheless so compli- 

 cates the case that interpretation becomes impossible. One thing is 

 certain, namely, that all the solubility curves of silicates yet deter- 

 mined show a marked increase of solubility with temperature, which 

 means a strong absorption of heat upon solution. No example of 

 retrograde solubility is known. In conclusion, then, we may state 

 that the solution of a silicate in a magma is usually accompanied by 

 a large absorption of heat, probably of the order of magnitude of the 

 heat of melting. 



THE QUESTION OF SUPERHEAT 



Having thus arrived at a general conception of the heat re- 

 quired for solution we may consider the question of the heat avail- 

 able. One aspect of this is concerned with the superheat of the 



