G. F. Becker — A new law of Thermo- Chemistry. 123 



The hypothesis that the order of genetic succession is that of 

 increasing acidity has been advocated, I believe, only for gran- 

 itic rocks. Even for these it is not rigidly true. The order in 

 which the feldspars appear in volcanic rocks, however, is also one 

 of increasing acidity, as well as of increasing fusibility. Such 

 relations between chemical composition and fusibility are of 

 course extremely frequent, especially among organic com- 

 pounds. Messrs. Fouque" and Michel-LeVy also found albite 

 more fusible than labradorite and less so than anorthite. Those 

 who regard the plagioclases as isomorphous mixtures of anor- 

 thite and albite must, therefore, consider oligoclase as the most 

 fusible mixture of the two, while it also appears to be com- 

 monly the last feldspar to crystallize. A similar maximum fusi- 

 bility is known to exist in other series. Thus the fundamental 

 slag of iron blast furnace practice is an exact bisilicate of the 

 form 4CaSi0 3 +A-lSi 3 9 . This is the most fusible compound of 

 these ingredients, any alteration in the acidity or in the ratio of 

 the bases rendering it more difficult of fusion. Oligoclase con- 

 tains more aluminium than the slag, but also sodium. 



It thus appears that while the chemical composition and the 

 fusibility of the rock forming minerals are most intimately 

 connected, neither of these properties affords an adequate ex- 

 planation of the order of genetic succession, nor does any 

 known combination of them explain the facts. That the fusi- 

 bility at least is not without effect upon the order of succession 

 seems highly probable, but, if so, there are other concomitant 

 influences which very materially modify the results. 



If the changes which occur in cooling eruptive magmas obey 

 the laws of thermo-chemistry, ihe somewhat confused relations 

 actually observed between genetic succession, fusibility and 

 acidity are just such as might be expected in advance. There ' 

 is every theoretical and experimental reason to suppose that the 

 fluid eruptive magma consists of one or more compounds differ- 

 ing essentially from the minerals eventually formed from it. 

 In cooling it must therefore undergo a series of chemical and 

 physical changes. The formation of any new stable chemical 

 compound, whether fluid or solid, in the mass converts other 

 forms of energy into heat; but, at the same time, the subtrac- 

 tion of any such group of molecules from the previously exist- 

 ing combination alters the chemical constitution of the residue. 

 It is quite conceivable that the change in the residue should in- 

 volve either an absorption or a liberation of heat, but if the 

 former and if this were to exceed the positive thermal effect of 

 the supposed stable compound, the whole operation would be 

 impossible. In general, if no physical change accompanies the 

 alteration of chemical configuration, that chemical change and 

 that only will take place through which heat is liberated most 

 rapidly. 



