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



It is not sufficient, however, to consider the chemical transfor- 

 mations by themselves, for these are in general accompanied by 

 physical alterations in the mass. Much the most important of 

 these is solidification, which is always accompanied by a liber- 

 ation of heat.* Now the amount of heat evolved by the solid- 

 ification of compounds is of the same order as the thermal 

 effect of chemical combination. Since therefore the sum of 

 the chemical and physical changes must be such as to develop 

 the maximum amount of heat per second, let us say, the fact 

 that heat is liberated by solidification establishes a distinct ten- 

 dency to the formation of solid precipitates ; but this tendency 

 is only operative on condition that by no other transformation 

 can heat be more rapidly liberated. 



In view of these considerations it seems almost certain that 

 the order of genetic succession of minerals will sometimes fol- 

 low the order of separate solidification, but that in a greater or 

 smaller proportion of cases there will be essential differences 

 between the two orders. Thus there is no difficulty in under- 

 standing that at a certain early stage in the cooling of an erup- 

 tive mass, the rapidity of the evolution of heat may be pro- 

 moted by a separation of a portion of the iron as magnetite, 

 while the separation of the entire mass of this mineral at this 

 stage would be incompatible with the most rapid possible evo- 

 lution of heat by the entire simultaneous chemical and physical 

 changes then progressing. So also if two magmas of similar 

 qualitative but different quantitative compositions are corn- 

 pared, it does not seem remarkable from a thermo-chernical 

 standpoint that augite should separate before labradorite in the 

 one and labradorite before augite in the other (andesite and 

 basalt). 



In one sense the above discussion is of little value to lithol- 

 ogy. The new law of thermo-chemistry does not enable one 

 to state what will be the order of genetic succession of miner- 

 als under a given set of chemical and physical conditions. 

 This is because the total thermal effect of many of the rock- 

 forming minerals is unknown, while, so far as I am aware, the 

 rate at which reactions evolve heat has not yet been studied by 

 exact methods. But the law at least explains why neither fusi- 

 bility nor basicity obtains complete control of the process and 

 how it may happen that the order of succession which prevails 

 in one set of cases may be reversed in another. The new 

 principle governs the genetic succession of minerals in erup- 

 tive rocks, but the constants of the equation in which it may 



* Solidification of course takes place at a lower temperature than fusion. This 

 appears to be a theoretical necessity, for if any small fraction of a mass were to 

 solidify at the melting point, the heat evolved by the act of solidification would 

 raise the surrounding temperature above the melting point and the solid particle 

 would melt again. 



