206 



SCIENCE 



[N. S. Vol. XXIX. No. 736 



enee of quartz. On the other hand, cer- 

 tain lower silicates do not form when there 

 is sufficient silica to form higher silicates 

 with the same bases. Thus KAlfSiOs), 

 (leucite) and NaAlSiO^ (nephelite) do not 

 occur pyrogenetically with SiO, (quartz). 



Moreover, it is well known that some rock 

 magmas, especially those of intermediate 

 composition, crystallize under one set of 

 conditions into certain combinations of 

 minerals, and under others into other com- 

 binations, certain minerals appearing in 

 one case and not in another, though the 

 magmas from which they formed were 

 chemically alike. 



In order to account for the production 

 of the mineral compounds known to occur 

 in igneous rocks, as well as for the absence 

 of others; and to understand the possi- 

 bility of variation in the production of 

 mineral compounds from any magma under 

 variable conditions ; and to comprehend the 

 act of separation and crystallization of such 

 minerals upon the solidification of the 

 magma; it is necessary to consider the 

 probable physical and chemical character 

 of liquid rock magmas, especially the 

 known physicochemical laws regarding 

 solutions. 



Discussions of the behavior of solutions 

 under varying conditions of temperature 

 and pressure involve theories of the pos- 

 sible molecular constitution of matter, 

 gaseous, liquid and solid, which must be 

 kept in mind in order to form any clear 

 conception of the processes under consid- 

 eration. The kinetic theory regarding the 

 behavior of molecules of gas, liquid or 

 solid, under variable temperatures and 

 pressures, furnishes definite pictures of 

 changes of state at transition points from 

 one phase to another. Those with which 

 the problems before us are most concerned 

 are the critical point of gases, the melting 

 point of solids, the solution or the separa- 

 tion points of solids in liquids and also the 



transition point between two crystal phases 

 of the same compound (such as that be- 

 tween quartz and tridymite). 



Since liquid rock magmas are solutions 

 of silicate compounds in one another, all 

 that is known of the physical and chemical 

 behavior of solutions is germane to the dis- 

 cussion. This includes the solution of 

 gases, liquids and solids, in liquids; and 

 eventually their solution in solids ; the solu- 

 bility of various substances in liquids of 

 other substances; the possible molecular 

 constitution of liquid solutions; the exist- 

 ence of molecules of different degrees of 

 complexity, and the dissociation or ioniza- 

 tion of some compound molecules ; the laws 

 relating to diffusion, and the relative dif- 

 fusibility of various compounds; those re- 

 lating to the molecular concentration — the 

 saturation and supersaturation of solutions. 

 The chief qualifying factors in this discus- 

 sion are the chemical composition of the 

 several compounds; the possibility of 

 changes in chemical equilibria; the viscos- 

 ity of the solution; the temperature, pres- 

 sure and the time through which any opera- 

 tion acts. The possibility of producing in a 

 colloidal condition one of the compounds, 

 A1(0H')3, Fe(0H)3 or Si(OH)„ by the 

 interaction of the hydroxyl (OH) and 

 aluminium' (Al), iron (Fe), or silicon 

 (Si), is also to be taken into consideration. 



In a solution containing the chemical 

 elements common to igneous rocks reactions 

 should take place between them in accord- 

 ance with known chemical laws, and with 

 results corresponding to observed pyrogen- 

 etic mineral compounds. Some of the fun- 

 damental laws relating to chemical reac- 

 tions among the elements are based upon 

 conceptions of chemical energy and activ- 

 ity, and of the conditions that modify their 

 effects. An important factor in chemical 

 processes is, often, a catalytic agent that 

 promotes reactions without itself appearing 

 as a component of the final products. The 



