270 PROCEEDINGS OF SECTIOA" C. 



Moreover, it is well known that some rock magmas, especially those 

 of intermediate composition, crystallise under one set of conditions 

 into certain combinations of minerals, and under others into othei' 

 combinations. 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 possibility of variation in the production of 

 mineral compounds from any magma under variable conditions; and 

 to comprehend the act of separation and crystallisation 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 solu- 

 tions. 



Discussions of the behaviour of solutions under varying conditions 

 of temperature and pressure involve theories of the possible 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 

 consideration. The kinetic theory regarding the behaviour of mole- 

 cules of gas, liquid, or solid, under variable temperatures ana 

 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 separation, points of solids 

 in liquids, and also the transition point between two solid phases 

 of the same compound, such as that between quartz and tridymite. 



Since liquid rock magmas are solutions of silicate compounds in 

 one another, all that is known of the physical and chemical behaviour 

 o: solutions is germane to the discussion. This includes the solution 

 of gases, liquids, and solids, in liquids; and eventually their solution 

 in solids. The solubility of various substances in liquids of other 

 substances; the possible molecular constitution of liquid solutions; 

 the existence of molecules of different degrees of complexity, and the 

 dissociation or ionisation of some compound molecules ; the laws relat- 

 ing to difi'usion, and the relative diffusibility of various compounds; 

 those relating to the molecular concentration — the saturation and 

 supersaturation of solutions. The chief qualifying factors in this dis- 

 cussion are the chemical composition of the several compounds; the 

 viscosity of the solution ; the temperature, pressure, and the time 

 through which any operation acts. The possibility of producing in a 

 colloidal condition one of the compounds : Al(OH)g, Fe(0H)3, or 

 Si(0H)4, by the interaction of 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 accordance with 

 known chemical laws, and with results corresponding to observed 

 pyrogenetic mineral compounds. Some of the fundamental laws relat- 

 ing to chemical reactions am.ong the elements are based upon concep- 

 tions of chemical energy and activity, 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 



