296 THE EVOLUTION OF PETKOLOQICAL IDEAS. 



substance. The temperature at which a definite sul)stance ciystallizes 

 from its own liquid depends only on the substance and on the pressure 

 to which it is subjected; whereas the temperature at which the same 

 su])stance separates from its solution in another snl)stance depends 

 principally on the relative proportions of the two substances. No 

 chemist Avill fall into the error of assumino- that a solution ceases to be 

 a solution at i^oo ,, 300'-, 400, .500 ,, or even when heated to a tempera- 

 ture at which it becomes self-luminous; or will suppose that a crystal- 

 line aggregate of ice and calcium chk)ride which has become tiuid is a 

 solution, but that a mixture of quartz and felspar which has been f nsed 

 is not." 



He then proceeds to point out that the laws which govern the solidi- 

 fication of aqueous solutions must hold good also for igneous solutions; 

 that by the addition of a certain amount of calcium chloride to water 

 the teuiperature may be lowered to —10*^ C. without the separation of 

 any solid substance; that by the addition of further amounts the tem- 

 perature of consolidation of water ma}' be lowered as much as oif- and 

 that of calcium chloride 100^. Other salts, such as the sulphates and 

 nitrates of potassium, may be made to separate from aqueous solutions 

 at temperatures from (JOO to 800 l)elow their freezing points; more- 

 over, the order of consolidation is determined by the relative amounts 

 of the two substances present; thus watei- may l)e made to consolidate 

 before or after a dissolved salt by varying the concentration of the 

 solution. 



I have given a somewhat full abstract of this important letter 

 because 1 believe that the expansion of the idea which it contains will 

 be the characteristic feature of the next great advance in petrological 

 science, an advance which will come about, not so nmch Iw adding to 

 our already large store of facts as b}^ dint of experiment controlled 

 by the modern theory of solution, and carried out for the express 

 purpose of testing the consequences of that theory and discovering the 

 modifications which may be necessary to adapt it to igneous magmas. 



Almost all recent writers on theoretical questions relating to the 

 igneous rocks have accepted the solution theoiy, and the condition of 

 foiination of minerals has been discussed from this point of view. 

 Crystals tend to form in a homogeneous liquid mass when the liquid 

 becomes supersaturated with any definite compound. As soon as crys- 

 tals arc developed the liquid in their unmediate neighborhood ceases 

 to- be supersaturated, and there is thus esta))lished an osmotic force 

 producing molecular flow from the supersaturated portions toward the 

 growing crystals. 



From a consideration of the work of Pelouze on glasses, comluned 

 with his own work on igneous rocks, Lagorio arrived at the conclusion 

 that the ordinary rock forming compounds tend to separate out in the 

 following order: Oxides, pure iron silicates, magnesian and ferro-mag- 

 nesian silicates (olivine and rhombic pyroxenes), calc-magnesian sili- 

 cates (monoclinic pyroxenes and hornblende), silicates of magnesium 



