488 Transactions. — Geology. 



For example, the osmotic pressure of a 1-per-cent. solution 

 of auro-potassic cyanide (AuKCy 2 ) at a temperature of 20° C. is 

 as under : — 

 Here r = 10 



and M = (197 x 1) + (39 x 1) + (26 x 2) = 288 



then p = (12 * *> < 20 + 273 > 



men p 2gg 



therefore p = 12-21 lb. per square inch. 



The osmotic pressure for a 4-per-cent. solution is 48-84 lb. 

 per square inch, and for a 10-per-cent. solution 122-1 lb. 



If we regard a molten magma as a mass of rock-material 

 in solid solution, it is manifest that the separation of the first 

 crop of minerals will result in a disturbance of the condition 

 of equilibrium — for we know that the osmotic pressure varies 

 directly as the concentration — and osmotic energy will at once 

 exert itself to again establish a state of equal concentration 

 throughout the magmatic solution. 



The temperature at the borders of the magma will be less 

 than that in the interior; and, since osmotic pressure varies 

 directly as the temperature, it follows that the osmotic 

 pressure will be less at the borders than in the interior. 

 But osmotic pressure holds good for the Boyle-Henry laws 

 and the law of Avogadro, and must therefore hold good for 

 the laws of thermo-dynamics.* Hence there will be a trans- 

 ference of osmotic energy from the interior to the borders. 

 The minerals first crystallized will be inert, and, being unable 

 to offer ionic resistance, will be carried towards the cooler 

 parts of the magma — that is, towards the borders. 



Manifestly this unequal struggle between different poten- 

 tials of osmotic energy will continue so long as the difference 

 of temperature exists ; but a point will be reached where the 

 pressure will be neutralised by the increasing viscosity of the 

 magma. 



The minerals which first crystallize are generally basic, 

 and these being carried outward as they form necessarily 

 leaves the interior more siliceous or acid than the borders. 



Further, it seems not improbable that molecular concen- 

 tration,! in accordance with the Ludwig-Soret principle, may 

 be the result of osmotic pressure due to unequal temperature 

 in a homogeneous solution. 



•Lord Rayleigh, "The Theory of Solution," Scientific Papers, 

 Cambridge Press, vol. iv., 1903. 



+ J. H. van t'Hoff, "The Role of Osmotic Pressure in the Analogy 

 between Solutions and Gases," " Tue Modern Theory of Solution," New 

 York, 1899, p. 21. 



