148 Scientific Proceedings, Royal Dublin Society. 



requires a temperature of at least 115° C, whereas by solution in 

 carbon bisulphide we may obtain crystals far more perfect at the 

 ordinary temperature of the air. We must, therefore, look upon 

 leucite as dissolved in a medium which is liquid at a bright red 

 heat, and only gives up this, as well as other minerals, by a lower- 

 ing of temperature, in the same way that a mixed boiling satu- 

 rated solution of salts of various solubilities separate out (far 

 below their fusing-point) as the solvent cools. Precipitation 

 might also depend upon withdrawal from the mixture of one 

 or more of its elements for the formation of a mineral that has 

 already commenced to separate. If we take a solution of mercuric 

 biniodide in a solution of potassic iodide, and add some substance 

 that will seize upon the iodine in the latter salt, such as argentic 

 nitrate, we have an immediate precipitate of the mercuric bin- 

 iodide proportional to the amount of potassic iodide broken up. 

 Stoppani gives the example of nitrate of potash dissolved in water, 

 which is precipitated immediately if alcohol is added. 1 The fact, 

 therefore, of leucite crystallizing far below its fusion-point proves 

 the solution of that mineral in that glass or some other. This 

 would explain the crystallization of the two minerals simultane- 

 ously, as at Roccamonfina ; for as the lowering of temperature 

 took place in the magma as the pyroxene crystallized out, the 

 remaining would become supersaturated with leucite, which 

 would have to separate. We might possibly imitate this con- 

 dition in freezing a saturated solution of a salt in water. It is 

 also possible that the leucite does not form until the potassic 

 chloride in the magma has been broken up, and the HC1 has 

 escaped in the vapour. 



In the formation of rocks we have a process of fractional ex- 

 haustion of the original amorphous medium, in which secondary 

 combinations can hardly be conceived to take place until some 

 portion assumes definite crystalline form, the kind of which will 

 depend upon the elements that enter into the composition of the 

 mixture, and the train of conditions which that undergoes in 

 passing from a higher to a lower temperature. Starting, for 

 example, from an amorphous mass of fused silicates, we may 

 suppose that condition 1 is favourable to the formation of mineral 



1 Corso di Geologia, vol. iii., p. 131. 



