»^' 4he^Cft€mktry of' Geology."'- ' 253 



olbkimbious 9oIUl)ou& ^8 an instance from the anima! kingdom. 

 Water cooled carefully below its usual point of congelation, and 

 saturated solutions of Glauber''s salts, were liquids in which a 

 similar instability of equilibrium was conspicuous. The lectu- 

 rer, in illustration, here showed two solutions of Glauber'*s salt, 

 4«>he explained that the mere pressure of the atmosphere, on re- 

 mbving the cork, or the slightest agitation, often caused such so- 

 lutions to become solid ; and that when these failed, the introduc- 

 tion of « solid body, especially a crystal of Glauber's^ salt, or of 

 any substance having even a feeble affinity for the salt or its sol- 

 vent, such as a globule of air or carbonic acid gas, generally de- 

 termined immediate crystallization. The solutions on the lecttire 

 table, retained their form after removal of the cork, and after 

 gentle agitation : one of them instantly became solid on the intro-. 

 duction of a glass tube ; and the other bore the introduction of the' 

 tube, but crystallised instantly when a globule of air from the 

 lungs was blown through the tube. The principle elucidated by 

 these facts was, he said, directly applicable to his argument. A' 

 solution of silica oozing slowly into the cavities of a porous or' 

 cellular rock might yield a deposite as a consequence of evapo- 

 ration, of a slight affinity between the silica and some substance 

 with which it accidentally came into contact, or of the solvent 

 power of an alkali which had contributed to its solution being 

 lessened by passing from the state of a simple carbonate to that 

 of a bicarbonate, or by entering into some other mode of combi- 

 nation. The siliceous matter, being once solid, would most pro- 

 bably be insoluble in the menstruum by which it had been origi- 

 nally dissolved, and in that state would promote the increase of 

 the deposit by its molecular attraction for the silex still remain- 

 ing in solution. In this manner, might cavities of considerable 

 size be gradually filled up with calcedony, flint, or rock-crystal. 

 It was difficult, he said, to indicate the precise circumstances 

 which determined the form assumed by the silex ; but it was proba- 

 ble, agreeably to the laws of crystallization, that the development 

 of regular crystals was owing to the extremely slow progress of 

 the same process, which, when less slow, might cause the deposit 

 to be amorphous. In the formation of calcedony and flint, it 

 was most likely as Brongniart supposed, that the silica, as in 



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