on the Chemistry of Geology, 27 



of the cork and after gentle agitation : one of them instantly 

 became solid on the introduction of a glass tube ; and the other 

 bore the introduction of the tube, but crystallized 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 evaporation, 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 bicar- 

 bonate, or by entering into some other mode of combination. 

 The siliceous matter, being once solid, would most probably be 

 insoluble in the menstruum by which it had been originally dis- 

 solved, and in that state would promote the increase of the de- 

 posite by its molecular attraction for the silex still remaining 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 

 probable, agreeably to the laws of crystallization, that the de- 

 velopment of regular crystals was owing to the extremely slow 

 progress of the same process which, when less slow, might 

 cause the deposite to be amorphous. In the formation of cal- 

 cedony and flint it was most likely, as Brongniart supposed, 

 that the silica, as in operations in the laboratory, was depo- 

 sited in a gelatinous form, hardening gradually by evapora- 

 tion and the cohesive attraction of its particles. The regu- 

 larly disposed lines which were so beautifully displayed in 

 some varieties of calcedony, seemed owing to successive de- 

 position, — one layer succeeding another, each assuming the 

 form and irregularities of the preceding, and differing in tint 

 according to the absence or presence of small varying quantities 

 of foreign matter, such as iron and manganese. In the case of 

 flint it was necessary, he said, to account for that remarkable 

 tendency which silica possessed, to occupy the place of or- 

 ganic matter, as exemplified by the specimens of flint,silicified 

 wood, and coral, on the lecture-table. This phenomenon the 

 lecturer thought might be explained on the principles which 

 had been developed that evening. Siliceous solutions, infil- 

 trating through organic masses in progress of decay, might 

 readily be decomposed by the affinity of gases or other com- 

 pounds generated during slow putrefaction, either for ihe silica 

 itself, or for its solvent. In either case a deposite of silex would 

 result. Consistently with this view, it was well known that 



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