204 Dr. Mac CuUoch 07i Mineral Veins. 



without the necessity of a dry or subliming heat, although it 

 is not easy to ascertain the exact nature of this process. 



If chemistry has not yet formed every complicated salt that 

 is found in the list of metallic saline minerals, it has pro- 

 duced so many that we may, with little hazard of error, con- 

 sider the aqueous process as fully competent to the production 

 of the whole. That nature can exhibit some of them, in a 

 crystallized form, such as the phosphat of iron, for example, 

 when we can only obtain them in our laboratories in a powdery 

 one, must be referred to the cause just noticed; namely, the 

 rapidity of our operations and the slowness of her's. As to 

 the silicats, our acquaintance with the real nature of this 

 combination, or the exact mode in which silica acts the part 

 of an acid, is as yet so recent and imperfect, that no opinion 

 can at present be given respecting them. 



The igneous theory of metallic veins was supposed to be 

 supported by an incontrovertible argument derived from the 

 sulphuret of iron, which, it was asserted, could not possibly be 

 formed from aqueous solution ; and the same rule was in con- 

 sequence extended to all the other sulphurets. We shall 

 shortly see that nature does produce it from aqueous solutions 

 abundantly. In the laboratory it can be procured, merely by 

 allowing the serum of blood to stand for some time ; and, it is 

 also obtained from the decomposition of sulphat of iron by 

 animal matters. There is little doubt that other metallic sul- 

 phurets may be formed in the same manner ; and, it is a subject 

 that requires to be further investigated by those who may 

 have leisure for this purpose. These combinations can also 

 be procured in the aqueous method, by means of sulphuretted 

 hydrogen ; a very probable agent in nature. In these latter 

 cases the sulphurets are only obtained in a powdery form ; but 

 in the former the iron pyrites is crystallized. 



Respecting the phosphurets, our direct experience is next to 

 nothing; but it must be remarked at the same time, that this 

 is at least a rare if not a doubtful modification of the metallic 

 minerals. But the methods of decomposing the sulphuric and 



