THE GEOLOGIST. 



71 



eavoiired to demonstrate that whenever, in nature, a chemical 

 decomposition takes place very slowly, the products are crystalline, and 

 assume the forms of the pre-existing body whenever the decomposi- 

 tion takes place abruptly we obtain, on the contrary, amorphous 

 precipitates or bodies devoid of crystalline structure. Amongst other 

 experiments, M. Kuhlmann has shown that a current of sulphuretted 

 hydrogen transforms many salts of lead into sulphide of lead without 

 producing a change in their structure, thus explaining certain anomalous 

 forms observed in nature. A. current of ammonia transforms pyrolusite 

 into protoxide of manganese without producing a change in the form of 

 the crystals of the first. Nascent hydrogen has the same effect whilst 

 reducing certain metallic salts, &c. 



The same author has, more recently, made a very interesting study of 

 the spontaneous crystallisation of amorphous bodies. It has no doubfe 

 happened to many of our readers to pick up pieces of common flint in 

 which certain parts are profusely crystallised, or studded with minute 

 transparent crystals of quartz. It has been M. Kuhlmann's object to 

 show how these crystals have been formed, and to explain other like 

 phenomena. Numerous observations have shown him that amorphous 

 or earthy matters in general have a great tendency to crystallise by slow 

 desiccation ; and he has shown by experiment that, in time, many 

 substances, at first apparently without structure of any sort or kind, 

 will take crystallised forms. This is perfectly illustrated by malate of 

 lead, which, as every chemist knows, is produced in the form of an 

 amorphous precipitate, but which after being allowed to repose for a 

 certain time, shoots out into crystals. M. Kuhlmann has shown that in 

 any substance the crystals produced in this manner are neater, or 

 better defined when the drying has proceeded very slowly. f The 

 siliceous deposits formed at the present time by the geysers of Iceland 

 furnish numerous examples of the spontaneous crystallisation of which 

 we speak. 



It has been frequently observed that when a piece of rock, or a 



Thus we see sulphate of potash placed in damp chalk, transform itself slowly 

 into gypsum without losing its crystalline form (according to Beudant). 



t The same holds true, to a certain extent, for volcanic lava, both ancient and 

 modern, which furnish very different products, according as their cooling has 

 been rapid or slow. In the first case they often form a black non-crystalline glass ; 

 in the latter, a stony mass of crystalline structure. 



