THE GEOLOGIST. 
71 
cavourod to demonstrate that whenever, in nature, a chemical 
decomposition takes place very slotcly, 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 doubt 
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. 
