as connected with the Theory of Substitutions. 361 
to the different modes in which the atoms of which their con- 
stituent molecules consist are grouped, and sometimes to the 
different allotropic states in which one or the other of those ele- 
ments is found. Thus as M. Millon has remarked, the intrinsic 
difference between carburetted hydrogen gas (CH), and otto of 
roses (CH), which are isomeric bodies, may perhaps consist in 
this, that in the former the carbon is under the form of common 
charcoal, and in the latter under the form of diamond. a9 
The following instances from Berzelius may serve as exam- 
ples of these allotropic states. 
Carbon is known under three forms—charcoal, plumbago, and 
diamond. They differ in specific gravity, in specific heat, and 
in their conducting power as respects caloric and electricity. In 
their relations to light, the one perfectly absorbs it, the second 
reflects it like a metal, the third transmits it like glass. In their 
relations with oxygen, they also differ surprisingly ; there are 
varieties of charcoal that spontaneously take fire in the air, but 
the diamond can only be burnt with difficulty at a high tempera- 
ture in pure oxygen gas. The second and third varieties do not 
belong to the same crystalline form: 
Silicium exists also under two forms. In its first it burns with 
facility in the air under a slight elevation of temperature. But, 
if it be previously exposed to a strong red heat, it changes into 
the second variety and becomes incombustible, so that it will not 
oxydize when placed with nitrate of potash in the hottest part of 
ablowpipe frame. As is well known, there are two forms of 
Silicic acid ; one soluble in water and hydrochloric acid, but pass- 
ing into the insoluble state by being previously made red hot. 
The silicium therefore carries in its combination the same proper- 
ties that it exhibits in the free state. 
In the same manner it might be shown that sulphur, selenium, 
phosphorus, titanium, chromium, uranium, tin, iridium, osmium, 
Copper, nickel, cobalt, and a variety of other bodies exist under 
several different forms, with distinctive properties that are often 
Well marked. In several of them the influence of this allotropic 
Condition is plainly carried into the compounds, as is well shown 
in the two varieties of arsenic which give rise to the two arseni- 
ous acids. 
The passage from one allotropic state to another takes tens 
commonly through the agency of apparently very trivial causes, 
