MINERAL CAVITIES AND THEIR CONTENTS. 
129 
than that of water. For instance, at 63°*7 C. (145°*4 F.) under a 
pressure of 223 atmospheres, the gas has been reduced to of 
its volume (Andrews, 4 On the Physical Properties of Matter,’ 
&c., Proc. Eoy. Soc., 1875). Under these circumstances it must 
be a gas at least as dense, if not denser than water. 
Certain specimens of rock crystal containing bubbles of car- 
bonic acid and also other gases floating on water have been 
heated under mercury to such a temperature that the relative 
densities of the gas and the water were reversed, and the bubbles 
sank. In one specimen this change took place at 150° C. (302° 
F.) It is an effect caused by the expansion of the water, and 
the consequent compression of the gas in the bubble. 
The diamond, which is the densest form of carbon, has never 
been artificially prepared, and its probable mode of formation 
has never been satisfactorily explained. It has been supposed 
to be derived from organic matter by a process of dehydro- 
genation, the hydrogen being separated from the carbon by the 
action of moist peroxide of iron and sulphates. A view which 
is equally tenable is that carbon is separated from carbonic 
acid, in a very highly compressed condition, by a process of 
deoxidation, accomplished by alkaline sulphides, metallic iron, 
or protoxide of iron at moderately high temperatures, or by 
the mutual reaction of hydrocarbons and highly compressed 
carbonic acid, which would result in the separation of water 
and condensed carbon. The combined actions of high tempera- 
tures and pressures are conditions under which no chemical 
reactions have ever been made in the laboratory. To imitate 
the supposed operations of nature is difficult, because, setting 
aside the factor time, chemical reactions in the bowels of the 
earth are reactions carried on under tremendous pressures in 
sealed tubes of vast thickness. 
