The diamond is the hardest of all known substances, leaving a scratch on any substance across which it may be drawn.  Yet it is one whose form can be changed, and whose hardness can be completely destroyed, by the simple process of combustion.  It can be deprived of its high lustre, and of its power of breaking up by refraction the light of the sun into the various tints of the solar spectrum, simply by heating it to a red heat, and then plunging it into a jar of oxygen gas.  It immediately expands, changes into a coky mass, and burns away.  The product left behind is a mixture of carbon and oxygen, in the proportions in which it is met with in carbonic-anhydride, or, carbonic acid gas deprived of its water.  This is indeed a strange transformation, from the most valuable of all our precious stones to a compound which is the same in chemical constituents as the poisonous gas which we and all animals exhale.  But there is this to be said.  Probably in the far-away days when the diamond began to be formed, the tree or other vegetable product which was its far-removed ancestor abstracted carbonic acid gas from the atmosphere, just as do our plants in the present day.  By this means it obtained the carbon wherewith to build up its tissues.  Thus the combustion of the diamond into carbonic-anhydride now is, after all, only a return to the same compound out of which it was originally formed.  How it was formed is a secret:  probably the time occupied in the formation of the diamond may be counted by centuries, but the time of its re-transformation into a mass of coky matter is but the work of seconds!

There is another form of carbon which was formerly of much greater importance than it is now, and which, although not a natural product, is yet deserving of some notice here.  Charcoal is the substance referred to.

In early days the word “coal,” or, as it was also spelt, “cole,” was applied to any substance which was used as fuel; hence we have a reference in the Bible to a “fire of coals,” so translated when the meaning to be conveyed was probably not coal as we know it.  Wood was formerly known as coal, whilst charred wood received the name of charred-coal, which was soon corrupted into charcoal.  The charcoal-burners of years gone by were a far more flourishing community than they are now.  When the old baronial halls and country-seats depended on them for the basis of their fuel, and the log was a more frequent occupant of the fire-grate than now, these occupiers of midforest were a people of some importance.

We must not overlook the fact that there is another form of charcoal, namely, animal charcoal or bone-black.  This can be obtained by heating bones to redness in closed iron vessels.  In the refining of raw sugar the discoloration of the syrup is brought about by filtering it through animal-charcoal; by this means the syrup is rendered colourless.

When properly prepared, charcoal exhibits very distinctly the rings of annual growth which may have characterised the wood from which it was formed.  It is very light in consequence of its porous nature, and it is wonderfully indestructible.