f linen (see Fig. 4). I told
you last time what a dear old friend told me, who from practical
experience is far more familiar with tinder-boxes and their working than
I am, that no material was better for making tinder than an old cambric
handkerchief. However, as I have no cambric handkerchief to operate
upon, I must use a piece of common linen rag. I want you to see
precisely what takes place. I set fire to my linen (which, by the bye, I
have taken care to wash carefully so that there should be no dirt nor
starch left in it), and while it is burning shut it down in my
tinder-box. That is my tinder. Let us now call this charred linen by its
proper name--my tinder is carbon in a state of somewhat fine
subdivision. Carbon is an elementary body. An element--I do not say this
is a very good definition, but it is sufficiently good for my
purpose--an element is a thing from which nothing can be obtained but
the element itself. Iron is an element. You cannot get anything out of
iron but iron; you cannot decompose iron. Carbon is an element; you can
get nothing out of carbon but carbon. You can combine it with other
things, but if you have only carbon you can get nothing out of the
carbon but carbon. But this carbon is found to exist in very different
states or conditions. For instance, it is found in the form of the
diamond. (Fig. 18 _a_). Diamonds consist of nothing more nor less than
this simple elementary body--carbon. It is a very different form of
carbon, no doubt you think, to tinder. Just let me tell you, to use a
very hard word, that we call the diamond an "allotropic" form of carbon.
Allotropic means an element with another _form_ to it--the diamond is
simply an allotropic form of carbon. Now the diamond is a very hard
substance indeed. You know perfectly well that when the glass-cutter
wants to cut glass he employs a diamond for the purpose, and the reason
why glass can be cut with a diamond is because the diamond is harder
than the glass. I dare say you have often seen the names of people
scratched on the windows of railway-carriages, with the object I suppose
that it may be known to all future occupants of these carriages that
persons of a certain name wore diamond rings. Well, in addition to the
diamond there is another form of carbon, which is called black-lead.
Black-lead--or, as we term it, graphite--of which I have several
specimens here--is simply carbon--an allotrope of carbon--the same
elementary substance,