presents each atom by the first
letter of its name and the points of attachment or bonds by straight
lines connecting the atoms of the different elements. Now it is one of
the rules of the game that all the bonds must be connected or hooked up
with atoms at both ends, that there shall be no free hands reaching out
into empty space. Carbon, for instance, has four bonds and hydrogen only
one. They unite, therefore, in the proportion of one atom of carbon to
four of hydrogen, or CH_{4}, which is methane or marsh gas and obviously
the simplest of the hydrocarbons. But we have more complex hydrocarbons
such as C_{6}H_{14}, known as hexane. Now if you try to draw the
diagrams or structural formulas of these two compounds you will easily
get

H H H H H H H
| | | | | | |
H-C-H H-C-C-C-C-C-C-H
| | | | | | |
H H H H H H H
methane hexane

Each carbon atom, you see, has its four hands outstretched and duly
grasped by one-handed hydrogen atoms or by neighboring carbon atoms in
the chain. We can have such chains as long as you please, thirty or more
in a chain; they are all contained in kerosene and paraffin.

So far the chemist found it east to construct diagrams that would
satisfy his sense of the fitness of things, but when he found that
benzene had the compostion C_{6}H_{6} he was puzzled. If you try to draw
the picture of C_{6}H_{6} you will get something like this:

| | | | | |
-C-C-C-C-C-C-
| | | | | |
H H H H H H

which is an absurdity because more than half of the carbon hands are
waving wildly around asking to be held by something. Benzene,
C_{6}H_{6}, evidently is like hexane, C_{6}H_{14}, in having a chain of
six carbon atoms, but it has dropped its H's like an Englishman. Eight
of the H's are missing.

Now one of the men who was worried over this benzene puzzle was the
German chemist, Kekule. One evening after working over the problem all
day he was sitting by the fire trying to rest, but he could not throw
it off his mind. The carbon and the hydrogen atoms danced like imps on
the carpet and as he watched them through his half-closed eyes he
suddenly saw that the chain of six carbon atoms had joined at the ends
and formed a ring while the six hydrogen atoms were holding on to the
outside hands, in this fashion:

H
|
C
/ \\
H-C C-H
|| |
H-C C-H
\ //
C
|
H

Professor Kekule