chemistry—­provided they retain their childish liking for puzzles.  It is really much like putting together the old six-block Chinese puzzle.  The chemist can work better if he has a picture of what he is working with.  Now his unit is the molecule, which is too small even to analyze with the microscope, no matter how high powered.  So he makes up a sort of diagram of the molecule, and since he knows the number of atoms and that they are somehow attached to one another, he represents 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 saw at once that the demons of his subconscious self had furnished him with a clue to