benzene, toluene, xylene, phenol, cresol, naphthalene, anthracene,
methyl anthracene, phenanthrene and carbazol.

There! I had to introduce you to the whole receiving line, but now that
that ceremony is over we are at liberty to do as we do at a reception,
meet our old friends, get acquainted with one or two more and turn our
backs on the rest. Two of them, I am sure, you've met before, phenol,
which is common carbolic acid, and naphthalene, which we use for
mothballs. But notice one thing in passing, that not one of them is a
dye. They are all colorless liquids or white solids. Also they all have
an indescribable odor--all odors that you don't know are
indescribable--which gives them and their progeny, even when odorless,
the name of "aromatic compounds."

[Illustration: Fig. 8. Diagram of the products obtained from coal and
some of their uses.]

The most important of the ten because he is the father of the family is
benzene, otherwise called benzol, but must not be confused with
"benzine" spelled with an _i_ which we used to burn and clean our
clothes with. "Benzine" is a kind of gasoline, but benzene _alias_
benzol has quite another constitution, although it looks and burns the
same. Now the search for the constitution of benzene is one of the most
exciting chapters in chemistry; also one of the most intricate chapters,
but, in spite of that, I believe I can make the main point of it clear
even to those who have never studied 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