ions have been, and are,
the principal objects of interest to the scientific student, and from
our strivings to understand them we have learnt most of what we know.
All three are quantities and all are expressible in terms of units.

Now there is one point which I have thought would especially interest
you. A very remarkable tendency of modern discovery shows more and more
clearly that not only are these things quantities which we can express
in units of our own choosing, but that Nature herself has already chosen
units for them. The natural unit does not, of course, bear any exact
connexion with our own. This being so, it must be of the utmost
importance that we should know what these natural units are and so be
able to understand what Nature is ready to tell us. Nature has chosen to
speak in a certain language; we must get to know that language.

In the first place we know surely that there are natural units of
matter. This was the great discovery made by Dalton in the beginning of
the nineteenth century. When he found that each of the known elements,
such as copper or oxygen or carbon, consisted ultimately of atoms, all
the atoms of any one element being alike, he laid the foundation on
which the huge structure of modern chemistry has been raised. The
chemist takes one or more atoms of one element, one or more of another,
and may be of a third or fourth, and he puts them together into a
compound which we call a molecule. The molecule for example of ordinary
salt contains always one atom of chlorine and one of sodium. Chlorine
and sodium are elements, salt is a compound. Six atoms of carbon and six
of hydrogen put together in a certain way make benzene. In the same way
every substance that we meet is capable of analysis, showing ultimately
the molecules as made up, according to a definite plan, of so many atoms
of the various elements. In analytical chemistry molecules are dissected
in order to discover the mode of their building; in synthetic chemistry
the atoms are put together to make a molecule which is already known to
have, or even may be anticipated to have, certain properties. This is
the work of the chemist. Sometimes enormous forces are concerned in this
pulling apart and putting together, witness the terrific power of modern
explosives. But the same kind of handling by the chemist may be devoted
to the delicate construction of a molecule which gives a certain colour
to the dyer's vat and so pleases the eye