er is a compound of hydrogen and oxygen,
and in its compound atom or molecule we have two atoms of hydrogen
combined with one of oxygen, symbolised as H_{2}O. We also learnt that
ammonia, or spirits of hartshorn, is a compound of hydrogen with
nitrogen, three atoms of hydrogen being combined with one of nitrogen,
thus, NH_{3}. An example of a hydrocarbon or compound of carbon and
hydrogen, is marsh gas (methane) or firedamp, CH_{4}. Nitric acid, or
_aqua fortis_, is a compound of nitrogen, oxygen, and hydrogen, one atom
of the first to three of the second and one of the third--NO_{3}H. But
this nitric acid question forces me on to a further statement, namely,
we have in this formula or symbol, NO_{3}H, a twofold idea--first, that
of the compound as a whole, an acid; and secondly, that it is formed
from a substance without acid properties by the addition of water,
H_{2}O, or, if we like, HOH. This substance contains the root or radical
of the nitric acid, and is NO_{2}, which has the power of replacing one
of the hydrogen atoms, or H, of water, and so we get, instead of HOH,
NO_{2}OH, which is nitric acid. This is chemical replacement, and on
such replacement depends our powers of building up not only colours, but
many other useful and ornamental chemical structures. You have all heard
the old-fashioned statement that "Nature abhors a vacuum." We had a very
practical example of this when in our first lecture on water I brought
an electric spark in contact with a mixture of free oxygen and hydrogen
in a glass bulb. These gases at once united, three volumes of them
condensing to two volumes, and these again to a minute particle of
liquid water. A vacuum was left in that delicate glass bulb whilst the
pressure of the atmosphere was crushing with a force of 15 lb. on the
square inch on the outside of the bulb, and thus a violent crash was the
result of Nature's abhorrence. There is such a kind of thing, though,
and of a more subtle sort, which we might term a chemical vacuum, and it
is the result of what we call chemical valency, which again might be
defined as the specific chemical appetite of each substance.

Let us now take the case of the production of an aniline colour, and let
us try to discover what aniline is, and how formed. I pointed to benzene
or benzol in the table as a hydrocarbon, C_{6}H_{6}, which forms a
principal colour-producing constituent of coal-tar. If you desire to
produce chemical appetite in benzene,