In our earlier
lectures we have learnt that water 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.
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