Before we go another step, I must ask and answer, therefore, a few questions.  Can we not get some little insight into the structure and general mode of developing the leading coal-tar colours which serve as types of whole series?  I will try what can be done with the little knowledge of chemistry we have so far accumulated.  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.  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, you must rob it of some of its hydrogen.  Thus C_{6}H_{5} is a group that would exist only for a moment, since it has a great appetite for H, and we may say