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 this appetite would go the length of at once absorbing either one atom of H (hydrogen) or of some similar substance or group having a similar appetite. Suppose, now, I place some benzene, C_{6}H_{6}, in a flask, and add some nitric acid, which, as we said, is NO_{2}OH. On warming the mixture we may say a tendency springs up in that OH of the nitric acid to effect union with an H of the C_{6}H_{6} (benzene) to form HOH (water), when an appetite is at once left to the remainder, C_{6}H_{5}--on the one hand, and the NO_{2}--on the other, satisfied by immediate union of these residues to form a substance C_{6}H_{6}NO_{2}, nitro-benzene or "essence of mirbane," smelling like bitter almonds. This is the first step in the formation of aniline. I think I have told you that if we treat zinc scraps with water and vitriol, or water with potassium, we can rob that water of its oxygen and set free the hydrogen. It is, however, a singular fact that if we liberate a quantity of fresh hydrogen amongst our nitrobenzene C_{6}H_{5}NO_{2}, that hydrogen tends to combine, or evinces an ungovernable appetite for the O_{2} of that NO_{2} group, the tendency being again to form water H_{2}O. This, of course, leaves the residual C_{6}H_{5}N: group with an appetite, and only the excess of hydrogen present to satisfy it. Accordingly hydrogen is taken up, and we get C_{6}H_{5}NH_{2} formed, which is aniline. I told you that ammonia is NH_{3}, and now in aniline we find an ammonia derivative, one atom of hydrogen (H) being replaced by the group C_{6}H_{5}. I will now describe the method of preparation of a small quantity of aniline, in order to illustrate what I have tried to explain in theory. Benzene from coal-tar is warmed with nitric acid in a flask. A strong action sets in, and on adding water, the nitrobenzene settles down as a heavy oil, and the acid water can be decanted off. After washing by decantation with water once or twice, and shaking with some powdered marble to neutralise excess of acid, the nitrobenzene is brought into contact with fresh hydrogen gas by placing amongst it, instead of zinc, some tin, and instead of vitriol, some hydrochloric acid (spirits of salt). To show you that aniline is formed, I will now produce a violet colour with it, which only aniline will give. This violet colour is produced by adding a very small quantity of the aniline, together with some bleaching powder, to a mixture of chalk and water, the chalk being added for the purpose of destroying acidity.