I must now come to a few circumstances which require careful consideration. We have already examined one of the effects of this chemical affinity; but to make the matter more clear we must point out some others. And here are two salts dissolved in water[19]. They are both colourless solutions, and in these glasses you cannot see any difference between them. But if I mix them, I shall have chemical attraction take place. I will pour the two together into this glass, and you will at once see, I have no doubt, a certain amount of change. Look, they are already becoming milky, but they are sluggish in their action--not quick as the others were--for we have endless varieties of rapidity in chemical action. Now, if I mix them together, and stir them, so as to bring them properly together, you will soon see what a different result is produced. As I mix them, they get thicker and thicker, and you see the liquid is hardening and stiffening, and before long I shall have it quite hard; and before the end of the lecture it will be a solid stone--a wet stone, no doubt, but more or less solid--in consequence of the chemical affinity. Is not this changing two liquids into a solid body a wonderful manifestation of chemical affinity? There is another remarkable circumstance in chemical affinity, which is, that it is capable of either waiting or acting at once. And this is very singular, because we know of nothing of the kind in the forces either of gravitation or cohesion. For instance, here are some oxygen particles, and here is a lump of carbon particles. I am going to put the carbon particles into the oxygen; they _can_ act, but they _do_ not--they are just like this unlighted candle. It stands here quietly on the table, waiting until we want to light it. But it is not so in this other case. Here is a substance, gaseous like the oxygen, and if I put these particles of metal into it, the two combine at once. The copper and the chlorine unite by their power of chemical affinity, and produce a body entirely unlike either of the substances used. And in this other case, it is not that there is any deficiency of affinity between the carbon and oxygen; for the moment I choose to put them in a condition to exert their affinity, you will see the difference. [The piece of charcoal was ignited, and introduced into the jar of oxygen, when the combustion proceeded with vivid scintillations.]