Another wonderful thing about this mode of changing the condition of the water is this--that we are able to get the separate parts of which it is composed, at a distance the one from the other, and to examine them, and see what they are like, and how many of them there are; and for this purpose I have here some more water in a slightly different apparatus to the former one (fig. 25), and if I place this in connection with the wires of the battery (at A B), I shall get a similar decomposition of the water at the two platinum plates. Now, I will put this little tube (O) over there, and that will collect the gas together that comes from this side (A), and this tube (H) will collect the gas that comes from the other side (B); and I think we shall soon be able to see a difference. In this apparatus the wires are a good way apart from each other, and it now seems that _each_ of them is capable of drawing off particles from the water and sending them off, and you see that one set of particles (H) is coming off twice as fast as those collected in the other tube (O). Something is coming out of the water _there_ (at H) which burns [setting fire to the gas]; but what comes out of the water _here_ (at O), although it will not burn, will support combustion very vigorously. [The Lecturer here placed a match with a glowing tip in the gas, when it immediately rekindled.] [Illustration: Fig. 25.] Here, then, we have two things, neither of them being water alone, but which we get out of the water. Water is therefore composed of two substances different to itself, which appear at separate places when it is made to submit to the force which I have in these wires; and if I take an inverted tube of water and collect this gas (H), you will see that it is by no means the same as the one we collected in the former apparatus (fig. 24). That exploded with a loud noise when it was lighted, but this will burn quite noiselessly--it is called _hydrogen_; and the other we call _oxygen_--that gas which so beautifully brightens up all combustion, but does not burn of itself. So now we see that water consists of two kinds of particles attracting each other in a very different manner to the attraction of gravitation or cohesion; and this new attraction we call _chemical affinity_, or the force of chemical action between different bodies. We are now no longer concerned with the attraction of iron for iron, water for water, wood for wood, or like bodies for each other, as we were when dealing with the force of cohesion: we are dealing with another kind of attraction,--the attraction between particles of a _different_ nature one to the other. Chemical affinity depends entirely upon the energy with which particles of _different_ kinds attract each other. Oxygen and hydrogen are particles of different kinds, and it is their attraction to each other which makes them chemically combine and produce water.