I must now shew you a little more at large what chemical affinity is. I can prepare these gases from other substances, as well as from water; and we will now prepare some oxygen. Here is another substance which contains oxygen--chlorate of potash. I will put some of it into this glass retort, and Mr. Anderson will apply heat to it. We have here different jars filled with water; and when, by the application of heat, the chlorate of potash is decomposed, we will displace the water, and fill the jars with gas. Now, when water is opened out in this way by means of the battery--which adds nothing to it materially, which takes nothing from it materially (I mean no _matter_; I am not speaking of _force_), which adds no _matter_ to the water--it is changed in this way: the gas which you saw burning a little while ago, called _hydrogen_, is evolved in large quantity, and the other gas, _oxygen_, is evolved in only half the quantity; so that these two areas represent water, and these are always the proportions between the two gases. +-----------+-----------+ | | | | | 8 | | | | Oxygen, 88.9 | | Oxygen. | | 1 | | Hydrogen, 11.1 | +-----------+ ---- | Hydrogen. | Water, 100.0 | | 9 | | | | | | +-----------+ But oxygen is sixteen times the weight of the other--eight times as heavy as the particles of hydrogen in the water; and you therefore know that water is composed of nine parts by weight--one of hydrogen and eight of oxygen; thus:-- Hydrogen, 46.2 cubic inches, = 1 grain. Oxygen, 23.1 cubic inches, = 8 grains. ---- -- Water (_steam_), 69.3 cubic inches, = 9 grains.