to it, when a sudden flash shewed how large a proportion of gunpowder had escaped combustion when falling through the flame of alcohol.]
These are all cases of chemical affinity; and I shew them to make you understand that we are about to enter upon the consideration of a strange kind of chemical affinity, and then to see how far we are enabled to convert this force of affinity into electricity or magnetism, or any other of the forces which we have discussed. Here is some zinc (I keep to the metal zinc, as it is very useful for our purpose), and I can produce hydrogen gas by putting the zinc and sulphuric acid together, as they are in that retort. There you see the mixture which gives us hydrogen—the zinc is pulling the water to pieces and setting free hydrogen gas. Now, we have learned by experience that, if a little mercury is spread over that zinc, it does not take away its power of decomposing the water, but modifies it most curiously. See how that mixture is now boiling; but when I add a little mercury to it, the gas ceases to come off. We have now scarcely a bubble of hydrogen set free, so that the action is suspended for the time. We
