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 have not _destroyed_ the power of chemical affinity, but modified it in a wonderful and beautiful manner. Here are some pieces of zinc covered with mercury, exactly in the same way as the zinc in that retort is covered; and if I put this plate into sulphuric acid, I get no gas--but this most extraordinary thing occurs, that if I introduce along with the zinc another metal which is _not_ so combustible, then I reproduce all the action. I am now going to put to the amalgamated zinc in this retort some portions of copper wire (copper not being so combustible a metal as the zinc), and observe how I get hydrogen again. As in the first instance, there the bubbles are coming over through the pneumatic trough, and ascending faster and faster in the jar. The zinc now is acting by reason of its contact with the copper.