I have _here_! And again, if I take this metal [a bar of aluminium[3] about eight times the bulk of the platinum], we find the water will balance that as well as it did the platinum; so that we get, even in the very outset, an example of what we want to understand by the words _forces_ or _powers_. I have spoken of water, and first of all of its property of falling downwards. You know very well how the oceans surround the globe--how they fall round the surface, giving roundness to it, clothing it like a garment; but, besides that, there are other properties of water. _Here_, for instance, is some quick-lime, and if I add some water to it, you will find another power or property in the water.[4] It is now very hot, it is steaming up, and I could perhaps light phosphorus or a lucifer match with it. Now, that could not happen without a _force_ in the water to produce the result; but that force is entirely distinct from its power of falling to the earth. Again, here is another substance [some anhydrous sulphate of copper[5]] which will illustrate another kind of power. [The Lecturer here poured some water over the white sulphate of copper, which immediately became blue, evolving considerable heat at the same time.] Here is the same water, with a substance which heats nearly as much as the lime does; but see how differently. So great indeed is this heat in the case of lime, that it is sufficient sometimes (as you see here) to set wood on fire; and this explains what we have sometimes heard, of barges laden with quick-lime taking fire in the middle of the river, in consequence of this power of heat brought into play by a leakage of the water into the barge. You see how strangely different subjects for our consideration arise, when we come to think over these various matters,--the power of heat evolved by acting upon lime with water, and the power which water has of turning this salt of copper from white to blue.