I now shall try to help your attention to what I may say by directing, to-day, our thoughts to one kind of power. You see what I mean by the term _matter_--any of these things that I can lay hold of with the hand, or in a bag (for I may take hold of the air by enclosing it in a bag)--they are all portions of matter with which we have to deal at present, generally or particularly, as I may require to illustrate my subject. Here is the sort of matter which we call _water_,--it is _there_ ice [pointing to a block of ice upon the table], _there_ water [pointing to the water boiling in a flask], _here_ vapour--you see it issuing out from the top [of the flask]. Do not suppose that that ice and that water are two entirely different things, or that the steam rising in bubbles and ascending in vapour _there_ is absolutely different from the fluid water. It may be different in some particulars, having reference to the _amounts_ of power which it contains; but it is the same, nevertheless, as the great ocean of water around our globe, and I employ it here for the sake of illustration, because if we look into it we shall find that it supplies us with examples of all the powers to which I shall have to refer. For instance, here is water--it is heavy; but let us examine it with regard to the _amount_ of its heaviness, or its gravity. I have before me a little glass vessel and scales [nearly equipoised scales, one of which contained a half-pint glass vessel], and the glass vessel is at present the lighter of the two; but if I now take some water and pour it in, you see that that side of the scales immediately goes down; that shews you (using common language, which I will not suppose for the present you have hitherto applied very strictly) that it is _heavy_: and if I put this additional weight into the opposite scale, I should not wonder if this vessel would hold water enough to weigh _it_ down. [The Lecturer poured more water into the jar, which again went down.] Why do I hold the bottle _above_ the vessel to pour the water into it? You will say, because experience has taught me that it is necessary. I do it for a better reason--because it is a law of nature that the water should fall towards the earth, and therefore the very means which I use to cause the water to enter the vessel are those which will carry the whole body of water down. That power is what we call _gravity_, and you see _there_ [pointing to the scales] a good deal of water gravitating towards the earth. Now _here_ [exhibiting a small piece of platinum[2]] is another thing which gravitates towards the earth as much as the whole of that water. See what a little there is of it--_that_ little thing is heavier than so much water [placing the metal in opposite scales to the water]. What a wonderful thing it is to see that it requires so much water as _that_ [a half-pint vessel full] to fall towards the earth, compared with the little mass of substance