Now, I have shewn you these things for the purpose of bringing your minds to see that bodies are not merely held together by this power of cohesion, but that they are held together in very curious ways. And suppose I take some things that are held together by this force, and examine them more minutely. I will first take a bit of glass, and if I give it a blow with a hammer, I shall just break it to pieces. You saw how it was in the case of the flint when I broke the piece off; a piece of a similar kind would come off, just as you would expect; and if I were to break it up still more, it would be as you have seen, simply a collection of small particles of no definite shape or form. But supposing I take some other thing, this stone for instance (fig. 15) [taking a piece of mica[12]], and if I hammer this stone, I may batter it a great deal before I can break it up. I may even bend it without breaking it; that is to say, I may bend it in _one particular direction_ without breaking it much, although I feel in my hands that I am doing it some injury. But now, if I take it by the edges, I find that it breaks up into leaf after leaf in a most extraordinary manner. Why should it break up like that? Not because all stones do, or all crystals; for there is some salt (fig. 16)--you know what common salt is[13]: here is a piece of this salt which by natural circumstances has had its particles so brought together that they have been allowed free opportunity of combining or coalescing; and you shall see what happens if I take this piece of salt and break it. It does not break as flint did, or as the mica did, but with a clean sharp angle and exact surfaces, beautiful and glittering as diamonds [breaking it by gentle blows with a hammer]; there is a square prism which I may break up into a square cube. You see these fragments are all square--one side may be longer than the other, but they will only split up so as to form square or oblong pieces with cubical sides. Now, I go a little further, and I find another stone (fig. 17) [Iceland, or calc-spar][14], which I may break in a similar way, but _not_ with the same result. Here is a piece which I have broken off, and you see there are plain surfaces perfectly regular with respect to each other; but it is not cubical--it is what we call a rhomboid. It still breaks in three directions most beautifully and regularly, with polished surfaces, but with _sloping_ sides, not like the salt. Why not? It is very manifest that this is owing to the attraction of the particles, one for the other, being less in the direction in which they give way than in other directions. I have on the table before me a number of little bits of calcareous spar, and I recommend each of you to take a piece home, and then you can take a knife and try to divide it in the direction of any of the surfaces already existing. You will be able to do it at once; but if you try to cut it _across_ the crystals, you cannot--by hammering, you may bruise