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 and break it up—but you can only divide it into these beautiful little rhomboids.
Now I want you to understand a little more how this is—and for this purpose I am going to use the electric light again. You see, we
