COHESION SI 



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, beauti- 

 ful 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 farther, and I find another 

 stone (Fie. 17) [Iceland or calc-spar] ( u ) 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 direc- 

 tions. 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 al- 

 ready existing. You will be able to do it at once ; but if you 

 try to cut it across the crystals, you can not; 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 can not look into the middle of a body 

 like this piece of glass. We perceive the outside form and 

 the inside form, and we look through it, but we can not well 

 find out how these forms become so, and I want you, there- 

 fore, to take a lesson in the way in which we use a ray of light 

 for the purpose of seeing what is in the interior of bodies. 

 Light is a thing which is, so to say, attracted by every sub- 

 stance that gravitates (and we do not know anything that 



18 Iceland or calc spar. Native carbonate of lime in its primitive crystal- 

 line form. 



