, CLEAVAGE OF CRYSTALS. 49 



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 frag- 

 ments 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 (fig. 17) [Iceland, or calc-spar] ( l4 ), 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 

 E 



