COHESION IN CRYSTALLINE BODIES. 47 



so make the least scratch upon the inside, the 

 whole bottle will break to pieces, it cannot 

 hold together. [The Lecturer here dropped a 

 small fragment of rock crystal into one of these 

 glass vessels, when the bottom immediately 

 came out and fell upon the plate.] There! it 

 goes through, just as it would through a sieve. 



Now, I have shown 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 to- 

 gether 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 col- 

 lection 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 



