30 FARADAY 



mediately 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 together in very curious ways. And suppose I take 

 some things that are held together by this force, and ex- 

 amine 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 (Fie. 15) [taking a piece of mica( u )], and if I 



FIG. 15 FIG. 1 6 FIG. 17 



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 ex- 

 traordinary manner. Why should it break up like that? 

 Not because all stones do, or all crystals; for there is some 

 salt (Fie. 16) you know what common salt is(") ; here 

 is a piece of this salt, which by natural circumstances has 

 had its particles so brought together that they have been 



n Mica. A silicate of alumina and magnesia. It has a bright metallic 

 lustre; hence its name, from mico, to shine. 



18 Common salt or chloride of sodium crystallizes in the form of solid 

 cubes, which, aggregated together, form a mass, which may be broken up 

 into the separate cubes. 



