COHESION 25 



the place of one. I will show you an experiment to prove 

 that this attraction does still exist in those particles; for 

 here is a piece of glass (for what was true of the flint and 

 the bar of iron is true of the piece of glass, and is true of 

 every other solid they are all held together in the lump 

 by the attraction between their parts), and I can show you 

 the attraction between its separate particles; for if I take 

 these portions of glass which I have reduced to very fine 

 powder, you see that I can actually build them up into a 

 solid wall by pressure between two flat surfaces. The 

 power which I thus have of building up this wall is due to 

 the attraction of the particles forming, as it were, the cement 

 which holds them together; and so in this case, where I 

 have taken no very great pains to bring the particles to- 

 gether, you see perhaps a couple of ounces of finely pounded 

 glass standing as an upright wall: is not this attraction 

 most wonderful? That bar of iron one inch square has 

 such power of attraction in its particles giving to it such 

 strength that it will hold up twenty tons' weight before 

 the little set of particles in the small space equal to one 

 division across which it can be pulled apart will separate. 

 In this manner suspension bridges and chains are held 

 together by the attraction of their particles, and I am going 

 to make an experiment which will show how strong is this 

 attraction of the particles. [The lecturer here placed his 

 foot on a loop of wire fastened to a support above, and 

 swung with his whole weight resting upon it for some 

 moments.] You see, while hanging here, all my weight is 

 supported by these little particles of the wire, just as in 

 pantomimes they sometimes suspend gentlemen and damsels. 

 How can we make this attraction of the particles a little 

 more simple? There are many things which, if brought 

 together properly, will show this attraction. Here is a 

 boy's experiment (and I like a boy's experiment). Get a 

 tobacco-pipe, fill it with lead, melt it, and then pour it out 

 upon a stone, and thus get a clean piece of lead (this is a 

 better plan than scraping it; scraping alters the condition 

 of the surface of the lead). I have here some pieces of 

 lead which I melted this morning for the sake of making 

 them clean. Now these pieces of lead hang together by the 



