TENACITY OF IRON. 39 



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 

 together, you see perhaps a couple of ounces of 

 finely pounded glass standing as an upright 

 wall is not this attraction most wonderful? 

 Thai 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 parti- 

 cles. [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 par- 



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