^ MATTER AND FORCE. 77 



a straight line ; but its manifestations are various, and some 

 times so complex as entirely to disguise its elementary con 

 stituents. Its different manifestations have received differ 

 ent names. Here, for example, is a magnet freely suspended. 

 I bring the end of a second magnet near one of the ends 

 of the suspended one attraction is the consequence. I re 

 verse the position of one of the magnets repulsion follows. 

 This display of power is called magnetic force. In the 

 case of gravitation we have a simple attraction, in the case 

 of magnetism attraction and repulsion always go together. 

 Thus magnetism is a double force, or, as it is usually called, 

 a polar force. I present a bit of common iron to the magnet, 

 the iron itself becomes a temporary magnet, and it now 

 possesses the power of attracting other iron. And if sev 

 eral pieces of iron be presented at the same time, not only 

 will the magnet act on them, but they will also act upon 

 each other. 



This leads me to j&if experiment which will give you 

 some idea of how bodies arrange themselves under the 

 operation of a polar force. Underneath this plate of glass 

 is placed a small magnet, and by an optical arrangement 

 comprising a powerful lamp, a magnified image of the mag 

 net is now cast upon the screen before you. I scatter iron 

 filings over the glass. You already notice a certain arrange 

 ment of the particles of iron. Their free action is, how 

 ever, hampered by friction. I therefore tap the glass, 

 liberate the particles, which, as I tap, arrange themselves in 

 these beautiful curves. This experiment is intended to 

 make clear to you how a definite arrangement of particles 

 a kind of incipient structure may result from the oper 

 ation of a polar force. We shall by-and-by see far more 

 wonderful exhibitions of the same structural action when 

 we come to deal with the force of crystallization. 



The magnetic force has here acted upon particles of 

 matter visible to the eye. But, as already stated, there are 



