20 Experimental Philosophy. [Lecture 2. 



in the cavities of a sponge. These experiments 

 will succeed equally in a space which is void of 

 air (such as the vacuum made by an air-pump) 

 as in the open air ; so that the effect cannot pro- 

 ceed from any pressure of the atmosphere, but 

 must be caused by attraction alone. 



Some bodies, however, in certain circumstances, 

 appear to possess a power the reverse of attrac- 

 tion; and this is called, in philosophical lan- 

 guage, repulsion. The repulsion of electricity 

 and of magnetism will be evinced when we come 

 to treat of those subjects ; and the -same feathers, 

 which were at first attracted by the excited or 

 electrified body, will be repelled or driven from 

 it; the magnet will repel at one end the same 

 bodies which it attracts at the other. Upon simi- 

 lar principles, if a small piece of iron is laid on a 

 bason of mercury, it will not sink, but will be 

 supported by it, while the mercury will be de- 

 pressed on each side ; and thus it is that a small 

 needle will swim upon the surface of water. 



