THE CONQUEST OF NATURE 



tides of the positive fluid are mutually repellent, as also 

 are the particles of the negative fluid, but, on the other 

 hand, positive particles attract and are attracted by 

 negative particles. We need not further elaborate the 

 details of this two-fluid theory, because the best modern 

 opinion considers it less satisfactory than Franklin's 

 one-fluid theory. Meantime, it will be observed that 

 the two theories have much in common; in particular 

 they agree in the essential feature of postulating an in- 

 visible something which is not matter, and which has 

 strange properties of attraction and repulsion. 



These properties of attraction and repulsion con- 

 stituted in the early day the only known manifestations 

 of electricity; and the same properties continue to hold 

 an important place in modern studies of the subject. 

 Electricity is so named simply because amber the 

 Latin electrum was the substance which, in the expe- 

 rience of the ancients, showed most conspicuously the 

 strange property of attracting small bodies after being 

 rubbed. Modern methods of developing electricity 

 are extremely diversified, and most of them are quite 

 unsuggestive of the rubbing of amber; yet nearly all 

 the varied manifestations of electricity are reducible, in 

 the last analysis, to attractions and repulsions among 

 the particles of matter. 



As to the alleged immaterial fluids which, according 

 to the theories just mentioned, make up the real sub- 

 stance of electricity, it was perfectly natural that they 

 should be invented by the physicists of the elder day. 

 All the conceptions of the human mind are developed 

 through contact with the material world; and it is 



