THE NATURE OF ELECTRICITY. 31 



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 This much we certainly know, that they all are forms 



of energy and that they radiate from the centers where 

 they are generated. Hence the term radiant energy 

 is equally applicable to each. 



THE WAVE THEORY. It has been assumed that a 

 subtle medium, termed ether, pervades all space ; that 

 it is so attenuated that it can insinuate itself between 

 the grosser molecules of material bodies ; so that solids 

 of the finest and closest texture, as well as liquids and 

 gases, are pervaded by it; and that light, and probably 

 electricity, are due to waves or undulations of this 

 ether. The evidence of the existence of such a medium 

 is almost wholly negative, and, like all negative evi- 

 dence, unsatisfactory. The assumption presupposes the 

 necessity of its existence. 



It has been stated that energy is a universal property 

 of matter, and the converse may be accepted, that 

 energy cannot exist without matter. And since light, 

 coming from the sun, must traverse the in'terplane- 

 tary spaces, there must be matter there ; else we shall 

 be compelled to admit that energy can exist without 

 matter, which is contrary to all our experience. 



The earth is surrounded by an atmosphere, to the 

 limits of which we cannot penetrate. In 1822, Dr. 

 Wollaston made a careful mathematical calculation, as 

 the result of which he claimed to have demonstrated 

 that the earth's atmosphere must have limits, beyond 

 which it cannot exist. And this apparent demonstra- 

 tion was accepted as authority, and remained unchal- 

 lenged for half a century. Meantime the wave theory 

 of light became prominent, and a medium being one of 

 its fundamental principles, the existence of the ether 

 was assumed, and is now generally accepted. 



