154 THE PRINCIPLES OF SCIENCE. 



the meaning of the word. Thus if we are to speak of 

 what constitutes electricity, we must search for the 

 nearest analogy, and as electricity is chiefly characterised 

 by the rapidity and facility of its movements, the notion 

 of a fluid of a very subtle character presented itself as 

 most appropriate. There is the single fluid and the 

 double fluid theory of electricity, and a great deal of 

 discussion has been uselessly spent upon them. The fact 

 is that if these theories be understood as more than con- 

 venient modes of describing the phenomena, they are 

 grossly invalid. The analogy extends only to the rapidity 

 of motion, and the fact that a phenomenon occurs suc- 

 cessively at different points of the body. The so-called 

 electric fluid adds nothing to the weight of the conductor, 

 and to suppose that it really consists of particles of matter 

 would be even more absurd than to reinstate the Corpus- 

 cular theory of light. An infinitely closer analogy exists 

 between electricity and light undulations, which are about 

 equally rapid in propagation ; and while we shall probably 

 continue for a long time to talk of the electric fluid, there 

 can be no doubt that this expression merely represents 

 some phase of molecular motion, some wave of disturbance 

 propagating itself at one time through material con- 

 ductors, at another time through the ethereal basis of 

 light. The invalidity of these fluid theories is moreover 

 shown in the fact that they have not led to the invention 

 of a single new experiment. When we speak of heat as 

 flowing from one body to another, we likewise use a 

 descriptive hypothesis merely ; for Lambert's theory of 

 the fluid motion of heat is no better than the Corpuscular 

 theory of light. 



Among these merely descriptive hypotheses I should 

 be inclined to place Newton's theory of Fits of Easy 

 Reflection and Refraction. That theory has been since ex- 

 ploded by actual discordance with fact, but even when 



