On Some Processes of Scientific Reasoning. 29 



motion of whose particles constitutes Light. Some 

 consider Light as the vibrations of ether — a substance 

 different from any substance known empirically — while 

 others consider it as the motion of particles of ordinary 

 matter ; others, I believe, hold a combination of these 

 theories, and consider that the vibrations of ether may be 

 communicated to the particles of ordinary matter. 



Acoustics calls for no special consideration ; like all other 

 physical sciences, it employs the method of Ideal construc- 

 tion. Some of its conclusions agree very closely with real 

 phenomena, while others do not accord very accurately with 

 experience. The metempirical conception of Sound gave 

 place, very early in the history of the Science, to the 

 empirical conception of a vibration of the particles of 

 sounding bodies. 



The Ideal constructions employed in Electricity and 

 Magnetism are of much the some character as those 

 employed in the Sciences of Heat and Light. As one example 

 I may mention that of soft iron, an abstraction convenient 

 for expressing certain general laws of electricity, which 

 are not accurately true for real iron. There are also 

 metempirical conceptions of Electricity and Magnetism as 

 the unknown causes of electric and magnetic phenomena. 

 These two are, however, now considered to be one and the 

 same. Electric and magnetic phenomena are intimately con- 

 nected, and, whatever Electricity itself may be, we have no 

 need to assume an additional entity as the cause of mag- 

 netic phenomena. None of the attempts to replace the 

 metempirical conception of Electricity by an empirical 

 one, similar to that to which Heat and Light have been 

 reduced, can at present be considered perfectly satisfactory. 

 The conception of Electricity as an imponderable fluid, 

 although applicable to many problems, presents considerable 

 difficulty. The most plausible theory seems to be that put 

 forward by Mr. Clerk Maxwell, who considers the attraction 

 between two electrified bodies to be caused by some sort of 

 strain of a medium between them, rather than to any 

 affection of the bodies themselves. From the action of 

 magnetism on polarised light, he is led to believe that the 

 ultimate cause of electrical phenomena is " the rotation of 

 very small portions of the medium, each rotating on its own 

 axis." * 



Clerk Maxwell's "Electricity and Magnetism," vol. ii. p. 408. 



