CHAP, i.] ELECTRICITY AND MAGNETISM. 61 



necessarily the material particles of the dielectric sub- 

 stance that are thus affected : hence it is believed that 

 electrical phenomena are due to stresses and strains in 

 the so-called " ether," the thin medium pervading all 

 matter and all space, whose highly elastic constitution 

 enables it to convey to us the vibrations of light though 

 it is millions of times less dense than the air. As the 

 particles of bodies are intimately surrounded by "ether," 

 the strains of the " ether " are also communicated to 

 the particles of bodies, and they too suffer a strain. 

 The glass between the two coatings of tinfoil in the 

 Leyden jar is actually strained or squeezed between the 

 attracting charges of electricity. When an insulated 

 charged ball is hung up in a room an equal amount of 

 the opposite kind of electricity is attracted to the inside 

 of the walls, and the air between the ball and the walls 

 is strained (electrically) like the glass of- the Leyden 

 jar. If a Leyden jar is made of thin glass it may give 

 way under the strain ; and when a Leyden jar is dis- 

 charged the layer of air between the knob of the jar and 

 the knob of the discharging tongs is more and more 

 strained as they are approached towards one another, 

 till at last the strain becomes too great, and the layer of 

 air gives way, and is "perforated" by the spark that 

 discharges itself across. The existence of such strains 

 enables us to understand the residual charge of Leyden 

 jars in which the glass does not recover itself all at once, 

 by reason of its viscosity, from the strain to which it 

 has been subjected. This hypothesis, that electric 

 force acts across space in consequence of the 

 transmission of stresses and strains in the 

 medium -with which space is filled, is now entirely 

 superseding the old theory of action -at -a- distance, which 

 was logically unthinkable, and whick, moreover, failed to 

 account for the facts of observatitffc^ 



