206 hertz's experiments. 



and magnetic forces were propagated instantaneously througbout 

 space; that if the sun became electrified it would instantaneously 

 begin to induce electricity on the earth; that there would be no delay 

 of eight minutes, such as occurs between a light occurring on the sun 

 and its acting on the earth. Similarly in the case of magnetic actions, 

 they were supposed to be propagated instantaneously throughout 

 space. It was, no doubt, known that it took time for an electric signal 

 to be transmitted along a conducting cable. This is however a very 

 much more complicated i^roblem than the simple one of supposing a 

 body surrounded by a non-conductor to be electrified. Will it or will 

 it not instantaneously act on all conductors in space, and begin to in- 

 duce electrification on them ? As far as was known such actions as 

 this, actions tlirough non-conducting sj^ace, were instantaneous. Such 

 an instantaneous action could not be transmitted by the air. Air can 

 not send on from point to point any effect more rapidly than a molecule 

 of air can moving carry it forward, and that is only a little faster than 

 the velocity of sound; and there was every reason to know that electric 

 induction through air was propagated much more rapidly than that. 

 There was every reason to believe that electric and magnetic forces 

 acted without any material intervention. 



In fact, in these older theories there was no thought of any medium 

 to transmit the actions; it was supposed that electricity acted across 

 any intervening space instantaneously. There is no real difficulty in 

 such a supposition. As far as we know gravitation is just such an 

 action, and as far as was then known there was no experiment that 

 disproved the supposition in the case of electric and magnetic actions. 

 It was known that no experiment had ever been devised that could test 

 whether this action was instantaneous or whether it was propagated 

 at a rate such as that of light. It was known that this action was 

 enormously more rapid than sound, but as light goes about 300,000 

 times as fast as sound there was plenty of spare velocity. Tiiese older 

 theories explained all that was known, and they suiJj)osed nothing as 

 to the existence of an intervening medium. Any theory that assumed 

 that induction was not instantaneous, but that energy having been 

 spent on electrification at one place work would be done at another 

 after some time, as in the case of light generated on the sun not reach- 

 ing the earth for eight minutes, any theory that assumed such a dis- 

 appearance of energy at one place and its re-appearance at another 

 after the lapse of some time must assume some medium in which the 

 energy exists after leaving the one place and before it reaches the 

 other. A theory that only supposes instantaneous action throughout 

 space need not assume the existence of a medium to transmit the action, 

 but any theory that supposes an action to take time in being trans- 

 mitted from one place to another nuist assume the existence of a 

 medium. Now, Maxwell's theory assumed the existence of a medium, 

 and along with that led to the conclusion that electric and magnetic 



