TRANSACTIOiNS OF SECTION A. 559 



general estimation. In fact, Rumford and Davy's experiments on heat, and 

 Young and Fiesnell's experiments on light had really decided these questions long 

 before the erroneous views were finally abandoned. I hope that science will not 

 be so slow in accepting the results of experiment in respect ofelectro-magnetism 

 as it was in the case of light and heat. Rowland's experiment proving an 

 electro-magnetic action between electric charges depending on their absolute and 

 not relative velocities has already proved the existence of a medium relative to 

 which the motion must take place, but the connection is rather metaphysical and 

 is too indirect to attract general attention. The importance of the striking ex- 

 periments was that they put the language of the wrong hypothesis out of fashion. 

 Elementary text-books that halted between two opinions, and, after the manner 

 of text-books, leant towards that enunciated in preceding text-books, had all 

 perforce to give prominence to the true theory, and the whole rising generation 

 beo-an their researches from a firm and true standpoint. I anticipate the same 

 results to follow Hertz's experimental demonstiation of a medium by which 

 electro-magnetic actions are produced. Text-books which have gradually been 

 invoking lines of force, in some re.spects to the aid of learners and in others to 

 their bewilderment, will now fearlessly discourse of the stresses in the ether that 

 cause electric and magnetic force, the younger generation will see clearly in 

 electro-magnetic phenomena the working of the all-pervading ether, and this will 

 give them a firm and true standpoint for further advances. 



And now I want to spend a short time in explaining- to you how the question 

 has been decided. An illustrative example may make the question itself clearer, 

 and so lead you to understand the answer better. In colloquial language we say 

 that balloons, hot air, &e., rise because they are light. In old times this was 

 stated more explicitly, and therefore much more clearly. It was said that they 

 possessed a quality called ' levity.' ' Levity ' was opposed to ' heaviness.' Heavi- 

 ness made things tend downwards, levity made things tend upwards. It wa,s a, 

 sort of action at a distance. At least it would have required such a hypothesis if 

 it had survived until it was known that heaviness was due to the action of the 

 earth. I expect levity would have been attributed to the direct action of heaven. 

 It was comparatively recently in the history of mankind that the rising of hot air, 

 flames, &c., was attributed to the air. Everybody knew that there was air, but it 

 was not supposed that the upward motion of flames was due to it. We now 

 know that this and the rising of balloons are due to the difference, of pressure at 

 different levels in the air. In a similar way we have long known that there is 

 an ether, an all-pervading medium, occupying all known space. Its existence is 

 a necessary consequence of the undulatory theory of light. People who think a little 

 but not much sometimes ask me, ' Why do you believe in the ether ? What's the 

 good of it ? ' ] ask them, ' What becomes of light for the eight minutes after it 

 has left the sun and before it reaches the earth ? ' When they consider that they 

 observe how necessary the ether is. If light took no time to come from the sun 

 there would be no need of the ether. That it is a vibratory phenomenon, that it is 

 affected by matter it acts through — these could be explained by action at a distance 

 very well. The phenomena of interference would, however, require such compli- 

 cated and curious laws of action at a distance as practically to put such a hypothesis 

 out of court, or else be purely mathematical expressions for wave-propagation. In 

 fact, anything except propagation in time is explicable by action at a distance. It 

 is the same m the case of electro-magnetic actions. There were two hypotheses 

 as to the causes of electro-magnetic actions. One attributed electric attraction 

 to a property of a thing called electricity, to attract at a distance, the other 

 attributed it to a pull exerted by . means of the ether, somewliat in the 

 way that air pushes balloons up. We do not know what the structure of 

 the ether is by means of which it can pull, but neither do we know what the structure 

 of a piece of indiarubber is by means of which it can pull, and we might as well 

 io-nore the indiarubber, though we know a lot about the laws of its action, because 

 we do not know its structure, as to ignore the ether because we do not know its 

 structure. Anyway, what was wanted was an experiment to decide between the 

 hypothesis of direct action at a distance and of action by means of a medium. 



