Sept. 6, 1888] 



NATURE 



447 



It has been decided in favour of the hypothesis that these actions 

 take place by means of an intervening medium. Although there 

 is nothing new about the question, and although most workers 

 at it have long been practically satisfied that electro-magnetic 

 actions are due to an intervening medium, I have thought it 

 worth while to try and explain to others who may not have 

 considered the problem, what the problem is and how it has 

 been solved. A Presidential address such as this is not for 

 specialists — it is for the whole Section ; and I would not have 

 thought of dealing with this subject, only that its immediate 

 consequences reach to all the bounds of physical science, and 

 are of interest to all its students. 



We are all familiar with this, that when we do not know all 

 about something there are generally a variety of explanations of 

 what we do know. Whether there is anything of which there 

 are in reality a variety of explanations is a deep question, 

 which some have connected with the freedom of the will, but 

 which I am not concerned with here. A notable example of 

 the possibility of a variety of explanations for us is recorded in 

 connection with an incident said to have occurred in the neigh- 

 bouring town of Clifton, where a remarkable meteorological 

 phenomenon, as it appeared to an observing scientist, was 

 explained by others as a bull's-eye lantern in the hands of Mr. 

 Pickwick. Another kind of example is the old explanation of 

 water rising in a pump, that "Nature abhors a vacuum," as 

 compared with the modern one. Nowadays, when we know as 

 little about anything, we say, " It is the property of electricity 

 to attract." This is really little or no advance on the old form, 

 and is merely a way of stating that we know a fact but not its 

 explanation. There are plenty of cases still where a variety of 

 explanations are possible. For example, we know of no experi- 

 ment it in crucis to decide whether the people I see around me are 

 conscious or are only automata. There are other questions 

 which have existed, but which have been experimentally de- 

 cided. The most celebrated of these are the questions between 

 the caloric and kinetic theories of heat, and between the emis- 

 sion and undulatory theories of light. The classical experiments 

 by which the case has been decided in favour of the kinetic 

 theory of heat and the undulatory theory of light are some of 

 the most important experiments that have ever been performed. 

 When it was shown that heat disappeared whenever work ap- 

 peared, and vice versd, and so the caloric hypothesis was dis- 

 proved ; when it was shown that light was propagated more 

 slowly in a dense medium than in a rare, the sciences of light 

 and heat were revolutionized. Not but that most who 

 studied the subjec: had given their adhesion to the true 

 theory before it was finally decided in general es:imation. 

 In fact, Rumford's and Davy's experiments on heat, and 

 Young and Fresnel'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 of 

 electro-magnetism as it was in the case of light and heat, and 

 that no Carnot will throw back science by giving plausible 

 explanations on a wrong hypothesis. 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 meta- 

 physical, and is too indirect to attract general attention. The 

 importance of these striking experiments 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 pre- 

 ceding text-books, had all perforce to give prominence to the 

 true theory, and the whole rising generation began their 

 researches from a firm and true stand-point. I anticipate the 

 same results to follow Hertz's experimental demonstration of a 

 medium by which electromagnetic actions are produced. Text- 

 books which have gradually been invoking lines of force, in some 

 respects 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 

 stand-point 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 under- 

 stand the answer better. In colloquial language we say that 



balloons, hot air, Sec, 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." Heaviness made 

 things tend downwards, levity made things tend upwards. It 

 was a sort of action at a distance. At least, it would have 

 required such an 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. Every- 

 body 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 

 anrl 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?" I 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 complicated and curious laws of action at a distance as 

 practically to put such an hypothesis out of court, or else be purely 

 mathematical expressions for wave propagation. In fact, any- 

 thing except propagation in time is explicable by action at a 

 distance. It is the same in 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 at- 

 tributed it to a pull exerted by means of the ether, somewhat 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 india-rubber 

 is by means of which it can pull ; and we might as well ignore 

 the india-rubber, 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. At the time that Clerk Maxwell delivered his 

 address no experiment was known that could decide between 

 the two hypotheses. Specific inductive capacity, the action 

 of intervening matter, the delay in telegraphing, the time 

 propagation of electro-magnetic actions by means of conducting 

 material — these were known, but he knew that they could be 

 explained by means of action at a distance, and had been so 

 explained. Waves in a conductor do not necessarily postulate 

 action through a medium such as the ether. When we are 

 dealing with a conductor and a thing called electricity running 

 over its surface, we are, of course, postulating a medium on or 

 in the conductor, but not outside it, which is the special point at 

 issue. Clerk Maxwell believed that just as the same air that 

 transmits sound is able by differences of pressure — i.e. by means 

 of its energy per unit volume — to move bodies immersed in it, so 

 the same ether that transmits light causes electrified bodies to 

 move by means of its energy per unit volume. He believed this, 

 but there was no experiment known then to decide between this 

 hypothesis and that of direct action at a distance. As I have 

 endeavoured to impress upon you, no experimentum crucis 

 between the hypotheses is possible except an experiment proving 

 propagation in time, either directly, or indirectly by an experi- 

 ment exhibiting phenomena like those of the interference of 

 light. A theorist may speak of propagation of actions in time 

 without talking of a medium. This is ail very well in mathe- 

 matical formulae, but, as in the case of light we must 

 consider what becomes of it after it has left the sun and 

 before it reaches the earth, so every hypothe-is assuming 

 action in time really postulates a medium whether we talk 

 about it or not. There are some difficulties surrounding the 

 complete interpretation of some of Hertz's experiments. The 

 conditions are complicated, but I confidently expect that they 

 will lead to a decision on most of the outstanding questions on 

 the theory of electro-magnetic action. However, there is no 

 doubt that he has observed the interference of electro-magnetic 



