SCIENCE. 



183 



worlds, and with all intervening space, by some ethereal 

 atmosphere, which embraces and holds them all. More- 

 over, the enormous velocity with which the vibrations of 

 this atmosphere are propagated proves that it is a substance 

 of the closest continuity, and of the highest tension. The 

 tremors which are imparted to it by luminous bodies rush 

 from particle to panicle at the rate of 186,000 miles in a 

 second of time ; and thus, although it is impalpable, intan- 

 gible, and imponderable, we know that it is a medium infi- 

 nitely more compact than the most solid substance which can 

 be felt and weighed. It is very difficult to conceive this, be- 

 cause the waves or tremors which constitute Light are not 

 recognizable by any sense but one : and the impressions of 

 that sense give us no direct information on the nature of the 

 medium by which those impressions are produced. We 

 cannot see the luminiferous medium except when it is in 

 motion, and not even then, unless that motion be in a cer- 

 tain direction toward ourselves. When this medium is at 

 rest we are in utter darkness, aud so are we also when its 

 movements are rushing past us, but do not touch us. The 

 luminiferous medium is, therefore, in itself, invisible; and 

 its nature can only be arrived at by pure reasoning — reason- 

 ing, of course, founded on observation, but observation of 

 rare phenomena, or of phenomena which can only be seen 

 under those conditions which Man has invented for analyz- 

 ing the operations of his own most glorious sense. And 

 never, perhaps, has Man's inventive genius been more sig- 

 nally displayed than in the long series of investigations which 

 first led up to the conception, and have now lurnished the 

 proof, that Light is nothing but the undulatory movement 

 of a substantial medium. It is very difficult to express in 

 language the ideas upon the nature of that medium which 

 have been built up from the facts of its behavior. It is diffi- 

 cult to do so, because all the words by which we express 

 the properties of Matter refer to its more obvious phenomena 

 —that is to say, to the direct impressions which Matter 

 makes upon the senses. And so, when we have to deal 

 with forms of Matter which do not make any impressions 

 of the same kind — forms of matter which can neither be 

 seen, nor felt, nor handled, which have neither weight, nor 

 taste, nor smell, nor aspect — we can only describe them by 

 the help of analogies as near as we can find. But as regards 

 the qualities of the medium which causes the sensation of 

 Light, the nearest analogies are remote, and what is worse, 

 they compel us to associate ideas which elsewhere are so 

 dissevered as to appear almost exclusive of each other. It is 

 now more than half a century since Dr. Thomas Young 

 astonished and amused the scientific world by declaring of 

 the luminiferous medium that we must conceive of it as 

 finding its way through all Matter as freely as the air moves 

 through a grove of trees. This suggests the idea of an ele- 

 ment of extreme tenuity. But that element cannot be 

 said to be thin in which a wave is transmitted with 

 the enormous velocity of Light. On the contrary, 

 its molecules must be in closest contact with each 

 other when a tremor is carried by them through a thickness 

 of 186,000 miles in a single second. Accordingly, Sir J. 

 Herschel has declared that the luminiferous ether must be 

 conceived of not as an air, nor as a fluid, but rather as a 

 solid — " in this sense at least, that its particles cannot be 

 supposed as capable of interchanging places, cr of bodily 

 transfer to any measurable distance from their own special 

 and assigned localities in the universe." 2 Well may Sir J. 

 Herschel add that "this will go far to realize (in however 

 unexpected a form) the ancient idea of a chrystalline orb." 

 and thus the wonderful result of all investigation is that 

 this earth is in actual rigid contact with the most distant 

 worlds in space — in rigid contact, that is to say, through a 

 medium which touches and envelops all, and which is in- 

 cessantly communicating from one world to another the 

 minutest vibrations it receives. 



The laws, therefore, and the constitution of Light, even 

 more than the law of gravitation, carry up to the highest 

 degree of certainty our conception of the Universe as one ; — 

 one, t*iat is to say, in virtue of the closest mechanical con- 

 nection, and of the prevalence of one universal medium. 



Mor it is now known that this medium is the vehicle 



not only of Light but also of Heat, whilst it has likewise a 

 special power of setting up, or of setting free, the myste- 



"2 Familiar Lectures on Scientific Subjects," p. 285. 



rious action of chemical affinity. The beautiful experiments 

 have become familiar by which these three kinds of ethereal 

 motion can be separated from each other in the solar spectrum, 

 and each of them can be made to exhibit its peculiar effects. 

 With these again the forces of galvanism and electricity 

 have some very intimate connection, which goes far to indi- 

 cate like methods of operation in some prevailing element. 

 Considering how all the forms of Matter, both in the organic 

 and in the inorganic worlds, depend on one or other, jr on 

 all of these — considering how Life itself depends upon 

 them, and how it flickers or expires according as they are 

 present in due proportion — it is impossible not to feel that 

 in this great group of powers, so closely bound up together, 

 we are standing very close indeed to some pervading, if not 

 universal, agency in the mechanism of Nature. 



This close connection of so many various phenomena 

 with different kinds of movement in a single medium is by 

 far the most striking and instructive discovery of modern 

 science. It supplies to some extent a solid physical basis, 

 and one veritable cause, for part, at least, of the general im- 

 pression of unity which the aspects of Nature leave upon 

 the mind. For all work done by the same implement 

 generally carries the mark of that implement, as it were of 

 a tool, upon it. Things made of the same material, what- 

 ever they may be, are sure to be like in those characteristics 

 which result from identical or from similar properties and 

 modes of action. And so far, therefore, it is easy to under- 

 stand the constant and close analogies which prevail in that 

 vast circle of phenomena which are connected with Heat, 

 Light, Electricity, Chemical and Vital Action. 



But although the employment of one and the same agency 

 in the production of a' variety of effects is, no doubt, one 

 cause of the visible unity which prevails in Nature, it is not 

 the only cause. The same close analogies exist where no 

 such identity of agency can be traced. Thus the mode in 

 which the atmosphere carries Sound is closely analogous to 

 to the mode in which the ether carries light. But the ether 

 and the atmosphere are two very different agents, and the 

 similarity of the laws which the undulations of both obey 

 is due to some other and some more general cause of unity 

 than identity of material. This more general cause is to be 

 found, no doubt, in one common law which determines the 

 forms of motion in all Matter, and especially in highly 

 elastic media. 



But, indeed, the mere physical unity which consists in 

 the action of one great vehicle of power, even if this were 

 more universally prevalent than it is known to be, is but 

 the lowest step in the long ascent which carries us up to a 

 unity of a more perfect kind. The means by which some 

 one single implement can be made to work a thousand dif- 

 ferent effects, not only without interference, and without 

 confusion, but with such relations between it and other 

 agents as to lead to complete harmonies of result, are 

 means which point to some unity behind and above the 

 implement itself — that is to say, they point to some unity 

 in the method of its handling, in the management of the 

 impulses which, receiving, it conveys, and in the arrange- 

 ment of the materials on which it operates. 



No illustration can be given of this higher kind of unity 

 which is half so striking as the illustration which is afforded 

 by the astonishing facts now familiar as to the composition 

 of solar light. When we consider that every color in the 

 spectrum represents the motion of a separate wave or rip- 

 ple, and that in addition to the visible series there are 

 other series, one at each end of the luminous rays, which 

 are non-luminous, and therefore invisible — all of which 

 consist of waves equally distinct ; when we consider farther 

 that all these are carried simultaneously with the same 

 speed across millions of miles ; that they are separable, and 

 yet are never separated ; that they are more accurately to- 

 gether, without jostling or confusion, in perfect combina- 

 tion, yet so that each shall be capable of producing its own 

 separate effect — it altogether transcends our faculties of 

 imagination to conceive how movements of such infinite 

 complication can be united in one such perfect order. 



And be it observed that the difficulty of conceiving this is 

 not diminished, but increased, by the fact that these move- 

 ments are propagated in a single medium ; because it is 

 most difficult to conceive how the particles of the medium 

 can be so arranged as to be capable of conveying so many 

 different kinds of motion with equal velocities and at the 



