SCIENCE. 
rS 3 
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 ofinvestigations which 
first led up to the conception, and have’ now furnished 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 J 
—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, that is to say, in virtue of the closest mechanical con- 
nection, and of the prevalence of one universal medium. 
Moreover, it is now known that this medium is the vehicle 
not only of Light but also of Heat, whilst ithas 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 
J different kinds of motion with equal velocities and at the 
