~ 
Fan. 25, 1883] 
NATURE 
395 
Remember then that whenever we see a thing being moved we 
must look for the rope; it may be visible or it may be invisible, 
but unless there is either “ pushee” or ‘‘ pullee” there can be no 
action. And if you further consider a pull it revolves itself intoa 
push ; to pulla thing towards you, you have to put your finger behind 
it and push ; a horse is said to pull a cart, but he is really push- 
ing at the collar ; an engine pushes a truck by means of a hook 
and eye; and soon. There is still the further very important 
and difficult question as to why the parts hang together, and why 
when you push one part the rest follows. : 
Cohesion is a very striking fact, and an explanation of it is 
much to be desired ; I shall have a little more to say about it 
later, but at present we have nothing more than an indication of 
the direction in which an explanation seems possible. We 
cannot speak distinctly about those actions which are as yet 
mysterious to us, but concerning those which are conparatively 
simple and intelligible we may make this general statement :— 
The only way of acting on a body directly is to push it behind. 
There must be contact between bodies before they can 
directly act on each other; and if they are not in contact with 
each other and yet act, they must both be in contact with some 
third body which is the medium of communication, the rope. 
Consider now for an instant the most complex case, the action 
of one animate body on another not touching it, To call the 
attention of a dog, for instance, there are several methods: one 
plan is to prod him with a stick, another is to heave a stone at 
him, a third is to whistle or call, while a fourth is to beckon 
him by gesture, or, what is essentially the same process, to flash 
sunlight into his eye with a mirror. In the first two of these 
methods the media of communication are perfectly obvious—the 
stick and the stone—in the third, the whistle, the medium is 
not so obvious, and this case might easily seem to a savage like 
action at a distance, but we know of course that it is the air, 
and that if the air between be taken away, all communication by 
sound is interrupted. But the fourth or optical method is not so 
interrupted ; the dog can see through a vacuum perfectly well, 
though he cannot hear through it ; but what the medium now is 
which conveys the impression is not so well known. ‘The sun’s 
light is conveyed to the earth by such a medium as this across 
the emptiness of planetary space. The only remaining typical 
plans of acting on the dog would be either by electric or mag- 
netic attractions, or by mesmerism, and I would have you seek 
for the medium which conveys these impressions with just as 
great a certainty that there is one as in any of the other cases. 
Leaving these more mysterious and subtle modes of communi- 
cation, let us return to the two most simple ones, viz., the stick 
and the stone. These two are representative of the only possible 
fundamental modes of communication between distant bodies, 
for one is compelled to believe that every more occult mode of 
action will ultimately resolve itself into one or other of these two. 
The stick represents the method of communication by con- 
tinuous substance; the stone represents the communication by 
actual transfer of matter, or, as I shall call it, the projectile 
method. There are no other known methods for one body to 
act on another than by these two—by continuous medium, and 
by projectile. 
We know one clear and well-established example of the pro- 
jectile method, viz., the transmission of pressure by gases. A 
gas consists of particles perfectly independent of each other, and 
the only way in which they can act on each other is by blows. 
The pressure of the air is a bombardment of particles, and 
actions are transmitted through gases as through a row of ivory 
balls. Sound is propagated by each particle receiving a knock 
and passing it on to the next, the final effect being much the 
same as if the first struck particles had been shot off through the 
whole distance. 
The explanation of the whole behaviour of gases in this 
manner is so simple and satisfactory, and moreover is so certainly 
the true account of the matter, that we are naturally tempted to 
ask whether this projectile theory is not the key to the universe, 
and whether every kind of action whatever cannot be worked 
out on this hypothesis of atoms blindly driving about in all 
directions at perfect random and with complete independence of 
each other except when they collide.!_ And accordingly we have 
the corpuscular theories of light and of gravitation ; both account 
for the respective phenomena by a battering of particles. The 
corpuscular theory of gravitation is, however, full of difficulties, 
for it is not obvious according to it why the weight of a plate is 
_ * To this hypothesis Mr. Tolver Preston has addressed himself with much 
ingenuity. 
the same when held edzeways as when held broadside on, in the 
stream of corpu cles ; while it is surprising (as indeed it perhaps 
is on any hypothesis) that the weight of a body is the same in 
the solid, liquid, and gaseous states. It has been attempted to 
explain cohesion also on the same hypothesis, but the difficulties, 
which were great enough before, are now enormous, and to me 
at any rate it seems that it is only by violent straining and by 
improbable hypotheses that we can explain all the actions of the 
universe by a mere battery of particles. 
Moreover, it is difficult to understand what the atoms them- 
selves can be like, or how they can strike and bound off one 
another without yielding to compression and then springing 
out again like two elastic balls ; it is difficult to understand the 
elasticity of really ultimate hard particles. And if the atoms are 
not such hard particles, but are elastic and yielding, and bound 
from one another according to the same sort of law that ivory 
balls do, of what are they composed? We shall have to begin 
all over again, and explain the cohesion and elasticity of the 
parts of the atom. 
The more we think over the matter, the more are we com- 
pelled to abandon mere impact as a complete explanation of 
action in general, But if this be so we are driven back upon 
the other hypothesis, the only other, viz. communication by con- 
tinuous medium. 
We must begin to imagine a continuous connecting medium 
between the particles—a substance in which they are imbedded, 
and which extends into all their interstices, and extends without 
break to the remotest limits of space. Once grant this and 
difficulties begin rapidly to disappear. There is now continuous 
contact between the particles of bodies, and if one is pushed the 
others naturally receive the motion, The atoms of gas are 
impinging as before, but we have now a different idea of what 
impact means. 
Gravitation is explainable by differences of pressure in the 
medium, caused by some action between it and matter not yet 
understood. Cohesion is explainable also probably in the same 
way. 
Light consists of undulation or waves in the medinm ; while 
electricity is turning out quite possibly to be an aspect of a part 
of the very medium itself. 
The medium is now accepted as a necessity by all modern 
physicists, for without it we are groping in the dark, with it we 
feel we have a clue which, if followed up, may lead us into the 
innermost secrets of nature, It has as yet been followed up 
very partially, but I will try and indicate the directions in which 
modern science is tending. 
The name you choose to give to the medium isa matter of 
very small importance, but ‘‘the Ether” is as good a name for 
it as another. 
As far as we know it appears to be a perfectly homogeneous 
incompressible continuous body incapable of being resolved 
into simple elements or atoms; it is, in fact, continuous, not 
molecular, There is no other body of which we can say this, 
and hence the properties of ether must be somewhat different 
from those of ordinary matter. But there is little difficulty in 
picturing a continuous substance to ourselves, inasmuch as the 
molecular and porous nature of ordinary matter is by no means 
evident to the senses, but 1s an inference of some difficulty. 
Ether is often called a fluid, or a liquid, and ayain it has been 
called a solid and has been likened to a jelly because of its 
rigidity; but none of these names are very much good; all 
these are molecular groupings, and therefore not like ether; let 
us think simply and solely of a continuous frictionless medium 
possessing inertia, and the vagueness of the notion will be 
nothing more than is proper in the present state of our 
knowledge. 
We have now to try and realise the idea of a perfectly con- 
tinuous, subtle, incompressible substance pervading all space and 
penetrating between the molecules of all ordinary matter, which 
are imbedded in it, and connected with one another by its 
means. And we must regard it as the one universal medium by 
which all actions between bodies are carried on, This, then, is 
its function—to act as the transmitter of motion and of energy. 
First consider the propagation of light. 
Sound is propagated by direct excursion and impact of the 
atoms of ordinary matter. Light is not so propagated. How 
do we know this ? 
I. Because of speed, 3 X 10, which is greater than anything 
transmissible by ordinary matter. 
2, Because of the kind of vibration, as revealed by the pheno- 
mena of polarisation. 
