472 
fe Leaving the metaphor, were we true physicists, and physicists 
-alone, we should, I suppose, be content to describe merely 
what we observe in the changes of energy. We should say, 
for instance, that so much kinetic energy ceases, and that so 
much heat appears, or that so much light comes to a surface, 
and that so much chemical energy takes its place. But we 
have to take ourselves as we are, and reckon with the fact that 
though our material is physical, we ourselves are psychical. 
And, as a mere matter of fact, we are not content with such dis- 
continuous descriptions. We dislike the discontinuity and we 
think of an underlying identity. We think of the heat as being 
that which a moment before was energy of visible motion, we 
think of the light as changing its form alone and becoming itself 
the chemical energy. Then to our passive dislike to discon- 
tinuity we join our active desire to form a mental picture of 
what may be going on, a picture like something which we 
already know. Coming on these discontinuities our ordinary 
method of explanation fails, for they are not obviously like 
‘those series of events in which we can trace every step. We 
then imagine a constitution of matter and modifications of it 
corresponding to the different kinds of energy, such that the 
discontinuities vanish, and such that we can picture one form of 
energy passing into another and yet keeping the same in kind 
‘throughout. We are no longer content to describe what we 
actually see or feel, but we describe what we imagine we should 
see or feel if our senses were on quite another scale of magnitude 
and sensibility. We cease to be physicists of the real and 
‘become physicists of the ideal. 
To form such mental pictures we naturally choose the sense 
which makes such pictures most definite, the sense of sight, and 
‘think of a constitution of matter which shall enable us to explain 
all the various changes in terms of visible motions and accelera- 
tions. We imagine a mechanical constitution of the universe. 
We are encouraged in this attempt by the fact that the rela- 
tions in this mechanical conception can be so exactly stated, that 
the equations of motion are so very definite. We have, too, 
examples of mechanical systems, of which we can give accounts 
far exceeding in accuracy the accounts of other physical systems. 
Compare, for instance, the accuracy with which we can describe 
and foretell the path of a planet with our ignorance of the move- 
ments of the atmosphere as dependent on the heat of the sun. 
‘The planet keeps to the astronomer’s time-table, but the wind 
still bloweth almost where it listeth. 
The only foundation which has yet been imagined for this 
‘mechanical explanation—if we may use ‘‘ explanation” to denote 
the likening of our imaginings to that which we actually 
observe—is the atomic and molecular hypothesis of matter. 
This hypothesis arose so early in the history of science that 
we are almost tempted to suppose that it is a necessity of 
thought, and that it has a warrant of some higher order than any 
other hypothesis which could be imagined. But I suspect that 
if we could trace its early development we should find that it 
arose in an attempt to explain the phenomena of expansion 
and contraction, evaporation and solution. Were matter a 
continuum we should have to admit all these as simple facts, 
inexplicable in that they are like nothing else. But imagine 
matter to consist of a crowd of separate particles with inter- 
spaces. Contraction and expansion are then merely a drawing 
in and a widening out of the crowd. Solution is merely the 
mingling of two crowds, ‘and evaporation merely a dispersal 
from the outskirts. The most evident properties of matter are 
then similar to what may be observed in any public meeting. 
For ages the molecular hypothesis hardly went further than 
this. The first step onward was the ascription of vibratory 
motion to the atoms to explain heat. Then definite qualities were 
cascribed, definite mutual forces were called into play to explain 
elasticity and other properties or qualities of matter. But I 
imagine its first really great achievement was its success in ex- 
plaining the law of combining proportions, and next to that we 
should put its success in explaining many of the properties of 
gases. 
While light was regarded aS corpuscular—in fact molecular, 
and while direct action at a distance presented no difficulty, the 
molecular hypothesis served as the one foundation for the 
mechanical representation of phenomena. But when it was 
shown that infinitely the best account of the phenomena of light 
could be given on the supposition that it consisted of waves, 
something was needed, as Lord Salisbury has said, to wave, 
both in the interstellar and in the intermolecular spaces. So 
the hypothesis of an ether was developed, a necessary comple- 
NO. 1559, VOL. 60] 
NATURE 
[SEPTEMBER 14, 1899 
ment of that form of the molecular hypothesis in which matter 
consists of discrete particles with matter-free intervening 
spaces. A 
Then Faraday’s discovery of the influence of the dielectric 
medium in electric actions led to the general abandonment of 
the idea of action at a distance, and the ether was called in to 
aid matter in the explanation of electric and magnetic pheno- 
mena. The discovery that the velocity of electro-magnetic 
waves is the same as that of light-waves is at least circumstantial 
evidence that the same medium transmits both. 
I suppose we all hope that some time we shall succeed in 
attributing to this medium such further qualities that it will be 
able to enlarge its scope and take in the work of gravitation. 
The mechanical hypothesis has not always taken this dualistic 
form of material atoms and molecules, floating in a quite distinct 
ether. I think we may regard Boscovich’s theory of point- 
centres surrounded by infinitely extending atmospheres of force 
as really an attempt to get rid of the dualism, and Faraday’s 
theory of point-centres with radiating lines of force is only Bos- 
covich’s theory in another form. But Lord Kelvin’s vortex- 
atom theory gives us a simplification more easily thought of. 
Here all space is filled with continuous fluid—shall we say a 
fluid ether 2—and the atoms are mere loci of a particular type of 
motion of this frictionless fluid. The sole differences in the 
atoms are differences of position and motion. Where there are 
whirls, we call the fluid matter ; where there are no whirls, we 
call it ether. All energy is energy of motion. Our visible 
kinetic energy, MV?/2, is energy in and round the central 
whirls; our visible energy of position, our potential energy, is 
energy of motion in the outlying regions. 
A similar simplification is given by Dr. Larmor’s hypothesis, 
in which, again, all space is filled with continuous substance all 
of one kind, but this time solid rather than fluid. The atoms 
are loci of strain instead of whirls, and the ether is that which is 
strained. 
So, as we watch the weaving of the garment of nature, we 
resolve it in imagination into threads of ether spangled over 
with beads of matter. We look still closer, and the beads of 
matter vanish ; they are mere knots and loops in the threads of 
ether. 
The question now faces us—How are we to regard these 
hypotheses as to the constitution of matter and the connecting 
ether? How are we to look upon the explanations they afford ? 
Are we to put atoms and ether on an equal footing with the 
phenomena observed by our senses, as truths to be investigated 
for their own sake? Or are they mere tools in the search for 
truth, liable to be worn out or superseded ? 
That matter is grained in structure is hardly more than an 
expression of the fact that in very thin layers it ceases to behave 
as in thicker layers. But when we pass on from this general 
statement and give definite form to the granules, or assume 
definite qualities to the intergranular cement, we are dealing 
with pure hypotheses. 
It is hardly possible to think that we shall ever see an atom or 
handle the ether. We make no attempt whatever to render 
them evident to the senses. We connect observed conditions 
and changes in gross visible matter by invisible molecular and 
ethereal machinery. The changes at each end of the machinery 
of which we seek to give an account are in gross matter, and this 
gross matter is our only instrument of detection, and we never 
receive direct sense-impressions of the imagined atoms or the 
intervening ether. To strictly descriptive physicists their only 
use and interest would lie in their service in prediction of the 
changes which are to take place in gross matter. 
It appears quite possible that various types of machinery 
might be devised to produce the known effects. The type we 
have adopted is undergoing constant minor changes, as new 
discoveries suggest new arrangements of the parts. Is it utterly 
beyond possibility that the type itself should change ? 
The special molecular and ethereal machinery which we have 
designed, and which we now generally use, has been designed 
because our most highly developed sense is our sense of sight. 
Were we otherwise, had we a sense more delicate than sight, 
one affording us material for more definite mental presentation, 
we might quite possibly have constructed very different hypo- 
theses. . Though, as we are, we cannot conceive any higher 
type than that founded on the sense of sight, we can imagine a 
lower type, and by way of illustration of the point let us take 
the sense of which my predecessor spoke last year—the sense of 
smell, In us it is very undeveloped. But let us imagine a 
