SEPTEMBER 14, 1899] 
NEAL TCT Ras 
473 
being in whom it is highly cultivated, say a very intellectual 
and very hypothetical dog. Let us suppose that he tries to 
frame an hypothesis as to light. Having found that his sense 
of smell is excited by surface exhalations, will he not naturally 
make and be content with a corpuscular theory of light? When 
he has discovered the facts of dispersion, will he not think of 
the different colours as different kinds of smell—insensible, 
perhaps, to him, but sensible to a still more highly gifted, still 
more hypothetical, dog ? 
Of course, with our superior intellect and sensibility, we can 
see where his hypothesis would break down; but unless we are 
to assume that we have reached finality in sense development, 
the illustration, grotesque as it may be, will serve to show that 
our hypotheses are in terms of ourselves rather than in terms of 
nature itself, they are ejective rather than objective, and so they 
are to be regarded as instruments, tools, apparatus only to aid 
us in the search for truth. 
To use an old analogy—and here we can hardly go except 
upon analogy—while the building of nature is growing spon- 
taneously from within, the model of it, which we seek to con- 
struct in our descriptive science, can only be constructed by means 
of scaffolding from without, a scaffolding of hypotheses. While 
in the real building all is continuous, in our model there are 
detached parts which must be connected with the rest by 
temporary ladders and passages, or which must be supported 
till we can see how to fill in the understructure. To give the 
hypotheses equal validity with facts is to confuse the temporary 
scaffolding with the building itself. 
But even if we take this view of the temporary nature of our 
molecular and ethereal imaginings, it does not lessen their 
value, their necessity to us. 
It is merely a true description of ourselves to say that we must 
believe in the continuity of physical processes, and that we must 
attempt to form mental pictures of those processes the details 
of which elude our observation. For such pictures we must 
frame hypotheses, and we have to use the best material at com- 
mand in framing them. At present there is only one funda- 
mental hypothesis—the molecular and ethereal hypothesis —in 
some such form as is generally accepted. 
Even if we take the position that the form of the hypothesis 
may change as our knowledge extends, that we may be able to 
devise new machinery—nay, even that we may be able to 
design some quite new type to bring about the same ends— 
that does not appear to me to lessen the present value of the 
hypothesis. We can recognise to the full how well it enables 
us to group together large masses of facts which, without it, 
would be scattered apart, how it serves to give working explan- 
ations, and continually enables investigators to think out new 
questions for research. We can recognise that it is the sym- 
bolical form in which much actual knowledge is cast. We 
might almost as well quarrel with the use of the letters of the 
alphabet, inasmuch as they are not the sounds themselves, but 
mere arbitrary symbols of the sounds. 
In this country there is no need for any defence of the use of 
the molecular hypothesis. But abroad the movement from the 
position in which hypothesis is confounded with observed truth 
has carried many through the position of equilibrium equally 
far on the other side, and a party has been formed which totally 
abstains from molecules as a protest against immoderate indul- 
gence in their use. Time will show whether these protesters 
can do without any hypothesis, whether they can build without 
scaffolding or ladders. I fear that it is only an attempt to build 
from balloons. 
But the protest will have value if it will put us on our guard 
against using molecules and the ether everywhere and every- 
where. There is, I think, some danger that we may get so 
accustomed to picturing everything in terms of these hypotheses 
that we may come to suppose that we have no firm basis for the 
facts of observation until we have given a molecular account of 
them, that a molecular basis is a firmer foundation than direct 
experience, 
Let me illustrate this kind of danger. The phenomena of 
capillarity can, for the most part, be explained on the assumption 
of a liquid surface tension. But if the subject is treated merely 
from this point of view, it stands alone—it is a portion of the 
building of science hanging in the air. The molecular hypo- 
thesis then comes in to give some explanation of the surface 
tension, gives, as it were, a supporting understructure con- 
necting capillarity with other classes of phenomena. But here, 
} think, the hypothesis should stop, and such phenomena as can 
NO. 1559, VOL. 60] 
be explained by the surface tension should be so explained 
without reference to molecules. They should not be brought 
in again till the surface-tension explanation fails. It is necessary 
to bear in mind what part is scaffolding, and what is the 
building itself, already firm and complete. 
Or, as another illustration, take the Second Law of Thermo- 
dynamics. I suspect that it is sometimes supposed that a 
molecular theory from which the Second Law could be deduced 
would be a better basis for it than the direct experience on 
which it was founded by Clausius and Kelvin, or that the mere 
imagining of a Maxwell’s sorting demon has already disproved 
the universality of the law ; whereas he is a mere hypothesis 
grafted on a hypothesis, and nothing corresponding to his action 
has yet been found. 
There is more serious danger of confusion of hypothesis 
with fact in the use of the ether: more risk of failure to see 
what is accomplished by its aid. In giving an account of light, 
for instance, the right course, it appears to me, is to describe 
the phenomena and lay down the laws under which they are 
grouped, leaving it an open question what it is that waves, 
until the phenomena oblige us to introduce something more 
than matter, until we see what properties we must assign to the 
ether, properties not possessed by matter, in order that it may 
be competent to afford the explanations we seek. We should 
then realise more clearly that it is the constitution of matter 
which we have imagined, the hypothesis of discrete particles 
which obliges us to assume an intervening medium to carry on 
the disturbance from particle to particle. But the vortex-atom 
hypothesis and Dr. Larmor’s strain-atom hypothesis both seem 
to indicate that we are moving in the direction of the abolition 
of the distinction between matter and ether, that we shall come 
to regard the luminiferous medium, not as an attenuated sub- 
stance here and there encumbered with detached blocks—the 
molecules of matter—but as something which in certain places 
exhibits modifications which we term matter. Or starting rather 
from matter, we may come to think of matter as no longer con- 
sisting of separated granules, but as a continuum with properties 
grouped round the centres, which we regard as atoms or 
molecules. 
Perhaps I may illustrate the danger in the use of the concep- 
tion of the ether by considering the common way of describing 
the electro-magnetic waves, which are all about us here, as 
ether waves. Now in all cases with which we are acquainted, 
these waves start from matter ; their energy before starting was, 
as far as we can guess, energy of the matter between the dif- 
ferent parts of the source, and they manifest themselves in the 
receiver as energy of matter. As they travel through the air, I 
believe that it is quite possible that the electric energy can be 
expressed in terms of the molecules of air in their path, that 
they are effecting atomic separations as they go. If so, then 
the air is quite as much concerned in their propagation as the 
ether between its molecules. In any case, to term them ether 
waves is to prejudge the question before we have sufficient 
evidence. 
Unless we bear in mind the hypothetical character of our 
mechanical conception of things, we may run some risk of 
another danger-—the danger of supposing that we have some- 
thing more real in mechanical than in other measurements. For 
instance, there is some risk that the work measure of specific 
heat should be regarded as more fundamental than the heat 
measure, in that heat is truly a ‘‘mode of motion.” On the 
molecular hypothesis, heat is no doubt a mixture of kinetic 
energy and potential energy of the molecules and their con- 
stituents, and may even be entirely kinetic energy ; and we may 
conceivably in the future make the hypothesis so definite that, 
when we heat a gramme of water I’, we can assign such a 
fraction of an erg to each atom. But look how much pure 
hypothesis is here. The real superiority of the work measure of 
specific heat lies in the fact that it is independent of any 
particular substance, and there is nothing whatever hypothetical 
about it. 
1 This risk of imagining one particular kind of measure more real than 
another, more in accordance with the truth of things, may be further illus- 
trated by the common idea that mass-acceleration is the only way to 
measure a force. We stand apart from our mechanical system and watch 
the motions and the accelerations of the various parts, and we find that 
mass-accelerations have acertain significance in our system, If we keep 
ourselves outside the system and only use our sense of sight, then mass- 
acceleration is the only way of describing that behaviour of one body in the 
presence of others which we term force on it. But if we go about in the 
system and pull and push bodies, we find that there is another conception 
of force, in which another sense than sight is concerned—another mode 
