SEPTEMBER 22, 1899. ] 
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 con- 
fusion 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 some- 
thing 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 
realize more clearly that it is the constitu- 
tion of matter which we have imagined, 
the hypothesis of discrete particles, which 
obliges us to assume an intervening me- 
dium 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 
substance here and there encumbered with 
detached blocks—the molecules of matter 
—hbut 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 consisting of separated granules, but 
as a continuum with properties grouped 
round the centers, which we regard as 
atoms or molecules. 
SCIENCE. 
393 
Perhaps I may illustrate the danger in 
the use of the conception 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 different 
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 something 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 en- 
ergy of the molecules and their constituents, 
and may even be entirely kinetic energy; 
and we may conceiyably in the future make 
the hypothesis so definite that, when we 
heat a gramme of water 1°, we can assign 
such fraction of an erg to each atom. But 
look how much pure hypothesis is here. 
The real superiority of the work meas- 
ure of specific heat lies in the fact that it is 
independent of any particular substance, 
and there is nothing whatever hypothetical 
about it.* 
*This risk of imagining one particular kind of 
