SEPTEMBER 22, 1899. ] 
it merely the old thread turned round and. 
presenting a new face to us? Wecan do 
little more than guess. We cannot get to 
the other side of the pattern, and our mi- 
nutest watching will not tell us all the 
working of the loom. 
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 con- 
tent with such discontinuous 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 a visible motion, we think of the 
light as changing its form alone and becom- 
ing itself the chemical energy. Then to our 
passive dislike to discontinuity we join our 
active desire to form a mental picture of 
what may be going on, a picture like some- 
thing which we already know. Coming on 
these discontinuities our ordinary method 
of explanation fails, for they are not ob- 
viously 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. 
Weare 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. 
SCIENCE, 
389 
To form such mental pictures we natur- 
ally 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 accelerations. We imagine a mechan- 
ical constitution of the universe. 
We are encouraged in this attempt by 
the fact that the relations in this mechan- 
ical conception can be so exactly stated, 
that the equations of motion are so very 
definite. We have, too, examples of me- 
chanical systems, of which we can give ac- 
counts far exceeding in accuracy the ac- 
counts 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 hy- 
pothesis arose so early in the history of sci- 
ence 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 imag- 
ined. But I suspect that if we could trace 
its early development we should find that 
it arose in an attempt to explain the phe- 
nomena 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 interspaces. Contraction 
and expansion are then merely a drawing 
in and a widening out of the crowd. Solu- 
