618 REPoRT— 1899. 
To take an old but never-worn-out metaphor, the physicist is examining the 
garment of Nature, learning of how many, or rather of how few, different kinds of 
thread it is woven, finding how each separate thread enters into the pattern, and 
seeking from the pattern woven in the past to know the pattern yet to come. 
How many ditferent kinds of thread does Nature use P 
So far, we have recognised some eight or nine, the number of different forms 
of energy which we are still obliged to count as distinct. But this distinction we 
cannot believe to be real. The relations between the different forms of energy, 
and the fixed rate of exchange when one form gives place to another, encourage 
us to suppose that if we could only sharpen our senses, or change our point of view, 
we could effect a still further reduction. We stand in front of Nature’s loom as 
we watch the weaving of the garment ; while we follow a particular thread in the 
pattern it suddenly disappears, and a thread of another colour takes its place. Is 
this a new thread, or is it merely the old thread turned round and presenting a new 
face to us? We can do little more than guess. We cannot get to the other side 
of the pattern, and our minutest 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 tale ourselves as we are, and reckon with 
the fact that though our material is physical, we ourselves are psychical. And, asa 
mere matter of fact, we are not content 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 visible motion, we think 
of the light as changing its form alone and becoming itself the chemical energy. 
Then to our passive dislile of discontinuity 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 corre- 
sponding 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 accelerations. We imagine a mechanical constitution of the universe. 
Weare encouraged in this attempt by the fact that therelations 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. Com- 
pare, for instance, the accuracy with which we can describe and foretell the path of 
a planet with our ignorance of the movements 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 explana- 
tion—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 
