Books: Five of Maxwell's Papers
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James Clerk Maxwell >> Five of Maxwell's Papers
But then how few of us are capable of deriving profit from such
studies. We cannot enter into full sympathy with these lower phases
of our nature without losing some of that antipathy to them which is
our surest safeguard against a reversion to a meaner type, and we
gladly return to the company of those illustrious men who by aspiring
to noble ends, whether intellectual or practical, have risen above the
region of storms into a clearer atmosphere, where there is no
misrepresentation of opinion, nor ambiguity of expression, but where
one mind comes into closest contact with another at the point where
both approach nearest to the truth.
I propose to lecture during this term on Heat, and, as our facilities
for experimental work are not yet fully developed, I shall endeavour
to place before you the relative position and scientific connexion of
the different branches of the science, rather than to discuss the
details of experimental methods.
We shall begin with Thermometry, or the registration of temperatures,
and Calorimetry, or the measurement of quantities of heat. We shall
then go on to Thermodynamics, which investigates the relations between
the thermal properties of bodies and their other dynamical properties,
in so far as these relations may be traced without any assumption as
to the particular constitution of these bodies.
The principles of Thermodynamics throw great light on all the
phenomena of nature, and it is probable that many valuable
applications of these principles have yet to be made; but we shall
have to point out the limits of this science, and to shew that many
problems in nature, especially those in which the Dissipation of
Energy comes into play, are not capable of solution by the principles
of Thermodynamics alone, but that in order to understand them, we are
obliged to form some more definite theory of the constitution of
bodies.
Two theories of the constitution of bodies have struggled for victory
with various fortunes since the earliest ages of speculation: one is
the theory of a universal plenum, the other is that of atoms and void.
The theory of the plenum is associated with the doctrine of
mathematical continuity, and its mathematical methods are those of the
Differential Calculus, which is the appropriate expression of the
relations of continuous quantity.
The theory of atoms and void leads us to attach more importance to the
doctrines of integral numbers and definite proportions; but, in
applying dynamical principles to the motion of immense numbers of
atoms, the limitation of our faculties forces us to abandon the
attempt to express the exact history of each atom, and to be content
with estimating the average condition of a group of atoms large enough
to be visible. This method of dealing with groups of atoms, which I
may call the statistical method, and which in the present state of our
knowledge is the only available method of studying the properties of
real bodies, involves an abandonment of strict dynamical principles,
and an adoption of the mathematical methods belonging to the theory of
probability. It is probable that important results will be obtained
by the application of this method, which is as yet little known and is
not familiar to our minds. If the actual history of Science had been
different, and if the scientific doctrines most familiar to us had
been those which must be expressed in this way, it is possible that we
might have considered the existence of a certain kind of contingency a
self-evident truth, and treated the doctrine of philosophical
necessity as a mere sophism.
About the beginning of this century, the properties of bodies were
investigated by several distinguished French mathematicians on the
hypothesis that they are systems of molecules in equilibrium. The
somewhat unsatisfactory nature of the results of these investigations
produced, especially in this country, a reaction in favour of the
opposite method of treating bodies as if they were, so far at least as
our experiments are concerned, truly continuous. This method, in the
hands of Green, Stokes, and others, has led to results, the value of
which does not at all depend on what theory we adopt as to the
ultimate constitution of bodies.
One very important result of the investigation of the properties of
bodies on the hypothesis that they are truly continuous is that it
furnishes us with a test by which we can ascertain, by experiments on
a real body, to what degree of tenuity it must be reduced before it
begins to give evidence that its properties are no longer the same as
those of the body in mass. Investigations of this kind, combined with
a study of various phenomena of diffusion and of dissipation of
energy, have recently added greatly to the evidence in favour of the
hypothesis that bodies are systems of molecules in motion.
I hope to be able to lay before you in the course of the term some of
the evidence for the existence of molecules, considered as individual
bodies having definite properties. The molecule, as it is presented to
the scientific imagination, is a very different body from any of those
with which experience has hitherto made us acquainted.
In the first place its mass, and the other constants which define its
properties, are absolutely invariable; the individual molecule can
neither grow nor decay, but remains unchanged amid all the changes of
the bodies of which it may form a constituent.
In the second place it is not the only molecule of its kind, for there
are innumerable other molecules, whose constants are not
approximately, but absolutely identical with those of the first
molecule, and this whether they are found on the earth, in the sun, or
in the fixed stars.
By what process of evolution the philosophers of the future will
attempt to account for this identity in the properties of such a
multitude of bodies, each of them unchangeable in magnitude, and some
of them separated from others by distances which Astronomy attempts in
vain to measure, I cannot conjecture. My mind is limited in its power
of speculation, and I am forced to believe that these molecules must
have been made as they are from the beginning of their existence.
I also conclude that since none of the processes of nature, during
their varied action on different individual molecules, have produced,
in the course of ages, the slightest difference between the properties
of one molecule and those of another, the history of whose
combinations has been different, we cannot ascribe either their
existence or the identity of their properties to the operation of any
of those causes which we call natural.
Is it true then that our scientific speculations have really
penetrated beneath the visible appearance of things, which seem to be
subject to generation and corruption, and reached the entrance of that
world of order and perfection, which continues this day as it was
created, perfect in number and measure and weight?
We may be mistaken. No one has as yet seen or handled an individual
molecule, and our molecular hypothesis may, in its turn, be supplanted
by some new theory of the constitution of matter; but the idea of the
existence of unnumbered individual things, all alike and all
unchangeable, is one which cannot enter the human mind and remain
without fruit.
But what if these molecules, indestructible as they are, turn out to
be not substances themselves, but mere affections of some other
substance?
According to Sir W. Thomson's theory of Vortex Atoms, the substance of
which the molecule consists is a uniformly dense _plenum_, the
properties of which are those of a perfect fluid, the molecule itself
being nothing but a certain motion impressed on a portion of this
fluid, and this motion is shewn, by a theorem due to Helmholtz, to be
as indestructible as we believe a portion of matter to be.
If a theory of this kind is true, or even if it is conceivable, our
idea of matter may have been introduced into our minds through our
experience of those systems of vortices which we call bodies, but
which are not substances, but motions of a substance; and yet the idea
which we have thus acquired of matter, as a substance possessing
inertia, may be truly applicable to that fluid of which the vortices
are the motion, but of whose existence, apart from the vortical motion
of some of its parts, our experience gives us no evidence whatever.
It has been asserted that metaphysical speculation is a thing of the
past, and that physical science has extirpated it. The discussion of
the categories of existence, however, does not appear to be in danger
of coming to an end in our time, and the exercise of speculation
continues as fascinating to every fresh mind as it was in the days of
Thales.