Conceptions of Ether: Studies in the History of Ether Theories 1740-1900. Edited by G. N. CANTOR and
M. J. S. HODGE. Cambridge: Cambridge University Press, 1981. Pp. 351. £30.00.
An ether, as rather reluctantly delimited by the editors of this collection, is imperceptible and all
pervading. It is present in, and moves through, both empty space and material bodies. It is almost
always weightless, but the other properties ascribed to it may be various. It may be mental or material,
active or passive, a homogenous substance or a cloud of particles. The facts, and fictions, which ethers
have been invoked to explain show an equal variety: the forces of gravitation, electricity and magnetism,
the transmission of light, heat and electromagnetic energy across empty space and of signals along
nerve fibres, the occurrence of chemical reactions, the interactions between minds and bodies and
between this world and its creator, a persons survival of his material body.
These theories stemmed indirectly from Descartes; the Aristotelian theory which first
employed the name was very different. Descartes’ belief that the phenomena of physics are at bottom
mechanical, that they arise from the interactions of material bodies by pressure and impact, remained
the central inspiration of ether theories throughout the succeeding two and a half centuries. The direct
inspiration of the mid-eighteenth century theorists who are the subject of the earlier essays in this book
was that same Newton who, true to his notorious words, set down a law of gravitation in his Principia,
but made no hypotheses as to its origin. In letters, and in other documents published only after his
death, he did speculate on the origins of gravitational attraction, examining the possibility that it arose
from repulsive forces between the particles of an ether which filled interplanetary space, and in a
number of Queries appended to the Optics he raises other speculations. Newton was not motivated by
Descartes’ mechanistic ideal; his ether derives its elasticity not from the momentum of its particles but
from the non-mechanical repulsive forces they exert upon one another. Newton’s motivation was, in
fact, the opposite of Descartes’; convinced, like Berkeley later, of the inactivity of matter, he sought to
explain away the activity with which his own theory endowed it. However, as the editors point out in
their valuable introduction, he had no reason for thinking the ethereal particles more active than the
material ones they so resembled.
Although the essays concern the years between 1740 and 1900, the introduction provides an
excellent account of earlier theories. However, both it and the essays stop short of the theory of special
relativity. That is an absurd point to end: special relativity had a decisive impact upon the ether theories
of electromagnetism, and the history of that impact would have made a natural conclusion. It is
astonishing to find historians arguing, as the editors do, that ‘the view that ether theories did not
survive 1905 is incorrect because there have been, and still are, many ether theories that, in principle,
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are perfectly compatible with special relativity’. A theory may fail to survive, not because it is refuted,
but because a compelling reason for accepting it has been removed.
The earlier essays show how prone eighteenth century scientists were to ascribe phenomena to
weightless, invisible fluids. Some of these explanations were well worked out, for instance, Le Sage’s
ingenious explanation of gravitation was substantial enough to interest Clerk Maxwell, but others were
little more than indications of the type of explanation that might be sought. The fact that ether was
given a role in both the fluid and the mechanical theories of heat demonstrates how readily it was
invoked to meet any difficulty which arose. The value of these speculations is not investigated but much
intriguing material is set before us. J. R. R. Christie, in his article on chemistry, does investigate the
contribution to Joseph Black’s discoveries about heat made by the ether theories of Black’s teacher,
Cullen. He proves the influence of Cullen’s experimental work but not, it seems to me, of his theories.
Roger French handles the concerns and disputes of eighteenth century physiologists with great
skill, relating them to the modification of the Aristotelian and Galenian traditions by Descartes. He
shows how ethereal fluids were exploited in solutions to different problems, for example, of the control
in contrast to the movement of animal limbs and, again, in mechanism and animism. He makes a
significant distinction between those who derived their Newtonianism from the Principle and knew
something of the laws of motion and those who derived it only from the Queries in the Optics and
knew little. Considering many of these theories, it is difficult not to sympathise with Thomas Reid’s
strictures:
First they suppose the Existence of certain things of whose existence we have no Evidence, and
Secondly when they are supposed to exist they do not account for the phenomena they are
brought to explain… [Nervous transmission and muscular contraction cannot] be accounted for
by any laws of Mechanism we know. It is something beyond Mechanism and of a superior
nature.
After a century and a bit, nearly everyone was of Reid’s opinion.
G. N. Cantor shows that two theological attitudes were elicited by one of the few experimental
consequences of ether speculations, that of the resistance to planetary motion which the particles of a
space-filling ether would present. Such a space-filling ether was postulated by Descartes and was
required by the theory that light is transmitted as vibration in an ethereal medium. Newton cited the
stability of the planetary orbits against Descartes’ theory but, when he himself flirted with the ether, set
the difficulty aside by stressing how very slight the resistance would be. ‘planets might be spiralling
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towards the sun but the turns of the spirals be too close to be distinguished. James Bradley’s discovery
of stellar aberration established that, if light was a wave motion in an ethereal medium, then the earth
moved through that medium, so that its resistance would, in a finite time, take it into the sun.
Aberration established the movement of the earth through the ether, not the fixedness of the stars, as
Cantor puts it. The nineteenth century philosopher of science, William Whewell, thought such
evidence of decay pointed to a beginning and, hence, a creation of the universe. Others saw it as an
imperfection and evidence against the existence of God.
Cantor deals, too, with the prisca sapientia tradition, which claimed to find ether theories
prefigured in ancient writings front many sources, Egyptian, Persian, Hebrew and Greek. Berkeley cites
this tradition in Siris to support his vital spirit ether, but another writer, Michael Ramsey, may have had
greater historical significance through a connection with Newton. This might explain Newton’s
renewed interest in ether theory in the Optics and, thence, that of the English eighteenth century
scientists. Ramsey presented the ancient wisdom in a book in which Zoroaster reveals the secrets of the
physical universe. Cantor discloses that the book, The Travels of Cyrus, exists in both an English and a
French version. In one, Zoroaster expounds a Newtonian system, in the other, a Cartesian one. Unless
Ramsey was quite dishonest, he can have regarded the prisca sapientia as no more than a fable; there is,
apparently, evidence that Newton took it more seriously.
‘Not a tolerant epistemology at the best of times, classical empiricism (by which I mean the
empiricism of Berkeley, Condillac, Hume and Reid) left no scope for entities like ether.’ ‘As regards
ether, their treatment allows the proposition that classical empiricism not only need not have frowned
upon the ether, but indeed could generate powerful arguments in its favour.’ Since the treaters referred
to in the second quotation are David Hume and Adam Smith, these opinions of two of the contributors,
Laudan and Christie respectively, stand in flat contradiction. Had Christie expressed himself more
circumspectly it should not be difficult to reconcile them. He writes about the ether in the work of the
scientists of the Scottish Enlightenment and traces the ether theories of the chemist William Cullen
back through Adam Smith to Hume. He is able to cite a discussion in which Smith illustrates Hume’s
account of the role of the imagination in the formation of belief by the example of belief in an ether,
though his evidence that Hume himself favoured the ether is thin, to say the least. Smith’s illustration
was not unfair: Hume does show how the imagination leads us to theoretical beliefs. It is wrong to infer
that those theoretical beliefs are compatible with Hume’s empiricism or that Hume held them to be.
However, Smith, adopting some things and discarding others, may have been led by Hume’s work to
look favourably on such theoretical speculations. Laudan, too, may be right that other scientists found
in the empiricist philosophy strong arguments against those speculations: they were there to be found.
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Rival scientific methods, according to Laudan, dominated the eighteenth and nineteenth
centuries. He describes how Le Sage was goaded by the neglect of his ether theory of gravitation to
attack the empiricist method which allowed generalization from observation but prohibited hypotheses
in which unobservables figured. Le Sage demonstrated that Newton himself employed such hypotheses
throughout his scientific work. Laudan sees the predictive successes of the wave theory of light as the
turning point of the debate. For Whewell, the theory was ‘recommended’ by ‘explaining what it was
meant to explain’ but ‘confirmed’ by ‘explaining what it was not meant to explain’. J. S. Mill was able to
resist such confirmation: ‘If the laws of the propagation of light accord with those of the vibrations of
an elastic fluid in as many ways as is necessary to make the hypothesis afford a correct expression of all
or most of the phenomena known at the time, it is nothing strange that they should accord with each
other in one respect more.’ Laudan says that he suspects that, while getting the better of the argument,
Mill missed Whewell’s point; yet he shows fairly conclusively that Whewell never made his point very
plain. If the justification of belief is what both were concerned with, I am unconvinced that a point
exists. Mill’s words should stand at the end of this book, for the outcome he thought unsurprising, that
the ether theory of light might provide in many ways a true picture and yet be false, was just what
everyone came to believe.
The ether had two tremendous successes: the theory that light was a mechanical vibration
transmitted through a molecular fluid and Clerk Maxwell’s mechanical model of the laws of
electromagnetism, which led him to the discovery of electromagnetic radiation and to the
electromagnetic theory of light. The second success annulled the first, for the incompleteness of the
model and the problems of movement through the ether led scientists to accept an electromagnetic, not
a mechanical, theory of light. Jed Buchwal contributes a good account of the wave theory developed by
Fresnal and Cauchy. His brief treatment of Thomas Young’s work seems to underrate the importance
of the discovery and explanation of wave interference.
Siegal, Wise and Stein investigate the history of electromagnetism in the latter half of the
nineteenth century in great detail. Siegal describes the origins of Maxwell’s work in that of Faraday and
Thomson. His attempt to provide insight into the workings of the mechanical theory is brave but, for
this reader, not really successful. The question of how central Maxwell’s mechanical theory was to his
formulation of the equations of the electromagnetic field remains without a definite answer. Wise
discusses the work of the German physicists of the period, including that of Wilhelm Weber which
anticipated later theories of the electron and the structure of the atom in a remarkable manner. Both he
and Stein treat Lorentz’s work but leave the reader puzzled as to whether Lorentz supposed that a
rarified gas of electrically charged particles could fulfil all the functions of the ether or only some of
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them. Stein deals with the British theorists, Thomson, FitzGerald and Larmor, who carried on
Maxwell’s attempt to provide a mechanical explanation of electromagnetism. He finds that Whittaker,
the classical ether historian, failed to appreciate what FitzGerald’s work added to Maxwell’s. He
describes the vortex-atom theory, common to Thomson and FitzGerald, according to which material
particles are vortices in the ether.
This book shows, in scholarly detail, how the ether occupied the minds of many scientists for
two centuries. No one in it asks the general question ‘Was it worth it?’ In the two most important fields,
celestial motion and electricity, success came to workers, Newton and Faraday, who set the ether aside.
Even in electromagnetism people came to think they would he better off without it. As the quotation
from Thomas Reid suggests, reducing things to what you can understand isn’t always the right way of
coming to understand them.
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