So much speculation, and not a little extravagant speculation, has been
devoted to the dynamics of the radiometer, that I feel some compunction
in adding another stone to the heap, my only apology and justification
for so doing being that I propose to regard the subject from a very
unsophisticated point of view, and with somewhat heretical directness
of vision—_i.e._, quite irrespective of atoms, molecules, or ether,
or any other specific preconceptions concerning the essential kinetics
of radiant forces, beyond that of regarding such forces as affections
or conditions of matter which are transmitted radially in constant
quantity, and therefore obey the necessary law of radial diffusion or
inverse squares.
The primary difficulty which appears to have generally been suggested
by the movements of the radiometer, is the case which it seems to
present of mechanical action without any visible basis of corresponding
reaction: a visible tangible object pushed forward, without any visible
pushing agent or resisting fulcrum against which the moving body reacts.
This difficulty has been met by the invocation of obedient and
vivacious molecules of residual atmospheric matter, which have been
called upon to bound and rebound between the vanes and the inner
surfaces of the glass envelope of the instrument.
How is it that the advocates of these activities have not sought to
verify their speculations by modifying the shape and dimensions of the
exhausted glass bulb or receiver?[10] If the motion of the radiometer
is due to such excursions and collisions, the length of excursion and
the angles of collision must modify its motions; and such modification
under given conditions would form a fine subject for the exercise of
the ingenuity of molecular mathematicians. If their hypothetical data
are sound, they should be able to predict the relative velocities or
torsion-force of a series of radiometers of similar construction in all
other respects, but with variable shapes and diameters of enclosing
vessels.
If we divest our minds of all visions of hypothetical atoms, molecules,
ethers, etc., and simply look at the facts of radiation with the
same humility of intellect as we usually regard gravitation, this
primary difficulty of the radiometer at once vanishes. The force
of gravitation is a radiant force acting somehow between, or upon,
or by distant bodies; and these bodies, however far apart, act and
react upon each other with mutual forces, precisely equal and exactly
contrary. We conceive the sun pulling the earth in a certain direction,
and receiving from the earth an equal pull in a precisely contrary
direction, and we have hitherto demanded no ethereal or molecular link
for the transmission of these mutually attractive forces. Why, then,
should we not regard radiant repulsive energy in the same simple manner?
If we do this there is no difficulty in finding the ultimate reaction
fulcrum of the radiometer vanes. It is simply the radiating body, the
match, the candle, the lamp, the sun, or whatever else may be the
source of the impelling radiations. According to this view, the radiant
source must be repelled with precisely the same energy as the arms or
pendulum of the radiometer; and it would move backward or in opposite
direction if equally free to move. If, by any means, we cause the glass
envelope of the radiometer to become the radiant source, it should be
repelled, and may even rotate in opposite direction to the vanes, or
_vice versâ_. This has been done with floating radiometers.
Viewed thus as simple matter of fact, irrespective of any preconceived
kinetics of intervening media, the net result of Mr. Crookes’s
researches become nothing less than the discovery of a new law of
nature of great magnitude and the broadest possible generality, viz.,
that the sun and all other radiant bodies—_i.e._, all the materials
of the universe—exert a mechanical repulsive force, in addition to
the calorific, luminous, actinic, and electrical forces with which
they have hitherto been credited. He has shown that this force is
refrangible and dispersible, that it is outspread with the spectrum,
but is most concentrated, or active, in the region of the ultra-red
rays, and progressively feeblest in the violet; or, otherwise stated,
it exists in closer companionship with heat than with light, and closer
with light than with actinism.
According to the doctrine of exchanges, which has now passed from the
domain of theory to that of demonstrated law, all bodies, whatever be
their temperature, are perpetually radiating heat-force, the amount
of which varies, _cæteris paribus_, with their temperature. If we now
add to this generalization that all bodies are similarly radiating
mechanical force and suffering corresponding mechanical reaction, the
theoretical difficulties of the radiometer vanish. What must follow in
the case of a freely suspended body unequally heated on opposite sides?
It must be repelled in a direction perpendicular to the surface of
its hottest side. If two rockets were affixed to opposite sides of a
pendant body, and were to exert unequal ejective forces, the reaction
of the stronger rocket would repel the body in the opposite direction
to its preponderating ejection. This represents the radiometer vane
with one side blackened and the other side bright. When exposed to
luminous rays the black side becomes warmer than the bright side by
its active absorption and conversion of light into heat, and thus the
blackened face radiates in excess and recedes.
We may regard it thus as acting by its own radiations, or otherwise as
acted upon by the more powerful radiant whose rays are differentially
received by the black and bright sides. These different modes of
regarding the action are perfectly consistent with each other, and
analogous to the two different modes of regarding gravitation, when
we describe the sun as attracting the earth, or, otherwise, the
earth as gravitating to the sun. Strictly speaking, neither of these
descriptions is correct, as the gravitation is mutual, and the total
quantity exerted between the sun and the earth is equal to the sum of
their energies, but it is sometimes convenient to regard the action
from a solar standpoint, and at others from a terrestrial. So with
the radiometer and the strictly mutual repulsions between it and the
predominating radiant.
It appears to me that this unsophisticated conception of radiant
mechanical repulsive force, and its necessary mechanical reaction
on the radiant body, meets all the facts at present revealed by the
experiments of Mr. Crookes and others.
The attraction which occurs when the disc of the radiometer is
surrounded with a considerable quantity of atmospheric matter is
probably due to inequality of atmospheric pressure. The absorbing
face of the disc becomes heated above the temperature of the opposite
face, the film of air in contact with the warmer face rises, leaving
a relatively vacuous space in front. This produces a rush of air from
back to front which carries the radiometer vane with it. When the
exhaustion of the radiometer is carried so far that the residual air
is only just sufficiently dense to neutralize the direct repulsion of
radiation, the neutral point is reached. When exhaustion is carried
beyond this, repulsion predominates.
Taking Mr. Crookes’s estimate of the mechanical energy of solar
radiation at 32 grains per square foot, 2 cwts. per acre, 57 tons per
square mile, etc., and accepting these as they are offered, _i.e._,
merely as provisional and approximate estimates, we are led to a
cosmical inference of the highest importance, one that must materially
modify our interpretations of some of the grandest phenomena of the
universe. Although the estimated sunlight pressure upon the earth, the
three thousand millions of tons, is too small a fraction of the earth’s
total weight to effect an easily measurable increase of the length of
our year, the case is quite otherwise with the asteroids and the zones
of meteoric matter revolving around the sun.
The mechanical repulsion of radiation is a superficial action, and
must, therefore, vary with the amount of surface exposed, while that of
gravitation varies with the mass. Thus the ratio of radiant repulsion
to the attraction of gravitation goes on increasing with the subdivison
of masses, and becomes an important fraction in the case of the smaller
bodies of the solar system. A zone of meteorites traveling around
the sun would be broken up, sifted, and sorted into different orbits,
according to their diameters, if this superficial repulsion operated
against gravitation without any compensating agency. Gravitation would
be opposed in various degrees, neutralized, and, in the case of cosmic
dust, even reversed. Comets presenting so large a surface in proportion
to their mass would either be driven away altogether or forced to move
in orbits utterly disobedient to present calculations. This would occur
if the inter-planetary spaces were as nearly vacuous as the torsion
instrument with which Mr. Crookes made his measurements.
Regarding the properties of our atmosphere only in the light
of experimental data, irrespective of imaginary molecules, and
their supposed gyrations or oscillations, we see at once that an
inter-planetary or inter-stellar vacuum must act like a Sprengel
pump upon our atmosphere, upon the atmosphere of other planets, and
upon those of the sun and the stars, and would continue such action
until an equilibrium between the repulsive energy of the gas and the
gravitation of the solid orbs had been established. Atmospheric matter
would thus be universally diffused, with special accumulations around
solid orbs, varying in quantity with their respective gravitating
energy. Such a universal atmosphere would accelerate orbital motion,
and this acceleration would vary with the surface of bodies. Its action
being thus exactly opposed to that of radiant repulsion, it must, at a
certain density, exactly neutralize it. That it does this is evident
from the obedience of all the elements of the solar system to the
calculated action of gravitation; and thus Mr. Crookes’s researches
not only confirm the idea of universal atmospheric diffusion, but they
afford a means by which we may ultimately measure the actual density of
the universal atmosphere. If, as I have endeavored to show in my essay
on “The Fuel of the Sun,” the initial radiant energy of every star
depends upon its mass, and its consequent condensation of atmospheric
matter, the density of inter-planetary atmosphere sufficient to
neutralize the radiant mechanical energy of our sun may be the same
as is demanded to perform the same function for all the stars of the
universe, and all their attendant worlds, comets, and meteors.
In order to prevent misunderstanding of the above, I must add that I
have therein studiously assumed a negative position in reference to all
hypothetical conceptions of the nature of heat, light, etc., and their
modes of transmission, simply because I feel satisfied that the subject
has hitherto been obscured and complicated by overstrained efforts to
fit the phenomena to the excessively definite hypotheses of modern
molecular mathematicians. The atoms invented by Dalton for the purpose
of explaining the demonstrated laws of chemical combination performed
this function admirably, and had great educational value, so long as
their purely imaginary origin was kept in view; but when such atoms are
treated as facts, and physical dogmas are based upon the assumption of
their actual existence, they become dangerous physical superstitions.
Regarding matter as continuous, _i.e._, supposing it to be simply as it
appears to be, and co-extensive with the universe, in accordance with
the experimental evidences of the unlimited expansibility of gaseous
matter, we need only assume that our sensations of heat, light, etc.,
are produced by active conditions of such matter analogous to those
which are proved to produce our sensations of sound. On this basis
there is no difficulty in conceiving the rationale of the reaction
which produces the repulsion of the radiometer. I may even go further,
and affirm that it is impossible to rationally conceive radiation
producing any mechanical effects without mechanical reaction. If heat
be motion, and actual motion of actual matter, mechanical force must be
exerted to produce it, and a body which is warmer on one side than the
other, _i.e._, which is exerting more outward motion-producing force on
one side than on the other, must be subject to proportionally unequal
reaction, and, therefore, if free to move, must retreat in a direction
contrary to that of its greater activity. Regarded thus, the residual
air of the radiometer does act, not by collisions of particles between
the vane and inside of the glass vessel, but by the direct reaction
of the radiant energy which would operate irrespective of vessels,
_i.e._, upon naked radiometer vanes if carried halfway to the moon, or
otherwise freed from excess of atmospheric embarrassment.
The recent experiments of Mr. Crookes, showing retardation of the
radiometer with extreme exhaustion, seem to indicate that heat-rays,
like the electric discharge, demand a certain amount of atmospheric
matter as their carrier.
I cannot conclude these hasty and imperfect notes, written merely with
suggestive intent, without quoting a passage from the preface to the
“Correlation of Physical Forces,” which, though written so long ago,
appears to me worthy of the profoundest present consideration.
“It appears to me that heat and light maybe considered as affections;
or, according to the undulatory theory, vibrations of matter itself,
and not of a distinct ethereal fluid permeating it: these vibrations
would be propagated just as sound is propagated by vibrations of wood
or as waves by water. To my mind all the consequences of the undulatory
theory flow as easily from this as from the hypothesis of a specific
ether; to suppose which, namely, to suppose a fluid _sui generis_ and
of extreme tenuity penetrating solid bodies, we must assume, first,
the existence of the fluid itself; secondly, that bodies are without
exception porous; thirdly, that these pores communicate; fourthly, that
matter is limited in expansibility. None of these difficulties apply
to the modification of this theory which I venture to propose: and no
other difficulty applies to it which does not equally apply to the
received hypothesis.”