experiments on permanent gases, and we have now to turn our attention
to the vapours of volatile liquids. Here, as in the case of the
gases, vast differences have been proved to exist between various
kinds of molecules, as regards their power of intercepting the
calorific waves. While some vapours allow the waves a comparatively
free passage, the faintest mixture of other vapours causes a
deflection of the magnetic needle. Assuming the absorption effected
by air, at a pressure of one atmosphere, to be unity, the following
are the absorptions effected by a series of vapours at a pressure of
1/60th of an atmosphere:
Name of vapour Absorption
Bisulphide of carbon 47
Iodide of methyl 115
Benzol 136
Amylene 321
Sulphuric ether 440
Formic ether 548
Acetic ether 612
Bisulphide of carbon is the most transparent vapour in this list; and
acetic ether the most opaque; 1/60th of an atmosphere of the former,
however, produces 47 times the effect of a whole atmosphere of air,
while 1/60th of an atmosphere of the latter produces 612 times the
effect of a whole atmosphere of air. Reducing dry air to the pressure
of the acetic ether here employed, and comparing them then together,
the quantity of wave-motion intercepted by the ether would be many
thousand times that intercepted by the air.
Any one of these vapours discharged into the free atmosphere, in front
of a body emitting obscure rays, intercepts more or less of the
radiation. A similar effect is produced by perfumes diffused in the
air, though their attenuation is known to be almost infinite.
Carrying, for example, a current of dry air over bibulous paper,
moistened by patchouli, the scent taken up by the current absorbs 30
times the quantity of heat intercepted by the air which carries it;
and yet patchouli acts more feebly on radiant heat than any other
perfume yet examined.
Here follow the results obtained with various essential oils, the
odour, in each case, being carried by a current of dry air into the tube
already employed for gases and vapours:
Name of perfume Absorption
Patchouli 30
Sandal wood 32
Geranium 33
Oil of cloves 34
Otto of roses 37
Bergamot 44
Neroli 47
Lavender 60
Lemon 65
Portugal 67
Thyme 68
Rosemary 74
Oil of laurel 80
Camomile flowers 87
Cassia 109
Spikenard 355
Aniseed 372
Thus the absorption by a tube full of dry air being 1, that of the
odour of patchouli diffused in it is 30, at of lavender 60, that of
rosemary 74, whilst that of aniseed amounts to 372. It would be idle
to speculate the quantities of matter concerned in these actions.
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12. Aqueous Vapour in relation to the Terrestrial Temperatures.
We are now fully prepared for a result which, without such
preparation, might appear incredible. Water is, to some extent, a
volatile body, and our atmosphere, resting as it does upon the surface
of the ocean, receives from it a continual supply of aqueous vapour.
It would be an error to confound clouds or fog or any visible mist
with the vapour of water, which is a perfectly impalpable gas,
diffused, even on the clearest days, throughout the atmosphere.
Compared with the great body of the air, the aqueous vapour it
contains is of almost infinitesimal amount, 99.5 out of every 100
parts of the atmosphere being composed of oxygen and nitrogen. In the
absence of experiment, we should never think of ascribing to this
scant and varying constituent any important influence on terrestrial
radiation; and yet its influence is far more potent than that of the
great body of the air. To say that on a day of average humidity in
England, the atmospheric vapour exerts 100 times the action of the air
itself, would certainly be an understatement of the fact. Comparing a
single molecule of aqueous vapour with an atom of either of the main
constituents of our atmosphere, I am not prepared to say how many
thousand times the action of the former exceeds that of the latter.
But it must be borne in mind that these large numbers depend, in part,
on the extreme feebleness of the air; the power of aqueous vapour
seems vast, because that of the air with which it is compared is
infinitesimal. Absolutely considered, however, this substance,
notwithstanding its small specific gravity, exercises a very potent
action. Probably from 10 to 15 per cent. of the heat radiated from
the earth is absorbed within 10 or 20 feet of the earth's surface.
This must evidently be of the utmost consequence to the life of the
world. Imagine the superficial molecules of the earth agitated with
the motion of heat, and imparting it to the surrounding aether; this
motion would be carried rapidly away, and lost for ever to our planet,
if the waves of aether had nothing but the air to contend with in
their outward course. But the aqueous vapour takes up the motion, and
becomes hereby heated, thus wrapping the earth like a warm garment,
and protecting its surface from the deadly chill which it would
otherwise sustain. Various philosophers have speculated on the
influence of an atmospheric envelope. De Saussure, Fourier,
M. Pouillet, and Mr. Hopkins have, one and all, enriched scientific
literature with contributions on this subject, but the considerations
which these eminent men have applied to atmospheric air, have, if my
experiments be correct, to be transferred to the aqueous vapour.
The observations of meteorologists furnish important, though hitherto
unconscious evidence of the influence of this agent. Wherever the air
is dry we are liable to daily extremes of temperature. By day, such
places, the sun's heat reaches the earth unimpeded, and renders the
maximum high; by night, on the other hand, the earth's heat escapes
unhindered to space, and renders the minimum low. Hence the
difference between the maximum and minimum is greatest where the air
is driest. In the plains of India, the heights of the Himalaya, in
central Asia, in Australia--wherever drought reigns, we have the heat
of day forcibly contrasted with the chill of night. In the Sahara
itself, when the sun's rays cease to impinge on the burning soil, the
temperature runs rapidly down to freezing, because there is no vapour
overhead to check the calorific drain. And here another instance
might be added to the numbers already known, in which nature tends as
it were to check her own excess. By nocturnal refrigeration, the
aqueous vapour of the air is condensed to water on the surface of the
earth; and, as only the superficial portions radiate, the act of
condensation makes water the radiating body. Now experiment proves
that to the rays emitted by water, aqueous vapour is especially
opaque. Hence the very act of condensation, consequent on terrestrial
cooling, becomes a safeguard to the earth, imparting to its radiation
that particular character which renders it most liable to be prevented
from escaping into space.
It might however be urged that, inasmuch as we derive all our heat
from the sun, the selfsame covering which protects the earth from
chill must also shut out the solar radiation. This is partially true,
but only partially; the sun's rays are different in quality from the
earth's rays, and it does not at all follow that the substance which
absorbs the one must necessarily absorb the other. Through a layer of
water, for example, one tenth of an inch in thickness, the sun's rays
are transmitted with comparative freedom; but through a layer half
this thickness, as Melloni has proved, no single ray from the warmed
earth could pass. In like manner, the sun's rays pass with
comparative freedom through the aqueous vapour of the air: the
absorbing power of this substance being mainly exerted upon the
invisible heat that endeavours to escape from the earth. In
consequence of this differential action upon solar and terrestrial
heat, the mean temperature of our planet is higher than is due to its
distance from the sun.
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13. Liquids and their Vapours in relation to Radiant Heat.
The deportment here assigned to atmospheric vapour has been
established by direct experiments on it taken from the streets and
parks of London, from the downs of Epsom, from the hills and sea-beach
of the Isle of Wight, and also by experiments on air in the first
instance dried, and afterwards rendered artificially humid by pure
distilled water. It has also been established in the following way: Ten
volatile liquids were taken at random and the power of these liquids, at a
common thickness, to intercept the waves of heat, was carefully
determined. The vapours of the liquids were next taken, in quantities
proportional to the quantities of liquid, and the power of the vapours
intercept the waves of heat was also determined.
Commencing with the substance which exerted the least absorptive
power, and proceeding onwards to the most energetic, the following
order of absorption was observed:
Liquids Vapours
Bisulphide of carbon. Bisulphide of carbon.
Chloroform. Chloroform.
Iodide of methyl. Iodide of methyl.
Iodide of ethyl. Iodide of ethyl.
Benzol. Benzol.
Amylene. Amylene.
Sulphuric aether. Sulphuric aether.
Acetic aether. Acetic aether.
Formic aether. Formic aether.
Alcohol. Alcohol.
Water.
We here find the order of absorption in both cases be the same. We
have liberated the molecules from the bonds which trammel them more or
less in a liquid condition; but this change in their state of
aggregation does not change their relative powers of absorption.
Nothing could more clearly prove that the act of absorption depends
upon the individual molecule, which equally asserts its power in the
liquid and the gaseous state. We may safely conclude from the above
table that the position of a vapour is determined by that of its
liquid. Now at the very foot of the list of liquids stands _water_,
signalising itself above all others by its enormous power of
absorption. And from this fact, even if no direct experiment on the
vapour of water had ever been made, we should be entitled to rank that
vapour as our most powerful absorber of radiant heat. Its
attenuation, however, diminishes its action. I have proved that a
shell of air two inches in thickness surrounding our planet, and
saturated with the vapour of sulphuric aether, would intercept 35 per
cent. of the earth's radiation. And though the quantity of aqueous
vapour necessary to saturate air is much less than the amount of
sulphuric aether vapour which it can sustain, it is still extremely
probable that the estimate already made of the action of atmospheric
vapour within 10 feet of the earth's surface, is under the mark; and
that we are indebted to this wonderful substance, to an extent not
accurately determined, but certainly far beyond what has hitherto been
imagined, for the temperature now existing at the surface of the
globe.
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14. Reciprocity of Radiation and Absorption.
Throughout the reflections which have hitherto occupied us, the image
before the mind has been that of a radiant source sending forth
calorific waves, which on passing among the molecules of a gas or
vapour were intercepted by those molecules in various degrees. In all
cases it was the transference of motion from the aether to the
comparatively quiescent molecules of the gas or vapour that occupied
our thoughts. We have now to change the form of our conception, and
to figure these molecules not as absorbers but as radiators, not as
the recipients but as the originators of wave-motion. That is to say,
we must figure them vibrating, and generating in the surrounding
aether undulations which speed through it with the velocity of light.
Our object now is to enquire whether the act of chemical combination,
which proves so potent as regards the phenomena of absorption, does
not also manifest its power in the phenomena of radiation. For the
examination of this question it is necessary, in the first place, to
heat our gases and vapours to the same temperature, and then examine
their power of discharging the motion thus imparted to them upon the
aether in which they swing.
A heated copper ball was placed above a ring gas-burner possessing a
great number of small apertures, the burner being connected by a tube
with vessels containing the various gases to be examined. By gentle
pressure the gases were forced through the orifices of the burner
against the copper ball, where each of them, being heated, rose in an
ascending column. A thermoelectric pile, entirely screened from the
hot ball, was exposed to the radiation of the warm gas, while the
deflection of a magnetic needle connected with the pile declared the
energy of the radiation.
By this mode of experiment it was proved that the selfsame molecular
arrangement which renders a gas a powerful absorber, renders it a
powerful radiator--that the atom or molecule which is competent to
intercept the calorific waves is, in the same degree, competent to
send them forth. Thus, while the atoms of elementary gases proved
themselves unable to emit any sensible amount of radiant heat, the
molecules of compound gases were shown to be capable of powerfully
disturbing the surrounding aether. By special modes of experiment the
same was proved to hold good for the vapours of volatile liquids, the
radiative power of every vapour being found proportional to its
absorptive power.
The method of experiment here pursued, though not of the simplest
character, is still easy to grasp. When air is permitted to rush into
an exhausted tube, the temperature of the air is raised to a degree
equivalent to the _vis viva_ extinguished. [Footnote: See above for a
definition of _vis viva_.] Such air is said to be dynamically heated,
and, if pure, it shows itself incompetent to radiate, even when a
rock-salt window is provided for the passage of its rays. But if
instead of being empty the tube contain a small quantity of vapour,
the warmed air communicates its heat by contact to the vapour, the
molecules of which convert into the radiant form the heat imparted to
them by the atoms of the air. By this process also, which I have
called Dynamic Radiation, the reciprocity of radiation and absorption
has been conclusively proved. [Footnote: When heated air imparts its
motion to another gas or vapour, the transference of heat is
accompanied by a change of vibrating period. The Dynamic Radiation of
vapours is rendered possible by this transmutation of vibrations.]
In the excellent researches of Leslie, De la Provostaye and Detains,
and Balfour Stewart, the same reciprocity, as regards solid bodies,
has been variously illustrated; while the labours, theoretical and
experimental, of Kirchhoff have given this subject a wonderful
expansion, and enriched it by applications of the highest kind. To
their results are now to be added the foregoing, whereby gases and
vapours, which have been hitherto thought inaccessible to experiments
with the thermo-electric pile, are proved by it to exhibit the
indissoluble duality of radiation and absorption, the influence of
chemical combination on both being exhibited in the most decisive and
extraordinary way.
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15. Influence of Vibrating Period and Molecular Form. Physical
Analysis of the Human Breath.
In the foregoing experiments with gases and vapours have employed
throughout invisible rays, and found some of these bodies so impervious
to radiant heat, that lengths of a few feet they intercept every ray
as actually as a layer of pitch. The substances, however, which show
themselves thus opaque to radiant heat perfectly transparent to light.
Now the rays of light differ from those of invisible heat merely in
point of period, the former failing to affect the retina because their
periods of recurrence are too slow. Hence, in one way or other, the
transparency of our gases and vapours depends upon the periods of the
waves which impinge upon them. What is the nature of this dependence?
The admirable researches of Kirchhoff help us an answer. The atoms
and molecules of every gas have certain definite rates of oscillation,
and those waves aether are most copiously absorbed whose periods
recurrence synchronise with those of the atomic groups amongst which they
pass. Thus, when we find invisible rays absorbed and the visible ones
transmitted by a layer of gas, we conclude that the oscillating
periods of the atoms constituting the gaseous molecules coincide with
those of the invisible, and not with those of the visible spectrum.
It requires some discipline of the imagination to form a clear picture
of this process. Such a picture is, however, possible, and ought to
be obtained. When the waves of aether impinge upon molecules whose
periods of vibration coincide with the recurrence of the undulations,
the timed strokes of the waves augment the vibration of the molecules,
as a heavy pendulum is set in motion by well-timed puffs of breath.
Millions of millions of shocks are received every second from the
calorific waves; and it is not difficult to see that as every wave
arrives just in time to repeat the action of its predecessor, the
molecules must finally be caused to swing through wider spaces than if
the arrivals were not so timed. In fact, it is not difficult to see
that an assemblage of molecules, operated upon by contending waves,
might remain practically quiescent. This is actually the case when
the waves of the visible spectrum pass through a transparent gas or
vapour. There is here no sensible transference of motion from the
aether to the molecules; in other words, there is no sensible
absorption of heat.
One striking example of the influence of period may be here recorded.
Carbonic acid gas is one of the feeblest absorbers of the radiant heat
emitted by solid bodies. It is, for example, to a great extent
transparent to the rays emitted by the heated copper plate already
referred to. There are, however, certain rays, comparatively few in
number, emitted by the copper, to which the carbonic acid is
impervious; and could we obtain a source of heat emitting such rays
only, we should find carbonic acid more opaque to the radiation from
that source, than any other gas. Such a source is actually found in
the flame of carbonic oxide, where hot carbonic acid constitutes the
main radiating body. Of the rays emitted by our heated plate of
copper, olefiant gas absorbs ten times the quantity absorbed by
carbonic acid. Of the rays emitted by a carbonic oxide flame,
carbonic acid absorbs twice as much as olefiant gas. This wonderful
change in the power of the former, as an absorber, is simply due to
the fact, that the periods of the hot and cold carbonic acid are
identical, and that the waves from the flame freely transfer their
motion to the molecules which synchronise with them. Thus it is that
the tenth an atmosphere of carbonic acid, enclosed in a tube four feet
long, absorbs 60 per cent. of the radiation from carbonic oxide
flame, while one-thirtieth of an atmosphere absorbs 48 per cent. of
the heat from the same source.
In fact, the presence of the minutest quantity of carbonic acid may be
detected by its action on the rays from the carbonic oxide flame.
Carrying, for example, the dried human breath into a tube four feet
long, the absorption there effected by the carbonic acid of the breath
amounts to 50 per cent. of the entire radiation. Radiant heat may
indeed be employed as a means of determining practically the amount of
carbonic acid expired from the lungs. My late assistant, Mr. Barrett,
while under my direction, made this determination. The absorption
produced by the breath freed from its moisture, but retaining its
carbonic acid, was first determined. Carbonic acid, artificially
prepared, was then mixed with dry air in such proportions that the
action of the mixture upon the rays of heat was the same as that of
the dried breath. The percentage of the former being known,
immediately gave that of the latter. The same breath, analysed
chemically by Dr. Frankland, and physically by Mr. Barrett, gave the
following results:
Percentage of Carbonic Acid in the Human Breath.
Chemical analysis Physical analysis
4.66 4.56
5.33 5.22
It is thus proved that in the quantity of aethereal motion which it is
competent to take up, we have a practical measure of the carbonic acid
of the breath, and hence of the combustion going on in the human
lungs.
Still this question of period, though of the utmost importance, is not
competent to account for the whole of the observed facts. The aether,
as far as we know, accepts vibrations of all periods with the same
readiness. To it the oscillations of an atom of free oxygen are just
as acceptable as those of the atoms in a molecule of olefiant gas;
that the vibrating oxygen then stands so far below the olefiant gas in
radiant power must be referred not to period, but to some other
peculiarity. The atomic group which constitutes the molecule of
olefiant gas, produces many thousand times the disturbance caused by
the oxygen, it may be because the group is able to lay a vastly more
powerful hold upon the aether than single atoms can. Another, and
probably very potent cause of the difference may be, that the
vibrations, being those of the constituent atoms of the molecule,
[Footnote: See 'Physical Considerations,' Art. iv.] are generated
in highly condensed aether, which acts like condensed air upon sound.
But whatever may be the fate of these attempts to visualise the physics
of the process, it will still remain true, that to account for the
phenomena of radiation and absorption we must take into consideration
the shape, size, and condition of the aether within the molecules, by
which the external aether is disturbed.
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16. Summary and Conclusion.
Let us now cast a momentary glance over the ground that we have left
behind. The general nature of light and heat was first briefly
described: the compounding of matter from elementary atoms, and the
influence of the act of combination on radiation and absorption, were
considered and experimentally illustrated. Through the transparent
elementary gases radiant heat was found to pass as through a vacuum,
while many of the compound gases presented almost impassable obstacles
to the calorific-waves. This deportment of the simple gases directed
our attention to other elementary bodies, the examination of which led
to the discovery that the element iodine, dissolved in bisulphide of
carbon, possesses the power of detaching, with extraordinary sharpness,
the light of the spectrum from its heat, intercepting all luminous
rays up to the extreme red, and permitting the calorific rays beyond
the red to pass freely through it. This substance was then employed
to filter the beams of the electric light, and to form foci of
invisible rays so intense as to produce almost all the effects
obtainable in ordinary fire. Combustible bodies were burnt, and
refractory ones were raised to a white heat, by the concentrated
invisible rays. Thus, by exalting their refrangibility, the invisible
rays of the electric light were rendered visible, and all the colours
of the solar spectrum were extracted from utter darkness. The extreme
richness of the electric light in invisible rays of low refrangibility
was demonstrated, one-eighth only of its radiation consisting of
luminous rays. The deadness of the optic nerve to those invisible
rays was proved, and experiments were then added to show that the
bright and the dark rays of a solid body, raised gradually to
incandescence, are strengthened together; intense dark heat being an
invariable accompaniment of intense white heat. A sun could not be
formed, or a meteorite rendered luminous, on any other condition. The
light-giving rays constituting only a small fraction of the total
radiation, their unspeakable importance to us is due to the fact, that
their periods are attuned to the special requirements of the eye.