are converged to a focus of dazzling brilliancy. Placing in the path
of the rays, between the light and the focus, a vessel of water, and
introducing at the focus a piece of ice, the ice is not melted by the
concentrated beam. Matches, at the same place, are ignited, and wood
is set on fire. The powerful heat, then, of this luminous beam is
incompetent to melt the ice. On withdrawing the cell of water, the
ice immediately liquefies, and the water trickles from it in drops.
Reintroducing the cell of water, the fusion is arrested, and the drops
cease to fall. The transparent water of the cell exerts no sensible
absorption on the luminous rays, still it withdraws something from the
beam, which, when permitted to act, is competent to melt the ice. This
something is the dark radiation of the electric light. Again, I place
a slab of pure ice in front of the electric lamp; send a luminous beam
first through our cell of water and then through the ice. By means of
a lens an image of the slab is cast upon a white screen. The beam,
sifted by the water, has little power upon the ice. But observe what
occurs when the water is removed; we have here a star and there a
star, each star resembling a flower of six petals, and growing visibly
larger before our eyes. As the leaves enlarge, their edges become
serrated, but there is no deviation from the six-rayed type. We have
here, in fact, the crystallisation of the ice reversed by the
invisible rays of the electric beam. They take the molecules down in
this wonderful way, and reveal to us the exquisite atomic structure of
the substance with which Nature every winter roofs our ponds and
lakes.
Numberless effects, apparently anomalous, might be adduced in
illustration of the action of these lightless rays. These two
powders, for example, are both white, and undistinguishable from each
other by the eye. The luminous rays of the sun are unabsorbed by
both--from such rays these powders acquire no heat; still one of them,
sugar, is heated so highly by the concentrated beam of the electric
lamp, that it first smokes and then violently inflames, while the
other substance, salt, is barely warmed at the focus. Placing two
perfectly transparent liquids in test-tubes at the focus, one of them
boils in a couple of seconds, while the other, in a similar position,
is hardly warmed. The boiling-point of the first liquid is 78°C,
which is speedily reached; that of the second liquid is only 48°C,
which is never reached at all. These anomalies are entirely due to
the unseen element which mingles with the luminous rays of the
electric beam, and indeed constitutes 90 per cent. of its calorific
power.
A substance, as many of you know, has been discovered, by which these
dark rays may be detached from the total emission of the electric
lamp. This ray-filter is a liquid, black as pitch to the luminous,
but bright as a diamond to the non-luminous, radiation. It
mercilessly cuts off the former, but allows the latter free
transmission. When these invisible rays are brought to a focus, at a
distance of several feet from the electric lamp, the dark rays form an
invisible image of their source. By proper means, this image may be
transformed into a visible one of dazzling brightness. It might,
moreover, be shown, if time permitted, how, out of those perfectly
dark rays, could be extracted, by a process of transmutation, all the
colours of the solar spectrum. It might also be proved that those
rays, powerful as they are, and sufficient to fuse many metals, can be
permitted to enter the eye, and to break upon the retina, without
producing the least luminous impression.
The dark rays being thus collected, you see nothing at their place of
convergence. With a proper thermometer it could be proved that even
the air at the focus is just as cold as the surrounding air. And mark
the conclusion to which this leads. It proves the aether at the focus
to be practically detached from the air,--that the most violent
aethereal motion may there exist, without the least aerial motion.
But, though you see it not, there is sufficient heat at that focus to
set London on fire. The heat there is competent to raise iron to a
temperature at which it throws off brilliant scintillations. It can
heat platinum to whiteness, and almost fuse that refractory metal. It
actually can fuse gold, silver, copper, and aluminium. The moment,
moreover, that wood is placed at the focus it bursts into a blaze.
It has been already affirmed that, whether as regards radiation or
absorption, the elementary atoms possess but little power. This might
be illustrated by a long array of facts; and one of the most singular
of these is furnished by the deportment of that extremely combustible
substance, phosphorus, when placed at the dark focus. It is
impossible to ignite there a fragment of amorphous phosphorus. But
ordinary phosphorus is a far quicker combustible, and its deportment
towards radiant heat is still more impressive. It may be exposed to
the intense radiation of an ordinary fire without bursting into flame.
It may also be exposed for twenty or thirty seconds at an obscure
focus, of sufficient power to raise platinum to a red heat, without
ignition. Notwithstanding the energy of the aethereal waves here
concentrated, notwithstanding the extremely inflammable character of
the elementary body exposed to their action, the atoms of that body
refuse to partake of the motion of the powerful waves of low
refrangibility, and consequently cannot be affected by their heat.
The knowledge we now possess will enable us to analyse with profit a
practical question. White dresses are worn in summer, because they
are found to be cooler than dark ones. The celebrated Benjamin
Franklin placed bits of cloth of various colours upon snow, exposed
them to direct sunshine, and found that they sank to different depths
in the snow. The black cloth sank deepest, the white did not sink at
all. Franklin inferred from this experiment that black-bodies are the
best absorbers, and white ones the worst absorbers, of radiant heat.
Let us test the generality of this conclusion. One of these two cards
is coated with a very dark powder, and the other with a perfectly
white one. I place the powdered surfaces before a fire, and leave
them there until they have acquired as high a temperature as they can
attain in this position. Which of the cards is then most highly
heated? It requires no thermometer to answer this question. Simply
pressing the back of the card, on which the white powder is strewn,
against the cheek or forehead, it is found intolerably hot. Placing
the dark card in the same position, it is found cool. The white
powder has absorbed far more heat than the dark one. This simple
result abolishes a hundred conclusions which have been hastily drawn
from the experiments of Franklin. Again, here are suspended two
delicate mercurial thermometers at the same distance from a gas-flame.
The bulb of one of them is covered by a dark substance, the bulb of
the other by a white one. Both bulbs have received the radiation from
the flame, but the white bulb has absorbed most, and its mercury
stands much higher than that of the other thermometer. This
experiment might be varied in a hundred ways: it proves that from the
darkness of a body you can draw no certain conclusion regarding its
power of absorption.
The reason of this simply is, that colour gives us intelligence of
only one portion, and that the smallest one, of the rays impinging on
the coloured body. Were the rays all luminous, we might with
certainty infer from the colour of a body its power of absorption; but
the great mass of the radiation from our fire, our gas-flame, and even
from the sun itself, consists of invisible calorific rays, regarding
which colour teaches us nothing. A body may be highly transparent to
the one class of rays, and highly opaque to the other. Thus the white
powder, which has shown itself so powerful an absorber, has been
specially selected on account of its extreme perviousness to the
visible rays, and its extreme imperviousness to the invisible ones;
while the dark powder was chosen on account of its extreme
transparency to the invisible, and its extreme opacity to the visible,
rays. In the case of the radiation from our fire, about 98 per cent
of the whole emission consists of invisible rays; the body, therefore,
which was most opaque to these triumphed as an absorber, though that
body was a white one.
And here it is worth while to consider the manner in which we obtain
from natural facts what may be called their intellectual value.
Throughout the processes of Nature we have interdependence and
harmony; and the main value of physics, considered as a mental
discipline, consists in the tracing out of this interdependence, and
the demonstration of this harmony. The outward and visible phenomena
are the counters of the intellect; and our science would not be worthy
of its name and fame if it halted at facts, however practically
useful, and neglected the laws which accompany and rule the phenomena.
Let us endeavour, then, to extract from the experiment of Franklin all
that it can yield, calling to our aid the knowledge which our
predecessors have already stored. Let us imagine two pieces of cloth
of the same texture, the one black and the other white, placed upon
sunned snow. Fixing our attention on the white piece, let us enquire
whether there is any reason to expect that it will sink in the snow at
all. There is knowledge at hand which enables us to reply at once in
the negative. There is, on the contrary, reason to expect that, after
a sufficient exposure, the bit of cloth will be found on an eminence
instead of in a hollow; that instead of a depression, we shall have a
relative elevation of the bit of cloth. For, as regards the luminous
rays of the sun, the cloth and the snow are alike powerless; the one
cannot be warmed, nor the other melted, by such rays. The cloth is
white and the snow is white, because their confusedly mingled fibres
and particles are incompetent to absorb the luminous rays. Whether,
then, the cloth will sink or not depends entirely upon the dark rays
of the sun. Now the substance which absorbs these dark rays with the
greatest avidity is ice,--or snow, which is merely ice in powder.
Hence, a less amounts of heat will be lodged in the cloth than in the
surrounding snow. The cloth must therefore act as a shield to the
snow on which it rests; and, in consequence of the more rapid fusion
of the exposed snow, its shield must, in due time, be left behind,
perched upon an eminence like a glacier-table.
But though the snow transcends the cloth, both as a radiator and
absorber, it does not much transcend it. Cloth is very powerful in
both these respects. Let us now turn our attention to the piece of
black cloth, the texture and fabric of which I assume to be the same
as that of the white. For our object being to compare the effects of
colour, we must, in order to study this effect in its purity, preserve
all the other conditions constant. Let us then suppose the black
cloth to be obtained from the dyeing of the white. The cloth itself,
without reference to the dye, is nearly as good an absorber of heat as
the snow around it. But to the absorption of the dark solar rays by
the undyed cloth, is now added the absorption of the whole of the
luminous rays, and this great additional influx of heat is far more
than sufficient to turn the balance in favour of the black cloth. The
sum of its actions on the dark and luminous rays, exceeds the action
of the snow on the dark rays alone. Hence the cloth will sink in the
snow, and this is the complete analysis of Franklin's experiments.
Throughout this discourse the main stress has been laid on chemical
constitution, as influencing most powerfully the phenomena of
radiation and absorption.
With regard to gases and vapours, and to the liquids from which these
vapours are derived, it has been proved by the most varied and
conclusive experiments that the acts of radiation and absorption are
molecular--that they depend upon chemical, and not upon mechanical,
condition. In attempting to extend this principle to solids I was met
by a multitude of facts, obtained by celebrated experimenters, which
seemed flatly to forbid such an extension. Mellon, for example, had
found the same radiant and absorbent power for chalk and lamp-black.
MM. Masson and Courtépée had performed a most elaborate series of
experiments on chemical precipitates of various kinds, and found that
they one and all manifested the same power of radiation. They
concluded from their researches, that when bodies are reduced to an
extremely fine state of division, the influence of this state is so
powerful as entirely to mask and override whatever influence may be
due to chemical constitution.
But it appears to me that through the whole of these researches an
oversight has run, the mere mention of which will show what caution is
essential in the operations of experimental philosophy; while an
experiments or two will make clear wherein the oversight consists.
Filling a brightly polished metal cube with boiling water, I determine
the quantity of heat emitted by two of the bright surfaces. As a
radiator of heat one of them far transcends the other. Both surfaces
appear to be metallic; what, then, is the cause of the observed
difference in their radiative power? Simply this: one of the surfaces
is coated with transparent gum, through which, of course, is seen the
metallic lustre behind; and this varnish, though so perfectly
transparent to luminous rays, is as opaque as pitch, or lamp-black, to
non-luminous ones. It is a powerful emitter of dark rays; it is also
a powerful absorber. While, therefore, at the present moment, it is
copiously pouring forth radiant heat itself, it does not allow a
single ray from the metal behind to pass through it. The varnish
then, and not the metal, is the real radiator.
Now Melloni, and Masson, and Courtépée experimented thus: they mixed
their powders and precipitates with gum-water, and laid them, by means
of a brush, upon the surfaces of a cube like this. True, they saw
their red powders red, their white ones white, and their black ones
black, but they saw these colours _through the coat of varnish which
surrounded every particle_. When, therefore, it was concluded that
colour had no influence on radiation, no chance had been given to it
of asserting its influence; when it was found that all chemical
precipitates radiated alike, it was the radiation from a varnish,
common to them all, which showed the observed constancy. Hundreds,
perhaps thousands, of experiments on radiant heat have been performed
in this way, by various enquirers, but the work will, I fear, have to
be done over again. I am not, indeed, acquainted with an instance in
which an oversight of so trivial a character has been committed by so
many able men in succession, vitiating so large an amounts of
otherwise excellent work. Basing our reasonings thus on demonstrated
facts, we arrive at the extremely probable conclusion that the
envelope of the particles, and not the particles themselves, was the
real radiator in the experiments just referred to. To reason thus,
and deduce their more or less probable consequences from experimental
facts, is an incessant exercise of the student of physical science.
But having thus followed, for a time, the light of reason alone
through a series of phenomena, and emerged from them with a purely
intellectual conclusion, our duty is to bring that conclusion to an
experimental test. In this way we fortify our science.
For the purpose of testing our conclusion regarding the influence of
the gum, I take two powders presenting the same physical appearance;
one of them is a compound of mercury, and the other a compound of
lead. On two surfaces of a cube are spread these bright red powders,
without varnish of any kind. Filling the cube with boiling water, and
determining the radiation from the two surfaces, one of them is found
to emit thirty-nine units of heat, while the other emits seventy-four.
This, surely, is a great difference. Here, however, is a second cube,
having two of its surfaces coated with the same powders, the only
difference being that the powders are laid on by means of a
transparent gum. Both surfaces are now absolutely alike in radiative
power. Both of them emit somewhat more than was emitted by either of
the unvarnished powders, simply because the gum employed is a better
radiator than either of them. Excluding all varnish, and comparing
white with white, vast differences are found; comparing black with
black, they are also different; and when black and white are compared,
in some cases the black radiates far more than the white, while in
other cases the white radiates far more than the black. Determining,
moreover, the absorptive power of those powders, it is found to go
hand-in-hand with their radiative power. The good radiator is a good
absorber, and the bad radiator is a bad absorber. From all this it is
evident that as regards the radiation and absorption of non-luminous
heat, colour teaches us nothing; and that even as regards the
radiation of the sun, consisting as it does mainly of non-luminous
rays, conclusions as to the influence of colour may be altogether
delusive. This is the strict scientific upshot of our researches. But
it is not the less true that in the case of wearing apparel--and this
for reasons which I have given in analysing the experiments of
Franklin--black dresses are more potent than white ones as absorbers
of solar heat.
Thus, in brief outline, have been brought before you a few of the
results of recent enquiry. If you ask me what is the use of them, I
can hardly answer you, unless you define the term use. If you meant
to ask whether those dark rays which clear away the Alpine snows, will
ever be applied to the roasting of turkeys, or the driving of
steam-engines--while affirming their power to do both, I would frankly
confess that they are not at present capable of competing profitably
with coal in these particulars. Still they may have great uses
unknown to me; and when our coal-fields are exhausted, it is possible
that a more aethereal race than we are may cook their victuals, and
perform their work, in this transcendental way. But is it necessary
that the student of science should have his labours tested by their
possible practical applications? What is the practical value of
Homer's Iliad? You smile, and possibly think that Homer's Iliad is
good as a means of culture. There's the rub. The people who demand
of science practical uses, forget, or do not know, that it also is
great as a means of culture--that the knowledge of this wonderful
universe is a thing profitable in itself, and requiring no practical
application to justify its pursuit.
But while the student of Nature distinctly refuses to have his labours
judged by their practical issues, unless the term practical be made to
include mental as well as material good, he knows full well that the
greatest practical triumphs have been episodes in the search after
pure natural truth. The electric telegraph is the standing wonder of
this age, and the men whose scientific knowledge, and mechanical
skill, have made the telegraph what it is, are deserving of all
honour. In fact, they have had their reward, both in reputation and
in those more substantial benefits which the direct service of the
public always carries in its train. But who, I would ask, put the
soul into this telegraphic body? Who snatched from heaven the fire
that flashes along the line? This, I am bound to say, was done by two
men, the one a dweller in Italy, [Footnote: Volta] the other a
dweller in England, [Footnote: Faraday] who never in their enquiries
consciously set a practical object before them--whose only stimulus
was the fascination which draws the climber to a never-trodden peak,
and would have made Caesar quit his victories for the sources of the
Nile. That the knowledge brought to us by those prophets, priests,
and kings of science is what the world calls 'useful knowledge,' the
triumphant application of their discoveries proves. But science has
another function to fulfil, in the storing and the training of the
human mind; and I would base my appeal to you on the specimen which
has this evening been brought before you, whether any system of
education at the present day can be deemed even approximately
complete, in which the knowledge of Nature is neglected or ignored.
********************
IV. NEW CHEMICAL REACTIONS PRODUCED BY LIGHT.
1868-69.
1 DECOMPOSITION BY LIGHT.
MEASURED by their power, not to excite vision, but to produce heat--in
other words, measured by their absolute energy--the ultra-red waves of
the sun and of the electric light, as shown in the preceding articles,
far transcend the visible. In the domain of chemistry, however, there
are numerous cases in which the more powerful waves are ineffectual,
while the more minute waves, through what may be called their
timeliness of application, are able to produce great effects. A
series of these, of a novel and beautiful character, discovered in
1868, and further illustrated in subsequent years, may be exhibited by
subjecting the vapours of volatile liquids to the action of
concentrated sunlight, or to the concentrated beam of the electric
light. Their investigation led up to the discourse on 'Dust and
Disease' which follows in this volume; and for this reason some
account of them is introduced here.
*****
A glass tube 3 feet long and 3 inches wide, which had been frequently
employed in my researches on radiant heat, was supported horizontally
on two stands. At one end of the tube was placed an electric lamp,
the height and position of both being so arranged, that the axis of
the tube, and that of the beam issuing from the lamp, were coincident.
In the first experiments the two ends of the tube were closed by
plates of rock-salt, and subsequently by plates of glass. For the
sake of distinction, I call this tube the experimental tube. It was
connected with an air-pump, and also with a series of drying and other
tubes used for the purification of the air.
A number of test-tubes, like F, fig. 2 (I have used at least fifty of
them), were converted into Woulf's flasks. Each of them was stopped
by a cork, through which passed two glass tubes: one of these tubes
(a) ended immediately below the cork, while the other (b) descended to
the bottom of the flask, being drawn out at its lower end to an
orifice about 0.03 of an inch in diameter. It was found necessary to
coat the cork carefully with cement. In the later experiments corks
of vulcanised India-rubber were invariably employed.
The little flask, thus formed, being partially filled with the liquid
whose vapour was to be examined, was introduced into the path of the
purified current of air. The experimental tube being exhausted, and
the cock which cut off the supply of purified air being cautiously
turned on, the air entered the flask through the tube b, and escaped
by the small orifice at the lower end of b into the liquid. Through
this it bubbled, loading itself with vapour, after which the mixed air
and vapour, passing from the flask by the tube a, entered the
experimental tube, where they were subjected to the action of light.
The whole arrangement is shown in fig. 3, where L represents the
electric lamp, ss' the experimental tube, pp' the pipe leading to the
air-pump, and F the test-tube containing the volatile liquid. The
tube tt' is plugged with cotton-wool intended to intercept the
floating matter of the air; the bent tube T' contains caustic potash,
the tube T sulphuric acid, the one intended to remove the carbonic
acid and the other the aqueous vapour of the air.
The power of the electric beam to reveal the existence of anything
within the experimental tube, or the impurities of the tube itself, is
extraordinary. When the experiments is made in a darkened room, a
tube which in ordinary daylight appears absolutely clean, is often
shown by the present mode of examination to be exceedingly filthy.
The following are some of the results obtained with this arrangement:
Nitrite of amyl. The vapour of this liquid was in the first instance
permitted to enter the experimental tube, while the beam from the
electric lamp was passing through it. Curious clouds, the cause of
which was then unknown, were observed to form near the place of entry,
being afterwards whirled through the tube.