Six Lectures on Light Part 9

[Ill.u.s.tration: Fig. 48.]

We will now adopt stronger measures with the radiation. In this larger camera of blackened tin is placed a lamp, in all particulars similar to those already employed. But instead of gathering up the rays from the carbon-points by a condensing lens, we gather them up by a concave mirror (_m_ _m'_, fig. 48), silvered in front and placed behind the carbons (P). By this mirror we can cause the rays to issue through the orifice in front of the camera, either parallel or convergent. They are now parallel, and therefore to a certain extent diffused. We place a convex lens (L) in the path of the beam; the light is converged to a focus (C), and at that focus paper is not only pierced, but it is instantly set ablaze.

Many metals may be burned up in the same way. In our first lecture the combustibility of zinc was mentioned. Placing a strip of sheet-zinc at this focus, it is instantly ignited, burning with its characteristic purple flame. And now I will subst.i.tute for our gla.s.s lens (L) one of a more novel character. In a smooth iron mould a lens of pellucid ice has been formed. Placing it in the position occupied a moment ago by the gla.s.s lens, I can see the beam brought to a sharp focus. At the focus I place, a bit of black paper, with a little gun-cotton folded up within it. The paper immediately ignites and the cotton explodes. Strange, is it not, that the beam should possess such heating power after having pa.s.sed through so cold a substance? In his arctic expeditions Dr. Scoresby succeeded in exploding gunpowder by the sun's rays, converged by large lenses of ice; here we have succeeded in producing the effect with a small lens, and with a terrestrial source of heat.

In this experiment, you observe that, before the beam reaches the ice-lens, it has pa.s.sed through a gla.s.s cell containing water. The beam is thus sifted of const.i.tuents, which, if permitted to fall upon the lens, would injure its surface, and blur the focus. And this leads me to say an antic.i.p.atory word regarding transparency. In our first lecture we entered fully into the production of colours by absorption, and we spoke repeatedly of the quenching of the rays of light. Did this mean that the light was altogether annihilated? By no means. It was simply so lowered in refrangibility as to escape the visual range.

It was converted into heat. Our red ribbon in the green of the spectrum quenched the green, but if suitably examined its temperature would have been found raised. Our green ribbon in the red of the spectrum quenched the red, but its temperature at the same time was augmented to a degree exactly equivalent to the light extinguished.

Our black ribbon, when pa.s.sed through the spectrum, was found competent to quench all its colours; but at every stage of its progress an amount of heat was generated in the ribbon exactly equivalent to the light lost. It is only when _absorption_ takes place that heat is thus produced: and heat is always a result of absorption.

Examine the water, then, in front of the lamp after the beam has pa.s.sed through it: it is sensibly warm, and, if permitted to remain there long enough, it might be made to boil. This is due to the absorption, by the water, of a certain portion of the electric beam.

But a portion pa.s.ses through unabsorbed, and does not at all contribute to the heating of the water. Now, ice is also in great part transparent to these latter rays, and therefore is but little melted by them. Hence, by employing the portion of the beam transmitted by water, we are able to keep our lens intact, and to produce by means of it a sharply defined focus. Placed at that focus, white paper is not ignited, because it fails to absorb the rays emergent from the ice-lens. At the same place, however, black paper instantly burns, because it absorbs the transmitted light.

And here it may be useful to refer to an estimate by Newton, based upon doubtful data, but repeated by various astronomers of eminence since his time. The comet of 1680, when nearest to the sun, was only a sixth of the sun's diameter from his surface. Newton estimated its temperature, in this position, to be more than two thousand times that of molted iron. Now it is clear from the foregoing experiments that the temperature of the comet could not be inferred from its nearness to the sun. If its power of absorption were sufficiently low, the comet might carry into the sun's neighbourhood the chill of stellar s.p.a.ce.

-- 4. _Combustion of a Diamond by Radiant Heat_.

The experiment of burning a diamond in oxygen by the concentrated rays of the sun was repeated at Florence, in presence of Sir Humphry Davy, on Tuesday, the 27th of March, 1814. It is thus described by Faraday:--'To-day we made the grand experiment of burning the diamond, and certainly the phenomena presented were extremely beautiful and interesting. A gla.s.s globe containing about 22 cubical inches was exhausted of air, and filled with pure oxygen. The diamond was supported in the centre of this globe. The Duke's burning-gla.s.s was the instrument used to apply heat to the diamond. It consists of two double convex lenses, distant from each other about 3 feet; the large lens is about 14 or 15 inches in diameter, the smaller one about 3 inches in diameter. By means of the second lens the focus is very much reduced, and the heat, when the sun s.h.i.+nes brightly, rendered very intense. The diamond was placed in the focus and anxiously watched. On a sudden Sir H. Davy observed the diamond to burn visibly, and when removed from the focus it was found to be in a state of active and rapid combustion.'

The combustion of the diamond had never been effected by radiant heat from a terrestrial source. I tried to accomplish this before crossing the Atlantic, and succeeded in doing so. The small diamond now in my hand is held by a loop of platinum wire. To protect it as far as possible from air currents, and also to concentrate the heat upon it, it is surrounded by a hood of sheet platinum. Bringing a jar of oxygen underneath, I cause the focus of the electric beam to fall upon the diamond. A small fraction of the time expended in the experiment described by Faraday suffices to raise the diamond to a brilliant red.

Plunging it then into the oxygen, it glows like a little white star; and it would continue to burn and glow until wholly consumed. The focus can also be made to fall upon the diamond in oxygen, as in the Florentine experiment: the result is the same. It was simply to secure more complete mastery over the position of the focus, so as to cause it to fall accurately upon the diamond, that the mode of experiment here described was resorted to.

-- 5. _Ultra-red Rays: Calorescence_.

In the path of the beam issuing from our lamp I now place a cell with gla.s.s sides containing a solution of alum. All the _light_ of the beam pa.s.ses through this solution. This light is received on a powerfully converging mirror silvered in front, and brought to a focus by the mirror. You can see the conical beam of reflected light tracking itself through the dust of the room. A sc.r.a.p of white paper placed at the focus s.h.i.+nes there with dazzling brightness, but it is not even charred. On removing the alum cell, however, the paper instantly inflames. There must, therefore, be something in this beam besides its light. The _light_ is not absorbed by the white paper, and therefore does not burn the paper; but there is something over and above the light which _is_ absorbed, and which provokes combustion. What is this something?

In the year 1800 Sir William Herschel pa.s.sed a thermometer through the various colours of the solar spectrum, and marked the rise of temperature corresponding to each colour. He found the heating effect to augment from the violet to the red; he did not, however, stop at the red, but pushed his thermometer into the dark s.p.a.ce beyond it.

Here he found the temperature actually higher than in any part of the visible spectrum. By this important observation, he proved that the sun emitted heat-rays which are entirely unfit for the purposes of vision. The subject was subsequently taken up by Seebeck, Melloni, Muller, and others, and within the last few years it has been found capable of unexpected expansions and applications. I have devised a method whereby the solar or electric beam can be so _filtered_ as to detach from it, and preserve intact, this invisible ultra-red emission, while the visible and ultra-violet emissions are wholly intercepted. We are thus enabled to operate at will upon the purely ultra-red waves.

In the heating of solid bodies to incandescence, this non-visual emission is the necessary basis of the visual. A platinum wire is stretched in front of the table, and through it an electric current flows. It is warmed by the current, and may be felt to be warm by the hand. It emits waves of heat, but no light. Augmenting the strength of the current, the wire becomes hotter; it finally glows with a sober red light. At this point Dr. Draper many years ago began an interesting investigation. He employed a voltaic current to heat his platinum, and he studied, by means of a prism, the successive introduction of the colours of the spectrum. His first colour, as here, was red; then came orange, then yellow, then green, and lastly all the shades of blue. As the temperature of the platinum was gradually augmented, the atoms were caused to vibrate more rapidly; shorter waves were thus introduced, until finally waves were obtained corresponding to the entire spectrum. As each successive colour was introduced, the colours preceding it became more vivid. Now the vividness or intensity of light, like that of sound, depends not upon the length of the wave, but on the amplitude of the vibration. Hence, as the less refrangible colours grew more intense when the more refrangible ones were introduced, we are forced to conclude that side by side with the introduction of the shorter waves we had an augmentation of the amplitude of the longer ones.

These remarks apply not only to the visible emission examined by Dr.

Draper, but to the invisible emission which precedes the appearance of any light. In the emission from the white-hot platinum wire now before you, the lightless waves exist with which we started, only their intensity has been increased a thousand-fold by the augmentation of temperature necessary to the production of this white light. Both effects are bound up together: in an incandescent solid, or in a molten solid, you cannot have the shorter waves without this intensification of the longer ones. A sun is possible only on these conditions; hence Sir William Herschel's discovery of the invisible ultra-red solar emission.

The invisible heat, emitted both by dark bodies and by luminous ones, flies through s.p.a.ce with the velosity of light, and is called _radiant heat_. Now, radiant heat may be made a subtle and powerful explorer of molecular condition, and, of late years, it has given a new significance to the act of chemical combination. Take, for example, the air we breathe. It is a mixture of oxygen and nitrogen; and it behaves towards radiant heat like a vacuum, being incompetent to absorb it in any sensible degree. But permit the same two gases to unite chemically; then, without any augmentation of the quant.i.ty of matter, without altering the gaseous condition, without interfering in any way with the transparency of the gas, the act of chemical union is accompanied by an enormous diminution of its _diathermancy_, or perviousness to radiant heat.

The researches which established this result also proved the elementary gases, generally, to be highly transparent to radiant heat.

This, again, led to the proof of the diathermancy of elementary liquids, like bromine, and of solutions of the solid elements sulphur, phosphorus, and iodine. A spectrum is now before you, and you notice that the transparent bisulphide of carbon has no effect upon the colours. Dropping into the liquid a few flakes of iodine, you see the middle of the spectrum cut away. By augmenting the quant.i.ty of iodine, we invade the entire spectrum, and finally cut it off altogether. Now, the iodine, which proves itself thus hostile to the light, is perfectly transparent to the ultra-red emission with which we have now to deal. It, therefore, is to be our ray-filter.

Placing the alum-cell again in front of the electric lamp, we a.s.sure ourselves, as before, of the utter inability of the concentrated light to fire white paper-Introducing a cell containing the solution of iodine, the light is entirely cut off; and then, on removing the alum-cell, the white paper at the dark focus is instantly set on fire.

Black paper is more absorbent than white for these rays; and the consequence is, that with it the suddenness and vigour of the combustion are augmented. Zinc is burnt up at the same place, magnesium bursts into vivid combustion, while a sheet of platinized platinum, placed at the focus, is heated to whiteness.

Looked at through a prism, the white-hot platinum yields all the colours of the spectrum. Before impinging upon the platinum, the waves were of too slow recurrence to awaken vision; by the atoms of the platinum, these long and sluggish waves are broken up into shorter ones, being thus brought within the visual range. At the other end of the spectrum, by the interposition of suitable substances, Professor Stokes _lowered_ the refrangibility, so as to render the non-visual rays visual, and to this change he gave the name of _Fluorescence_.

Here, by the intervention of the platinum, the refrangibility is _raised_, so as to render the non-visual visual, and to this change I have given the name of _Calorescence_.

At the perfectly invisible focus where these effects are produced, the air may be as cold as ice. Air, as already stated, does not absorb radiant heat, and is therefore not warmed by it. Nothing could more forcibly ill.u.s.trate the isolation, if I may use the term, of the luminiferous ether from the air. The wave-motion of the one is heaped up to an extraordinary degree of intensity, without producing any sensible effect upon the other. I may add that, with suitable precautions, the eye may be placed in a focus competent to heat platinum to vivid redness, without experiencing any damage, or the slightest sensation either of light or heat.

The important part played by these ultra-red rays in Nature may be thus ill.u.s.trated: I remove the iodine filter, and concentrate the total beam upon a test tube containing water. It immediately begins to splutter, and in a minute or two it _boils_. What boils it? Placing the alum solution in front of the lamp, the boiling instantly ceases.

Now, the alum is pervious to all the luminous rays; hence it cannot be these rays that caused the boiling. I now introduce the iodine, and remove the alum: vigorous ebullition immediately recommences at the invisible focus. So that we here fix upon the invisible ultra-red rays the heating of the water.

We are thus enabled to understand the momentous part played by these rays in Nature. It is to them that we owe the warming and the consequent evaporation of the tropical ocean; it is to them, therefore, that we owe our rains and snows. They are absorbed close to the surface of the ocean, and warm the superficial water, while the luminous rays plunge to great depths without producing any sensible effect. But we can proceed further than this. Here is a large flask containing a freezing mixture, which has so chilled the flask, that the aqueous vapour of the air of this room has been condensed and frozen upon it to a white fur. Introducing the alum-cell, and placing the coating of h.o.a.r-frost at the intensely luminous focus of the electric lamp, not a spicula of the dazzling frost is melted.

Introducing the iodine-cell, and removing the alum, a broad s.p.a.ce of the frozen coating is instantly melted away. Hence we infer that the snow and ice, which feed the Rhone, the Rhine, and other rivers with glaciers for their sources, are released from their imprisonment upon the mountains by the invisible ultra-red rays of the sun.

-- 6. _Ident.i.ty of Light and Radiant Heat. Reflection from Plane and Curved Surfaces. Total Reflection of Heat_.

The growth of science is organic. That which today is an _end_ becomes to-morrow a _means_ to a remoter end. Every new discovery in science is immediately made the basis of other discoveries, or of new methods of investigation. Thus about fifty years ago OErsted, of Copenhagen, discovered the deflection of a magnetic needle by an electric current; and about the same time Thomas Seebeck, of Berlin, discovered thermoelectricity. These great discoveries were soon afterwards turned to account, by n.o.bili and Melloni, in the construction of an instrument which has vastly augmented our knowledge of radiant heat.

This instrument, which is called a _thermo-electric pile_, or more briefly a thermo-pile, consists of thin bars of bis.m.u.th and antimony, soldered alternately together at their ends, but separated from each other elsewhere. From the ends of this 'thermo-pile' wires pa.s.s to a galvanometer, which consists of a coil of covered wire, within and above which are suspended two magnetic needles, joined to a rigid system, and carefully defended from currents of air.

The action of the arrangement is this: the heat, falling on the pile, produces an electric current; the current, pa.s.sing through the coil, deflects the needles, and the magnitude of the deflection may be made a measure of the heat. The upper needle moves over a graduated dial far too small to be directly seen. It is now, however, strongly illuminated; and above it is a lens which, if permitted, would form an image of the needle and dial upon the ceiling. There, however, it could not be conveniently viewed. The beam is therefore received upon a looking-gla.s.s, placed at the proper angle, which throws the image upon a screen. In this way the motions of this small needle may be made visible to you all.

The delicacy of this apparatus is such that in a room filled, as this room now is, with an audience physically warm, it is exceedingly difficult to work with it. My a.s.sistant stands several feet off. I turn the pile towards him: the heat radiated from his face, even at this distance, produces a deflection of 90. I turn the instrument towards a distant wall, a little below the average temperature of the room. The needle descends and pa.s.ses to the other side of zero, declaring by this negative deflection that the pile has lost its warmth by radiation against the cold wall. Possessed of this instrument, of our ray-filter, and of our large Nicol prisms, we are in a condition to investigate a subject of great philosophical interest; one which long engaged the attention of some of our foremost scientific workers--the substantial _ident.i.ty of light and radiant heat_.

That they are identical in _all_ respects cannot of course be the case, for if they were they would act in the same manner upon all instruments, the _eye_ included. The ident.i.ty meant is such as subsists between one colour and another, causing them to behave alike as regards reflection, refraction, double refraction, and polarization. Let us here run rapidly over the resemblances of light and heat. As regards reflection from plane surfaces, we may employ a looking-gla.s.s to reflect the light. Marking any point in the track of the reflected beam, cutting off the light by the dissolved iodine, and placing the pile at the marked point, the needle immediately starts aside, showing that the heat is reflected in the same direction as the light. This is true for every position of the mirror.

Recurring, for example, to the simple apparatus employed in our first lecture (fig. 3, p. 11); moving the index attached to the mirror along the divisions of our graduated arc (_m_ _n_), and determining by the pile the positions of the invisible reflected beam, we prove that the angular velocity of the heat-beam, like that of the light-beam, is twice that of the mirror.

[Ill.u.s.tration: Fig. 49.]

As regards reflection from curved surfaces, the ident.i.ty also holds good. Receiving the beam from our electric lamp on a concave mirror (_m_ _m_, fig. 49), it is gathered up into a cone of reflected light rendered visible by the floating dust of the air; marking the apex of the cone by a pointer, and cutting off the light by the iodine solution (T), a moment's exposure of the pile (P) at the marked point produces a violent deflection of the needle.

The common reflection and the total reflection of a beam of radiant heat may be simultaneously demonstrated. From the nozzle of the lamp (L, fig. 50) a beam impinges upon a plane mirror (M N), is reflected upwards, and enters a right-angled prism, of which _a_ _b_ _c_ is the section. It meets the hypothenuse at an obliquity greater than the limiting angle,[23] and is therefore totally reflected. Quenching the light by the ray-filter at F, and placing the pile at P, the totally reflected heat-beam is immediately felt by the pile, and declared by the galvanometric deflection.

[Ill.u.s.tration: Fig. 50.]

-- 7. _Invisible Images formed by Radiant Heat._

Perhaps no experiment proves more conclusively the substantial ident.i.ty of light and radiant heat, than the formation of invisible heat-images. Employing the mirror already used to raise the beam to its highest state of concentration, we obtain, as is well known, an inverted image of the carbon points, formed by the light rays at the focus. Cutting off the light by the ray-filter, and placing at the focus a thin sheet of platinized platinum, the invisible rays declare their presence and distribution, by stamping upon the platinum a white-hot image of the carbons. (See fig. 51.)

[Ill.u.s.tration: Fig. 51.]

-- 8. _Polarization of Heat_.

Whether radiant heat be capable of polarization or not was for a long time a subject of discussion. Berard had announced affirmative results, but Powell and Lloyd failed to verify them. The doubts thus thrown upon the question were removed by the experiments of Forbes, who first established the polarization and 'depolarization' of heat.

The subject was subsequently followed up by Melloni, an investigator of consummate ability, who sagaciously turned to account his own discovery, that the obscure rays of luminous sources are in part transmitted by black gla.s.s. Intercepting by a plate of this gla.s.s the light from an oil flame, and operating upon the transmitted invisible heat, he obtained effects of polarization, far exceeding in magnitude those which could be obtained with non-luminous sources. At present the possession of our more perfect ray-filter, and more powerful source of heat, enables us to pursue this ident.i.ty question to its utmost practical limits.

[Ill.u.s.tration: Fig. 52.]

Mounting our two Nicols (B and C, fig. 52) in front of the electric lamp, with their princ.i.p.al sections crossed, no light reaches the screen. Placing our thermo-electric pile (D) behind the prisms, with its face turned towards the source, no deflection of the galvanometer is observed. Interposing between the lamp (A) and the first prism (B) our ray-filter, the light previously transmitted through the first Nicol is quenched; and now the slightest turning of either Nicol opens a way for the transmission of the heat, a very small rotation sufficing to send the needle up to 90. When the Nicol is turned back to its first position, the needle again sinks to zero, thus demonstrating, in the plainest manner, the polarization of the heat.

When the Nicols are crossed and the field is dark, you have seen, in the case of light, the effect of introducing a plate of mica between the polarizer and a.n.a.lyzer. In two positions the mica exerts no sensible influence; in all others it does. A precisely a.n.a.logous deportment is observed as regards radiant heat. Introducing our ray-filter, the thermo-pile, playing the part of an eye as regards the invisible radiation, receives no heat when the eye receives no light; but when the mica is so turned as to make its planes of vibration oblique to those of the polarizer and a.n.a.lyzer, the heat immediately pa.s.ses through. So strong does the action become, that the momentary plunging of the film of mica into the dark s.p.a.ce between the Nicols suffices to send the needle up to 90. This is the effect to which the term 'depolarization' has been applied; the experiment really proving that with both light and heat we have the same resolution by the plate of mica, and recompounding by the a.n.a.lyzer, of the ethereal vibrations.

Removing the mica and restoring the needle once more to 0, I introduce between the Nicols a plate of quartz cut perpendicular to the axis; the immediate deflection of the needle declares the transmission of the heat, and when the transmitted beam is properly examined, it is found to be circularly polarized, exactly as a beam of light is polarized under the same conditions.

-- 9. _Double Refraction of Heat_.