Chapter V: Section II (2)

Thus it is evident that the tension of the ether in these experiments was continually diminished by the 0·2 of an inch, consequently its quantity was continually diminished by its 1/150th part, accompanied by a corresponding decrease in the deflections of the needle. The final result of this process showed that the radiation of an amount of vapour in the tube possessing a tension of less than the thousand millionth of an atmosphere is perfectly measurable. The temperature imparted to this infinitesimal quantity of matter did not exceed 0·75 of a centesimal degree. The molecules which constituted this intensely attenuated vapour, though inconceivable, had as true an existence as the suns which constitute the star-dust of the nebulæ. ‘A platinum wire raised to whiteness in a vacuum by an electric current, becomes comparatively cold in a second after the current has been interrupted; yet that wire, while ignited, was the repository of an immense amount of mechanical force. What has become of this? It has been conveyed away by a substance so attenuated that its very existence must for ever remain an hypothesis. But here is matter that we can weigh, measure, taste, and smell; that we can reduce to a tenuity which, though expressible by numbers, defeats the imagination to conceive of it. Still we see it competent to arrest and originate quantities of force which on comparison with its own mass are almost infinite, a small fraction of this force causing the double needle of the galvanometer to swing through considerable arcs. When we find ponderable matter producing these effects, we have less difficulty in investing the luminiferous ether with those mechanical properties which have long excited the interest and wonder of all who have reflected upon the circumstances involved in the undulatory theory of light.’

The dynamical principle was next applied to determine the radiation of a gas through itself; or through any other gas having the same period of vibration. For that purpose Mr. Tyndall made use of the hollow cylinder 49·4 inches long already mentioned, closed at both ends by plates of rock-salt, and divided internally into two chambers by a movable plate of the same substance. All sources of heat being dispensed with, the chamber next the voltaic pile contained the gas which was to act as an absorber, and the more remote as a radiator.

Heat is evolved in air when its motion is arrested; on entering an exhausted tube, the more rapid the motion the greater the heat. Both chambers of the cylinder were at first filled with the vapour to be examined, the usual pressure being the 1/60 part of an atmosphere. But the vapour entered so slowly, and the quantity was so small that the radiation due to the warming of the vapour by its own collision was insensible. The needle of the goniometer being at zero, dry air was allowed to enter the chamber most distant from the pile; this air became heated dynamically by the collision of its particles against the sides of the tube, communicated its heat to the vapour, and the vapour immediately discharged the heat thus communicated to it against the pile. This case not only resembles, but is actually of the same mechanical character as, that in which a vibrating tuning fork is brought into contact with a surface of some extent. The fork, which before was inaudible, becomes at once a copious source of sound. What the sounding board is to the fork, the compound molecule is to the elementary atom. The tuning fork vibrating alone is in the condition of the atom radiating alone; the sound of the one and the heat of the other being insensible. But in association with sulphuric or acetic ether vapour the elementary atom is in the condition of the tuning fork applied to its sounding board, communicating motion to the luminiferous ether through the molecules, as the fork through the board communicates its motion to the air.

Mr. Tyndall’s experiments show the great opacity of a gas to radiations from the same gas, and may likewise show the remarkable influence of attenuation in the case of vapour. The individual molecules of a vapour may be powerful absorbers and radiators, but in their strata they constitute an open sieve through which a great quantity of radiant heat may pass. In such thin strata, therefore, the vapours as used in the experiments were generally found far less energetic than the gases, while in thick strata the same vapours showed an energy greatly superior to the same gases, but the gases were always employed at a pressure of one atmosphere.

Lastly Mr. Tyndall examined the diathermancy of the liquids from which his vapours were derived, and the result leaves not a doubt that both absorption and radiation are phenomena irrespective of aggregation. If any vapour is a strong absorber and radiator, the liquid from whence it comes is also a strong absorber and radiator.

Perfectly dry pure air is as pervious to light and heat as a vacuum itself; consequently, if the atmosphere was quite pure and dry, the rays of the sun would fall on the earth with unmitigated force during the day, and would be radiated back again and dissipated in space during the night to the destruction of vegetation. But the earth is protected from these extremes by the absorptive power of aqueous vapour, which is always present more or less in the atmosphere; even when the air is so transparent that distant objects seem to be near, it is loaded with vapour in an elastic invisible state, which a change of temperature may condense into cloud or precipitate in rain.

The absorptive power of aqueous vapour was determined by placing tubes containing fragments of glass moistened with water between the drying apparatus and the experimental glass tube of the instrument, so that perfectly pure dry air in passing over the wet fragments of glass carried a portion of aqueous vapour with it into the exhausted experimental tube, and the deflection of the needle of the goniometer showed that the absorptive power of the aqueous vapour exceeded that of the dry air 80 times. Now since in the atmosphere there is one molecule of aqueous vapour with an absorptive power of 80 for every 200 atoms of oxygen and nitrogen whose absorptive power is 1 like that of one of its constituent atoms, it follows by comparison that the absorptive power of the molecule is 16,000 times greater than that of an atom of either oxygen or nitrogen. From this enormous opacity to obscure heat ‘it is certain that more than ten per cent. of the terrestrial radiation from the soil of England is stopped within ten feet of the surface of the soil; remove for a single summer night the aqueous vapour from the air which overspreads the country, and you would assuredly destroy every plant capable of being destroyed by a freezing temperature.’

The quantity of vapour in each place varies with the latitude, the season, and other circumstances; but whenever the amount of heat radiated from the earth surpasses the absorption, the remainder passes through the vapour into space, and for the same reason the residue of that coming from the sun passes through the vapour and comes to the earth, so that whatever may be the local differences it has been decidedly proved with regard to the whole globe, that the quantity of heat annually received from the sun is annually radiated into space; the latter is a force lost to the earth, nevertheless it does not interfere with the law of the conservation of force which extends to the universe.

By observations made during ten scientific ascents in a balloon to very great altitudes, Mr. Glaisher has proved, that the theory of the uniform decrease of temperature with increase of elevation is no longer tenable. Since the absorptive force of aqueous vapour is 16,000 times that of dry air, the whole of the heat radiated by the full moon is intercepted by our atmosphere. It raises the temperature of the higher regions, dissolves the vapour, dissipates the clouds, prevents the formation of more, and allows the heat radiated from the earth to pass freely into space: thus confirming the common, and almost universal, belief that the full moon dispels the clouds. The absorptive power of aqueous vapour is so enormous that even the planet Mercury may be habitable should his atmosphere contain a sufficient quantity of it to mitigate the heat of the sun.

No doubt all the heat from the stars must be absorbed by the atmosphere, but their photographs show that it is pervious to the chemical rays. Those from Sirius, the nearest and brightest of the stars, travelling through 180 millions of millions of miles and decreasing in quantity inversely as the square of the distance, still have sufficient energy to give a perfect photographic impression of its spectrum; but Sirius is sixty times larger than the sun, and is many times more luminous. A photograph of the spectrum of Capella has been taken, though three times more distant than Sirius. Photographs of double stars of the sixth and seventh magnitude show that actinic rays from immeasurable distances in space have power sufficient to decompose matter in unstable equilibrium on the surface of the earth.

The chemical power of the moon’s light only surpasses that of Jupiter in the ratio of 6 to 4 or 5, and Jupiter’s light has twelve times more actinic energy than that of Saturn. For such comparisons a standard of photographic intensity is requisite.

A paper coated with chloride of silver can be prepared which has a constant degree of sensitiveness, and Dr. Roscoe has proved that a constant dark tint is produced on this standard paper by a constant quantity of light, the tint being the same, whether light of the intensity represented by 1 acts for the time represented by 50, or light represented by 50 acts for the time represented by 1; or in other words the amount of the chemical action of light is directly proportional to the intensity of the light, and when the light is constant, the amount of action is exactly proportional to the time of exposure.

The ratio of the chemical action of the rays of light falling directly from the sun to the chemical action of the light diffused over the whole sky can be determined by means of an instrument, in which the shadow of a little ball is made to fall on a sensitive paper so as to intercept the direct rays of the sun, and allow it to be impressed by an action of the light diffused over sky alone; this compared with a similar paper, on which both the direct and indirect light has fallen, gives the ratio required. From this it appears, that the relative amount of chemically active light which comes directly from the sun, is very much less than the amount of his direct visible light. For while Professor Roscoe was making experiments at Manchester on the maximum effect of the chemical action of light, he found when the sun had an altitude of 20°, that of 100 chemical rays which fell on a piece of standard paper, only about 8 came from the direct light of the sun; while on the contrary, of 100 rays of visible light, 66 came directly from the sun, and only 40 from the light diffused over the whole sky, so that the diffused light is richer in chemical rays than the direct solar beam, ‘a startling result,’ but borne out by observations not only made at Manchester and in its vicinity, but at Kew, Heidelberg, and at Pará on the Amazon nearly under the equator.

On account of the increasing rarity of the atmosphere, the greater the height above the level of the sea, the less the amount of diffused light and consequently of actinic power. Hence photographers have to expose their plates for a much longer time to the light on the snowy peaks of the Alps and other great heights than in England or at the level of the sea. During Mr. Glaisher’s tenth balloon ascent simultaneous observations were made at Greenwich Observatory and in the balloon, when at more than three miles above the surface of the earth, the standard paper exposed to the full rays of the sun was not as much coloured in half an hour as the corresponding paper at Greenwich in one minute.

By a series of observations at Heidelberg, Kew, and Manchester, it has been proved that the very small relative chemical action of the sun’s direct light decreases rapidly with his altitude, and at these three places of observation, it has frequently happened when the sun’s altitude was very low, as at 12°, that his direct light made no impression on a sensitive paper. ‘The sun’s light had been robbed of its chemical power in passing through the air.’ This singular result is ascribed by Professor Roscoe to what he calls the opalescence of the atmosphere.

Opalescent glass, slightly milky liquids, pure water with particles of sulphur floating in it, are impervious to the chemical rays, whence Professor Roscoe infers that the atmosphere, more especially its lower regions, possesses that property in consequence of multitudes of solid particles floating in it. What they are is unknown, but infinitesimal particles of soda seem to be everywhere, and no doubt particles of other substances mixed with them may be often seen as motes dancing in the sunbeams. Besides, it is clearly proved that myriads of the eggs and germs of organized beings, though invisible to the naked eye, are continually floating in the air, and that they are more abundant in the lower than in the higher strata of the atmosphere. Since opalescent matter reflects the blue rays of light and transmits the red, Professor Roscoe ascribes the blue colour of the sky and the bright tints at sunrise and sunset to the opalescent property of the air.

The atmosphere is permeable to every kind of chemical rays, which is far from being the case with bodies on earth, some of which though transparent to all the visible rays, vary greatly in their transparency to the chemical rays.

The atoms and molecules of matter not only have the power of turning the rays of the solar beam out of their rectilinear path, but of changing their refrangibility.

The myriads of ethereal waves or rays of light that constitute the seven colours of the solar spectrum, decrease in refrangibility and increase in rapidity of vibration and length of wave from the extreme violet to the end of the red; each ray having its own rate of vibration, its own length of wave, and its own colour. From the middle of the yellow, which is the luminous part of the spectrum, the chemical spectrum extends invisibly, but with increasing refrangibility and increasing velocity of vibration, to a point far beyond the violet. On the contrary, the heat spectrum, which may also be said to begin in the yellow light, extends invisibly but with decreasing refrangibility, and decreasing velocity of vibration to some distance beyond the visible red.

The rays of heat are absorbed by the humours of the eye, but were they to reach the retina we should see that they differ from one another as much as those of the luminous spectrum; the chemical spectrum from its greater length is still more diversified.

The whole of the solar spectrum, visible and invisible, is crossed at right angles to its length by innumerable dark rayless lines, differing in breadth and intensity. Sir John Herschel discovered vacant spaces in the extra-luminous part of the heat spectrum, and more recently M. Edouard Becquerel, by throwing the solar spectrum upon a daguerreotype plate, discovered that the chemical spectrum given by a glass prism, from its beginning in the yellow to its extreme point beyond the violet, is crossed by rayless lines, and that the lines in the part passing through the visible spectrum coincide exactly with the rayless lines in the luminous part. This coincidence was confirmed by the independent researches of Dr. Draper at New York. By means of the rayless spaces or black lines in the visible spectrum, M. Kirchhoff has proved that thirteen terrestrial substances are constituents of the sun’s atmosphere.

The length of the undulations of the ether which produce the impression of the extreme violet rays of the solar spectrum on our eyes, is the seventeen millionth part of an inch; the length of the ethereal undulation that produces the sensation of the extreme red is the twenty-six millionth part of an inch; the ethereal undulations beyond these limits are invisible to human eyes. Nevertheless certain substances have the power of increasing the length of the vibrations, and reducing the rays of the spectrum to a lower grade in the scale of refrangibility, so that the invisible rays of the chemical spectrum have thus been brought within the limit of human vision.

For example, the chemical rays shine as visible light when they fall on glass tinged with the oxide of uranium. When these dark rays fall upon the glass, they put the whole of its molecules into vibrations, the same with their own, while at the same time they give a more rapid vibration to a certain number of the same molecules. The whole of the molecules restore their vibrations to the surrounding ether. Those having the same velocity with the chemical rays make no sensible impression on our eyes; but the more rapid vibrations come within the limits of the visible spectrum; they have consequently a lower refrangibility, and shine as visible light. It is called degraded light on account of its lower position in the prismatic scale, but more frequently fluorescent light, because fluor spar was the first solid known to possess the property. A number of substances are fluorescent, both solid and liquid, organic and inorganic.

If in a dark room a non-fluorescent body be illuminated by a sunbeam passing through glass stained deep blue by cobalt, it will reflect blue light; but it will appear to be perfectly black if it be viewed through glass tinged yellow by silver; while a piece of canary glass, which is highly fluorescent, will shine with a vivid light under the same circumstances. All the molecules of the canary glass give back to the ether the undulations that have been impressed on them by the blue light; while a certain number of them possess the power of receiving and giving back more rapid vibrations to the ether. The yellow glass held before the eye is impervious to the undulations of the blue rays, but transmits those of the fluorescent light, which emanate from the smaller number of molecules, and which thus become in reality new centres of light, different from the sun’s light, though dependent upon it: the one terrestrial, the other celestial. Since the vibrations of the fluorescent light are more rapid than those of the blue light their colour is lower in the prismatic scale. The vibrations of the molecules in a fluorescent substance are analogous to those of a musical cord, which give the fundamental note or pitch and its harmonics, for the whole of the musical cord while vibrating the fundamental note divides itself spontaneously into parts having more rapid vibrations, which give the harmonics. Professor Stokes of Cambridge, who made this beautiful experiment, computed that the vibrations which produced the fluorescent light were a major or minor third below the pitch or vibrations of the blue light.

One of the first discoveries of fluorescence was made by Sir John Herschel—certainly the first who observed the property in a liquid. He found that the blue light which emanates from all parts of a solution of the sulphate of quinine, especially from its surface, is fluorescent, and that the light transmitted through the liquid, though sensibly like the incident white light, is no longer capable of producing fluorescence; it has been deprived of its chemical rays by absorption.

The chemical rays having been rendered visible by an increase in the length of the periods of vibration, unsuccessful attempts have been made to change the periods of the rays of heat beyond the red end of the spectrum so as to bring them within the limits of vision. The idea of effecting such a change by employing a substance opaque to light, but pervious to heat, is due to Dr. Akin; but it has since been accomplished by Dr. Tyndall, who, in the course of his experiments on radiant heat, found that a solution of iodine in the bisulphide of carbon excludes the most dazzling light, but transmits the rays of heat freely. He employed a mirror, lined in front with silver, to concentrate the rays emitted from the charcoal points of the electric lamp, and interposed a vessel containing the solution in question, so that the rays of heat alone were brought to a focus almost undiminished. When the solar spectrum was examined, the point of maximum heat was found to be as far beyond the extreme red on one side as the green rays on the other. In the spectrum of the electrical light the point of maximum heat was also found to lie beyond the extreme red, but the augmentation of intensity was so sudden and enormous as far to exceed the maximum heat of the sun previously determined by Professor Müller. Aqueous vapour powerfully absorbs radiant heat; so a solar spectrum beyond the earth’s atmosphere might probably exhibit as great intensity as the electrical light. With the apparatus described oxidizable substances burst into the flame of common combustion when put into the focus; but when the chemical action of the oxygen of the atmosphere was excluded by igniting substances in vacuo by the invisible rays of heat, their periods of vibration were so changed as to bring them within the limits of vision. When the electric light is very powerful, a plate of platinized platinum in vacuo is raised to white heat at the focus of invisible rays; and when the incandescent platinum is looked at through a prism, its light yields a complete and brilliant spectrum. ‘In all these cases we have a perfectly invisible image of the charcoal points formed by the mirror; and no experiment illustrates the change of heat into light’ more strongly than the following:—When the plate of platinum or one of charcoal is placed in the focus, the invisible image raises it to incandescence, and thus prints itself visibly on the plate. On drawing the coal points of the lamp apart, or causing them to approach each other, the thermograph follows their motion. By cutting the plate of carbon along the boundary of the thermograph, a second pair of coal points may be formed of the same shape as the original ones, but turned upside down; and thus by the rays of the one pair of coal points which are incompetent to excite vision, we may cause a second pair to emit all the rays of the spectrum. Fluorescence and calorescence act in contrary directions. Fluorescence causes the molecules of a fluorescent substance to oscillate in slower periods than the incident light, while calorescence causes the molecules of a substance to oscillate in longer periods than the incident light. The refrangibility of the rays is lowered in the first case, and raised in the second.

Substances differ as much in their transmission of the chemical rays as those of light and heat. Glass is impervious to the most highly refrangible chemical rays, while rock crystal transmits them with the greatest facility; and on that account the absolute length of the spectrum was not known till the light was refracted by prisms of rock crystal. Besides, the number, position, and intensity of the chemical rays vary with the source of light. Some flames have scarcely any chemical rays; that of the oxy-hydrogen blowpipe, though intensely hot, has very few, and even the solar light is inferior in that respect to electricity. The electric spark from the prime conductor of a common electrifying machine, or the discharge of a Leyden jar, emits rays of very high refrangibility, far surpassing those which emanate from the sun. For, when the electric light from a highly charged Leyden jar was refracted by two quartz prisms and thrown by Professor Stokes on a plate of uranium glass, the chemical spectrum was highly luminous, and six or eight times as long as the visible spectrum. An equally extensive spectrum was obtained from the voltaic arc taken between copper points; it consisted entirely of bright lines. The long spectrum also appeared on the uranium glass when the spark refracted by quartz prisms was obtained from the secondary terminals of an induction coil in connection with the coatings of a Leyden jar. It consisted of bright lines, but was not so luminous as that from a powerful voltaic battery. On changing the metals of the points between which the sparks passed, the bright lines were changed, which showed that they were due to the particular metals.

The heat of the electric spark volatilizes the metals which form the points of the conducting wires; and all volatilized metals give characteristic spectra, both visible and chemical. The visible part differs from that of the solar spectrum in being crossed by bright lines instead of dark ones; but the number, intensity, and position of both the visible and invisible lines change with each metal. The changes in the invisible part under consideration may be readily observed by throwing the spectra either on a fluorescent or collodion plate. For example: in the spectrum from the spark between thallium points thrown on the latter, Dr. Miller found that there were two strong groups of lines in the least refrangible part of the spectrum; at a little distance from these there were three groups, the two first less intense than the third; several rows of feeble dots followed, and the chemical spectrum terminated rather abruptly with four nearly equidistant groups. This spectrum bears a resemblance to those of zinc and cadmium, less strongly to that of lead. Dr. Miller found that the photographic spectra of iron, cobalt, and nickel, also have a strong analogy, but that the metals arsenic, antimony, and tin showed as great a difference in the invisible as in the visible part of their spectrum.

The fluorescent spectra of seventeen metals were examined by Professor Stokes of Cambridge; several of them showed luminous lines of extraordinary strength, especially zinc, cadmium, magnesium, aluminium, and lead, which in a spectrum not generally remarkable contains one line surpassing perhaps all other metals in brilliancy. Some other metals exhibit in certain parts of their spectra lines that are both bright and numerous; on the whole some parts of the spectra are strong and tolerably continuous, while in others they are weak. This grouping of the lines is most remarkable in copper, nickel, cobalt, iron, and tin. Of all the metals examined, magnesium gave the shortest spectrum, ending in a very bright line, beyond which however excessively faint light extended to a distance equal to that of the long spectra. Aluminium, on the other hand exceeded all the other metals in richness of the rays of the very highest refrangibility. All the strong lines mentioned lie in that part of the spectra.

In the course of these experiments Professor Stokes observed that even quartz of a certain thickness is not transparent to invisible lines of the highest refrangibility, for the highest aluminium line, which is double, could only be seen by rays passing through the edge of the prism. This leads to another branch of the subject, namely, the absorption of the invisible rays by solids, liquids and gases. Mr. Wm. Allen Miller has shown from his own experiments that bodies pervious to the chemical rays in the solid form, are so also in the liquid and gaseous form; that colourless transparent solids which absorb the photographic rays, absorb them more or less also in their liquid and gaseous states. He has moreover found that the following substances have the same maximum transparency:—rock crystal, ice, and fluor spar among solids, water among liquids, the three elementary gases and carbonic acid among gaseous substances. The most opaque to the invisible rays are, nitrate of potash, bisulphide of carbon, and sulphuretted hydrogen. It appears that a thin plate of mica is intensely opaque to all the invisible rays except a small portion of them of the lowest refrangibility.

The absorptive property however is partial: an absorptive substance either cuts off a portion of the light of a fluorescent spectrum or stripes it with dark lines: each substance absorbs rays peculiar to itself. Those employed by Professor Stokes were the alkaloids and glucosides, and he assumed the spectrum of tin for their examination because it has a long interval of continuity.

The fluorescent property of yellow uranite was discovered by Professor Stokes some years ago, and now he has added another fluorescent mineral in adularia or moonstone; from its natural faces and planes of cleavage alike a beautiful blue fluorescence emanated under the induction spark. As the same was observed in colourless felspars generally, Professor Stokes concluded that fluorescence is an inherent property in the silicate of alumina and potash constituting the crystal of moonstone. The blue fluorescence extended to a very sensible though small depth within the substance.

A particular variety of fluor spar found at Alston Moor in Cumberland, which is very pale by transmitted light, shows a strong blue fluorescence, and is eminently phosphorescent on exposure to the electric spark. It is the same kind of crystal in which Sir David Brewster originally discovered the property of fluorescence. On presenting such a crystal to the spark passing between aluminium terminals, besides the usual blue fluorescence, there was another of a reddish colour extending not near so far into the crystal, produced by the rays belonging to the strong lines of aluminium of extreme refrangibility.

The cube of fluor spar which showed these effects was externally colourless to the depth of 1/20 of an inch; then came one or two strata parallel to the faces of the cube showing the ruddy fluorescence, while the blue fluorescence extended to a much greater depth and had a stratified appearance. This crystal was eminently phosphorescent, its blue phosphorescence being arranged in strata parallel to the face of the cube like the blue fluorescence, but it was not perceptible beyond a very moderate distance below the surface at which the exciting cause entered, that cause being the photographic rays of extremely high refrangibility of the electric spark—taken, as in all these experiments, between the secondary terminals of an induction coil in connection with the coatings of a Leyden jar, and refracted by quartz prisms.

Mr. Stokes has employed fluorescence as a means of tracing substances in impure chemical solutions. When a pure fluorescent substance is examined in a pure spectrum it is found that on passing from the extreme red to the violet and beyond, the fluorescence commences at a certain point of the spectrum, varying from one substance to another, and continues from thence onwards more or less strongly in one part or another according to the particular substance. The colour of the fluorescent light is found to be nearly constant throughout the spectrum. ‘Hence when in a solution examined in a pure spectrum we notice the fluorescence taking as it were a fresh start _with a different colour_, we may be pretty sure that we have to deal with a mixture of two fluorescent substances.’

Experience as well as theory shows that rapid absorption is accompanied by copious fluorescence. But experience has hitherto also shown what could not have been predicted, and may not be universally true, that conversely absorption is accompanied in the case of a fluorescent substance by fluorescence.

The phosphorescent light of insects, fish, and plants is owing to chemical action, which produces many luminous phenomena; but a great number of inorganic and organic substances shine in the dark with a phosphorescence which is nearly allied to fluorescence. It is produced by exposure to the sun, by heat, electricity, insulation, cleavage, friction, and motion. For if a bottle containing nitrate of uranium be shaken, it shines spontaneously with a vivid light; even the hand shows phosphorescence in the dark after being exposed to the sun.

The essential difference between fluorescence and phosphorescence consists in the time during which the light lasts. Fluorescence ceases almost immediately after the exciting cause is withdrawn, while a phosphorescent body whether excited by heat, solar light, or electricity, lasts a much longer time; besides, the fluorescent rays are generally of lower refrangibility. Light and heat are temporarily absorbed and given out again by every body on the surface of the earth, more or less, that are exposed to the sun’s light. The nights would be much darker even when illuminated by the stars were it not for earth light, for the molecules restore to the ether, in the form of phosphorescence, the undulations they have received from the sun’s light during the day. The snow and ice blink of the sailors is a striking instance; generally, however, it is of much shorter duration. The phosphorescent property is nearly allied to electricity, for bodies that are bad conductors are apt to become phosphorescent, while good conductors of electricity rarely if ever show it. Ozone must be phosphorescent, for oxygen exhibits persistent light when electric discharges are sent through it, and Mr. Faraday saw a flash of lightning leave a luminous trace on a cloud which lasted for a short time.

In the solar spectrum the chemical or actinic rays produce phosphorescence, which the red rays have the power to extinguish. M. Nièpce de St-Victor found that solar light impresses its vibrations so strongly on substances exposed for a short time to its influence that they not only shine in the dark, but that the phosphorescent light they radiate has chemical energy enough to decompose substances in unstable equilibrium, and leave daguerreotype impressions of great delicacy and beauty.

The polarization of light and heat affords a remarkable instance of the elective power of matter. Light and heat are said to be polarized, which, having been once reflected, are rendered incapable of being again reflected at certain angles. For example, a ray incident on a plate of flint glass at an angle of 57° is rendered totally incapable of being reflected at that same angle from another plate of flint glass in a plane at right angles to the first. At the incidence of 57° the whole of the ray is polarized: it is the maximum of polarization for flint glass, but there is a partial polarization for every other angle; the portion of the ray polarized increases gradually up to the maximum, as the incidence approaches to 57°. All reflecting surfaces are capable of polarizing light and heat, but the angle of incidence at which the ray is totally polarized is different in each substance. Thus, the angle of incidence for the maximum polarization of crown glass is 56° 55ʹ, and no ray can be totally polarized by reflection from the surface of water unless the angle of incidence is 53° 11ʹ. As each substance has its own maximum polarizing angle, the effect is evidently owing to the action of the molecules of matter, and not to any peculiarity in the light or heat.[8]

Light and heat are also polarized by refraction, for certain substances, especially irregularly crystallised minerals like Iceland spar, possess the property of dividing a ray of light or heat passing through them in certain directions into two pencils, namely, the ordinary and extraordinary rays. The first of these is refracted according to the same law as in glass or water, never quitting the plane perpendicular to the refracting surface, while the second does quit that plane, being refracted according to a different and more complicated law. Hence, if a crystal of Iceland spar be held to the eye, two images of the same object will generally be seen of equal brightness. But when they are viewed through a plate of tourmaline it will be found that while the spar remains in the same position the images vary in relative brightness as the tourmaline is made to revolve in the same plane; one increases in intensity till it arrives at a maximum, at the same time that the other diminishes till it vanishes, and so on alternately at each quarter revolution of the tourmaline, proving both rays to be polarized. For in one position the tourmaline transmits the ordinary ray and reflects the extraordinary, and after revolving 90°, the extraordinary ray is transmitted and the ordinary ray is reflected.

The undulations of the ethereal medium which produce the sensation of common light, are performed in every plane at right angles to the direction in which the ray is moving, but the case is very different after the ray has been polarized by passing through a substance like Iceland spar, for the light then proceeds in two parallel pencils whose undulations are still indeed transverse to the direction of the rays, but they are accomplished in planes at right angles to one another. The ray of common light is like a round rod, whereas the parallel polarized rays resemble two long flat rulers, one of which lies on its broad surface, and the other on its edge. By a simple mechanical law, each vibratory motion of the common light is resolved into vibratory motions at right angles to one another.

The polarization of light and heat by refraction is not owing to the chemical composition, but to a want of homogeneity in the molecular structure of the substances through which they pass; for regular crystals and substances which are throughout of the same temperature, density, and structure, are incapable of double refraction. The effect of molecular structure is strikingly exhibited by the circular polarisation in the dimorphic crystals of quartz. In one form the plane of polarization revolves from right to left, and in the other that plane revolves from left to right, although the crystals themselves differ apparently by a very slight and often almost imperceptible variety of forms.

Thus polarization forms the most admirable connection between light, heat, and crystalline structure; showing peculiar arrangements of the molecules in regions otherwise unapproachable, and too refined for our perceptions. Besides, the gorgeously coloured images displayed by depolarization are splendid examples of the power of matter in decomposing light.

The perfect correspondence of the properties of the symmetrical, elastic, and optical axes of crystals with light and heat is another instance of the connection between the latter and crystalline form.

The axis of symmetry is that direction or imaginary line within a crystal, round which all the parts or particles are symmetrically arranged. A medium is said to be elastic which returns to its original form with a resilient force after being relieved from compression, and the axis of elasticity of a crystal is that direction in which it is most elastic. The optic axis is that line or direction through which light passes in one beam according to the law of ordinary refraction. Crystals may have one, two or more optical axes according to their form. Doubly refracting crystals such as Iceland spar have only one principal optic axis in which the whole beam passes according to the ordinary law; in every other direction the beam of light is divided into two polarized rays, one of which called the ordinary ray passes according to the ordinary law, while the other, known as the extraordinary ray, traverses the crystal in a different direction, with more rapidity and according to a different and more complicated law. The velocity of this extraordinary ray is a maximum when at right angles to the principal optical axis, and a minimum when parallel to it.

In perfectly regular crystals like the cube or die, the octohedron, &c., there are three axes of symmetry and of equal elasticity at right angles to one another. In these regular crystals all the axes are optical, so that they have no double refraction.

Right square prisms have two equal rectangular axes of symmetry, two axes of equal elasticity, and one optical axis.

All crystals of the pyramidal and rhomboidal systems have one axis of symmetry, two axes of elasticity, one optical axis; and form coloured circular rings traversed by a black cross when viewed by depolarized light.

Lastly oblique prismatic crystals which have three unequal axes of symmetry have three axes of unequal elasticity, two optical axes; and by depolarization give coloured lamnescata, that is coloured figures having the form of the figure 8 which are traversed by a black cross in two opposite quadrants, and when the crystal is made to revolve, the same figure, but in the complementary colours and traversed by a white cross, appears in the other two quadrants.

The right and left-handed circular polarization of quartz, according as certain facettes of the crystal are turned to the right or left, and the property of double refraction being exclusively possessed by crystals of the rhomboidal form, are striking instances of the connection between the geometrical arrangement of the molecules of matter and the optical and thermal forces, for the polarization of heat and all its consequences are in every respect analogous to those of light, and similar phenomena would be seen were heat visible.

Heat changes the position of the optical axes of crystals. When applied to a crystal of sulphate of lime, the two optical axes gradually approach to each other and at last coincide; if the heat be continued and increased, the axes open again, but in a direction at right angles to their former position. Thus the force of heat throws every molecule in the body into correlative motion. The angles of all crystals that are not of the octohedral group are changed by heat and vary with the intensity; the difference between the length of the greatest and least optic axes in such crystals diminishes as the temperature is raised, increases when it is lowered, and is constant at a given heat. In Iceland spar heat indirectly affects the doubly refracting power, for the expansion of the crystal in the direction of its axis is accompanied by contraction at right angles to it, which brings the crystal nearer to the cubical form, and consequently diminishes its doubly refracting power.

According to the researches of M. Angström, in crystals with different axes of elasticity the velocity of the molecular vibrations is different in different directions when they are heated. In rock crystal and tourmaline the heat radiates from a surface cut parallel to the axis of the crystal; in felspar the radiating surface is at right angles to the symmetrical axis.

The optical axes of crystals are also affected by pressure. Doubly refracting crystals with one principal axis acquire two when the pressure is perpendicular to it. The new principal axis coincides with the line of pressure or is at right angles to it according as the crystal is positive or negative, that is, according as the extraordinary ray is refracted to or from the optic axis of the crystal. The colours produced by polarization are affected by compression and dilatation according as the crystal is positive or negative.

Sir David Brewster is of opinion that all the properties of double refraction and the gorgeous phenomena of polarization, whether by crystals or produced in various substances permanently or transiently by heat, cold, rapid cooling, compression, dilatation, and induration, are wholly the result of the forces by which the atoms are held together; but these phenomena may rather be said to depend upon a reciprocal action between an irregular molecular structure and the agency of light and heat: which indeed seems to be confirmed by the transit of these two forces through right and left-handed quartz, for there is no reason to believe that there is any difference in the form of the particles in these two crystalline substances.

The experiments of M. Becquerel show that electricity is a power which makes the atoms of matter aggregate in crystalline forms; for he has succeeded in forming crystals of gold, silver, cobalt, nickel, platinum, and a variety of the gems undistinguishable from those in nature, by exposing saturated solutions of these substances for a very long time to feeble voltaic electricity; and crystals of earthy matter have been obtained in the same manner. The electric and magnetic state of mineral veins in mines which contain a vast variety of crystals, metallic and non-metallic, strongly favours this view of the origin of crystalline form.

M. Regnault has proved that the ratio between the specific heat and the weight of the atoms of matter is intimately connected with the mode of their aggregation; and indeed if it be considered that the atoms have not only specific heat and weight, but specific affinity, electricity, magnetism, consequently polarity, and probably specific forms, these peculiar forces must necessarily influence the structure of crystals according as they combine with or oppose the natural or artificial forces acting upon them, or upon their dissimilar faces, and this may be the cause of the great variety of forms that matter appears under. Carbonate of lime alone assumes more than 1,200 different modifications of its primitive type, but whatever be the variety of forms which any one substance may take, they are found to be all compatible with and derivative from a common type. The circumstances which have caused dimorphous crystals to deviate from the general law have not yet been explained.

It is very singular that when chlorate of soda is dissolved in water the solution does not possess the property of circular polarization, but when evaporated and allowed to crystallise, some of the crystals turn light to the right, and others to the left. Now if all the crystals that have the same property be picked out and dissolved in water a second time, the liquid will still have no circular polarization, but when allowed to crystallise, some of the crystals make light revolve through them to the right and others to the left as before. From this it is supposed that the atoms of liquids, which are free to move in every direction, already possess part of the characters which the change to solidity renders evident and permanent.

Although the relations between the force of magnetism and the atoms of matter do not exhibit such brilliant phenomena as light does, they are nevertheless most interesting and wonderful. Mr. Faraday discovered that all substances, whether solid, liquid, or aëriform, are either magnetic like iron, or diamagnetic like bismuth, the latter being by far the most numerous. Thus if a bar of iron be freely suspended between the poles of an extremely powerful magnet or electro-magnet, it will be attracted by both poles and will rest or sit axially, that is, with its length between the poles or in the line of magnetic force; whereas an equal and similar bar of bismuth so suspended will be repelled by both poles and will rest or sit equatorially, that is with its length perpendicular to the line of magnetic force. Magnetism and diamagnetism are both dual forces, but they are in complete antithesis to one another, which is strikingly illustrated by their action on crystalline matter.

A sphere of amorphous substance freely suspended under magnetic influence is indifferent, that is to say it has no tendency to set one way more than another; but a sphere cut out of a crystal whether magnetic or diamagnetic, is more powerfully attracted or repelled in one direction than in any other, which shows a connection between the magnetic forces and crystalline structure.

Crystals of carbonate of iron and carbonate of lime are isomorphous, that is, they have exactly the same crystalline form, but the carbonate of iron being highly magnetic is most powerfully attracted in the direction of its greatest optical axis which therefore sets axially, that is, in the line of magnetic force; while the principal optic axis of the carbonate of lime, which is diamagnetic, is most powerfully repelled and therefore sets equatorially. In both cases the antithetic forces follow the same law of decrease in intensity from the greatest optical axis to the least.

A bar of soft iron sets with its longest dimensions axially, but a bar of highly compressed iron-dust, whose shortest dimensions coincide with the line of pressure, sets equatorially, because it is most powerfully attracted in the line of greatest density. A bar of bismuth sets equatorially, but a bar of highly compressed bismuth dust, whose shortest dimensions coincide with the line of pressure, sets with its length axially, because it is most strongly repelled in the direction of its greatest density. Hence the action of magnets upon matter is most powerful in the line of maximum density, the force being attractive or repulsive according to the kind of magnetism possessed by the atoms. It follows therefore that the density is greatest in the line of the principal optical axis, and gradually decreases to the least optical axis, where it is a minimum.

The position which crystals take with regard to the magnetic force depends also upon their natural joints of cleavages, and upon their power of transmitting electricity. The diamagnetic force is inversely as the conducting power of bodies, and the conducting power of crystals is a maximum in the planes of their principal natural joints. Hence the action of the diamagnetic power is least in the natural joints, and conversely the magnetic force is greatest. In fact, the magnetic phenomena of crystals depends upon unequal conductibility in different directions, and their set is determined by the difference between the forces of attraction and repulsion of the poles, for one pole of the magne-crystallic axis is attracted and the other repelled. It is unnecessary to give more examples to show the action of the magnetic forces upon the atomic structure of crystals.[9]

Magnetism changes the relations and distances between the ultimate atoms of matter, a circumstance which probably depends upon their polarity. It changes steel permanently, iron temporarily, and it elongates a bar of iron, which loses in breadth what it gains in length; and as heat is developed in one direction and absorbed in the other, the temperature of the bar remains the same. Heat being an expansive force, diminishes the magnetism of iron and nickel in proportion as it increases the distance between their atoms, till at length they lose their cohesive force altogether. But there seems to be a temperature at which the magnetic force is a maximum, above and below which temperature it diminishes. Thus the magnetism of cobalt increases with the temperature up to a certain point; it then decreases as the temperature increases, and it loses its magnetism altogether when the heat amounts to 1996°.

Sir Humphry Davy and M. Arago noticed that the voltaic arc takes a rotatory motion on the approach of a magnet; and the effect of magnetism on the stratified appearance of the electric light in highly rarefied air shows how powerful its action is. In the year 1858, Mr. Gassiot published a series of observations on stratified light; subsequently various publications appeared on the subject both by Mr. Gassiot and by Professor Plücker, who made a series of very interesting observations on the nature of the stratifications, but more especially on the effects produced when they are under the influences of magnetism. Since that time, Mr. Gassiot has published several papers on the subject, and still continues his experiments on the stratifications of electric light, which give a visible proof of the connection between electricity and magnetism. He first showed that the stratified character of the electric discharge through highly attenuated media is remarkably developed in the Torricellian vacuum; latterly he has made his experiments by passing electricity through closed glass tubes of various lengths and internal diameters, filled with highly attenuated gases and vapours.[10] Two among the many brilliant experiments of this gentleman may be selected as illustrations of the property of electric light.

One of these closed glass tubes containing a highly attenuated gas was 38 inches long with an internal diameter of about an inch, and had the extremities of two platinum wires fused into the same side 32 inches apart. When these wires were put in connection with the wires of an induction battery and brought into contact, and the electricity passed through the tube, the luminous appearances at the extremities or poles of the platinum wires were very different, but simultaneous. A glow surrounded the negative pole, and in close approximation to the glow, a well defined black space appeared, while from the positive pole there issued in rapid succession a series of alternate dark and brilliantly luminous curved strata, which formed a column of stratified light, the concavities of the strata being turned to the positive pole. The stratifications do not extend to the black band round the negative wire or ball, which is quite different to the dark intervening space between the stratified discharge and the luminous negative glow. On making and breaking the electric circuit, the stratified discharge emanates from each pole alternately, the concavities of the strata turning alternately in different directions; in fact the whole phenomena are reversed, but not changed. ‘The stratified discharge arises from the impulses of a force acting on highly attenuated but resisting media,’ a new proof of the wonderful power inherent in highly attenuated gases; the number of stratifications given out at each discharge, depending upon the intensity of the electricity and rarity of the gas.