89.]

LIGHT PRODUCED BY IGNITION.

159

within the barrel. The materials submitted to experiment were, platinum, brass, antimony, lead, and gas carbon; to these I may add, porcelain, black lead ware, copper, and palladium. Chalk and marble became visible before the barrel was red hot, and the phosphorescence of fluor spar was still more marked. At a temperature which, from the expansion of the platinum, Draper estimated at 1210° (659° C.), the light of a strip of platinum, heated by the voltaic current, was red, and extended up to the line F of the solar spectrum (106), where the colour of the emitted light was greenish grey. At 1325° (718° C.) the spectrum was prolonged into the bluish green. At 1440° (782° C.) the blue extended beyond Fraunhofer's line G; and at 2130° (1165° C.) a pure and intense spectrum, reaching as far as H, was obtained.

This observation may be carried still further by noting the effects produced upon the increase in the extent of the chemical action in the more refrangible portion of the spectrum, as the temperature is pushed still higher. The temperature of the voltaic are and of the electric spark may thus be inferred greatly to transcend that of the sun and oxyhydrogen jet.

All our artificial lights depend upon the ignition of solid or dense gaseous matter, in the high temperature developed by the chemical changes attendant on combustion. One of the most remarkable instances of the production of light, in this manner, is afforded by directing an ignited jet of mixed oxygen and hydrogen gases upon a piece of lime; the burning gas alone gives scarcely any sensible light, but as soon as the lime has become thoroughly heated, the brilliancy of the light is too great for the eye to bear.

3. Phosphorescence by Heat.-Some substances of mineral origin, when gently heated, emit a feeble light, which in a short time ceases, and cannot be again renewed until after the body has been exposed to the light of the sun, or to that emitted by the discharge of a Leyden jar (112). Native tricalcic diphosphate or phosphorite, and a variety of fluor spar known as chlorophane, exhibit the phenomenon very distinctly. Some organic compounds, such as quinine sulphate, and well-dried flour, particularly maize flour, also exhibit this phenomenon.

4. Luminous Animals.-The existence of phosphorescence may be recognised in the animal kingdom. The waters of the ocean in different parts of the globe, and at different times, appear to be luminous throughout, from the presence of countless hosts of luminous animalculæ : but usually the light of the sea appears to be developed only by agitation, and the crest of every wave may often be seen to be tipped with a beautiful fringe of pale green light. The glow-worm and the fire-fly offer other familiar instances of the same nature. Some kinds of scolopendra, in passing over the ground, leave a luminous trail behind them. Within certain limits, this power of emitting light appears to be under the control of the animal, and it ceases in a few hours after vitality is destroyed. Matteucci extracted from the glow-worm a yellowish phosphorescent matter, the light of

160

SOURCES OF LIGHT-THEORIES OF LIGHT.

[89.

which became extinct at 18° (-8° C.), and recovered its luminosity as the temperature rose, but it disappeared when the temperature rose to 122° (50° C.). The presence of oxygen appears in this case to be necessary, as the luminosity is temporarily suspended by immersion in nitrogen, or in carbonic anhydride.

5. Phosphorescence of Decaying Organic Matter.-Sea fish, in general, and whiting, herring, and mackerel in particular, soon after death, exhibit a luminous appearance; the light is most intense before putrefaction commences, and gradually disappears as decomposition proceeds. In order to observe the phenomenon distinctly, the fish should be gutted, and the roes and scales removed. The entire fish, and especially the soft roe, exhibits the light. By placing such luminous fish in weak saline solutions, such as those of Epsom salts, Glauber's salts, or common salt, these solutions likewise become luminous, and the appearance continues for some days; it is particularly visible when the liquids are agitated. The light is quickly extinguished by the addition of pure water, of lime water, of fermented liquids, of acid and alkaline liquids, and of strong saline solutions in general: the saline solutions, however, on being diluted, recover their luminosity. If the fish be exposed to a cold sufficient to freeze it, the luminosity disappears, but it returns when it is thawed; luminous wood also ceases to emit light below o° C. A temperature of about 100° (38° C.) seems to be that most favourable to the appearance of this remarkable light; it disappears considerably below 100° C., and the faculty of again becoming luminous on cooling is speedily destroyed by the continuance of the heat. (Hulme, Phil. Trans. 1800, 161.) This phosporescence does not appear to be dependent upon the process of oxida tion, for Matteucci found that the light is not sensibly diminished by immersion in nitrogen, hydrogen, or carbonic anhydride.

6. Electricity. The transient light of the electric spark, and the intense glare attendant on a flash of lightning, are familiarly known; but electricity may likewise be made to give a continuous and abundant supply of light: the ignition of charcoal-points between the wires of a voltaic battery may be made to yield a light which dazzles the unprotected eye. Attempts have been made to apply this light to the purposes of illumination on a large scale (280). Other less important sources of light, such as the friction of two pieces of quartz or of loaf sugar, may also possibly be of electrical origin.

7. Crystallization.-Light is likewise developed, under certain circumstances, in the act of crystallization. When the transparent form of arsenious anhydride is dissolved in hot hydrochloric acid, the liquid as it cools deposits crystals of opaque white arsenious anhydride: if the process be watched in a darkened room, the separation of each crystal will be seen to be accompanied by a faint flash. Fused sodic sulphate, and one or other vitrified salts, when dissolved in water and crystallized, exhibit the same phenomenon, which appears to accompany the transformation of a vitreous into a crystalline solid.

§ I. THEORIES OF LIGHT-REFLECTION-REFRACTION.

(90). Theories of Light-Undulations.-Two hypotheses have been proposed to account for the phenomena of light. Upon the first of these, the theory of emission, it is imagined that all

91.]

THEORIES OF LIGHT-UNDULATIONS.

161

luminous bodies are constantly throwing off into space a luminous matter, the particles of which are inconceivably minute, and are projected with a velocity equally inconceivable. These particles, when they fall upon any object, are reflected more or less completely from its surface; and, entering the transparent portions of the eye, form images upon the retina or expanded termination of the optic nerve, and are by it transmitted to the brain; the result enabling us to see the object from which the light was scattered.

Upon the second hypothesis, that of undulation, recourse is had to the supposition of an elastic medium or ether of inconceivable tenuity, filling all space, and the interstices of all material objects. This medium is not light itself, but it is susceptible of being thrown into the vibrations which constitute light; the undulations which fall upon the eye are converged by the lenses on to the retina producing the sensation of light. Upon this theory, therefore, the phenomena are explicable upon a mechanism similar to that by which the vibrations of elastic media are known to be propagated; such, for example, as that by which the undulations of the atmosphere are conveyed to the ear and excite the sensation of sound. The ether by means of which light is supposed to be transmitted, though possessed of inertia, is not admitted, like the atmosphere, to be affected to any sensible extent by gravity.

At present the theory of undulation is universally adopted, as it affords the most complete explanation of the facts upon which the science of optics is based. The analogies between light and sound are not the least striking and interesting amongst the proofs adduced in its support. Indeed, it will greatly facilitate the comprehension of the mechanism by which light is supposed to be propagated, if we first examine some of the phenomena of sound which admit of being traced in a manner more directly appreciable to common apprehension than that of light.

(91) Illustrations of Undulations from the Phenomena of Sound.-We have abundant evidence of the fact that sound, whenever produced, arises from a series of vibrations which are occasioned by any sudden impulse, such as a blow, communicated to any substance possessed of even a very small elasticity. In other words, the impression which we receive is due to the vibration into which the particles of the sounding body are thrown; these vibrations react upon an elastic medium, such as the air: the impulses are communicated by the motions of the particles of air to the ear, and by reaction upon the auditory nerves they excite the sense of hearing.

These motions of sounding bodies are frequently not too rapid to be traced by the eye; for example, a stretched string whilst sounding may be easily seen to be in rapid vibration. Again, if a goblet be dusted over with a little sand, or any

162

VIBRATION-VARIETIES OF Sound.

[91.

fine powder, and a violin bow be drawn across its edge so as to elicit a sound, the particles of dust will be briskly agitated. And in the common experiment of half filling a finger-glass with water, and producing a sound by drawing the moistened finger along its edge, the water within, whilst the sound lasts, is beautifully rippled, to an extent corresponding with the loudness of the tone. These motions are also distinctly visible in the prongs of a tuning-fork whilst it is in the act of producing sound (fig. 66). Such vibrations, however, to render them audible, require the intervention of an elastic medium to convey them to the ear. If a bell be suspended in the receiver of the air-pump, and struck, it will be distinctly heard whilst the vessel is full of air; but, as the exhaustion proceeds, on repeating the stroke it will gradually become feebler, and at last will be inaudible, or nearly so.

FIG. 66.

A 73

Other media besides air may, however, be employed for the transmission of sound. A bell may be rung, for instance, under water, and will be heard by a person also under the water at even a greater distance than in the air. Wood will likewise transmit sound freely, and to still greater distances than atmospheric air.

These impulses require time for their propagation, and the rate of propagation varies in different bodies. Sound travels, for example, at the rate of 1120 feet (341 metres) in a second through air at the temperature of 16° C., of 4708 feet (1435 metres) through water, and of 16,130 feet (4916 metres) per second through iron wire.

The intensity of sound, like that of all forces radiating from a centre, diminishes as the inverse square of the distance; and as it is propagated in waves or undulations, it is subject to reflection from obstacles interposed in its course, producing the various kinds and forms of echo.

(92) Varieties of Sound.-Sounds differ from each other in loudness, quality, and pitch. The loudness of a sound depends upon the extent of the vibration. A tuning-fork vibrating freely in the air produces only a feeble sound; but if the handle be placed upon a table whilst the prongs are vibrating, the wooden surface is thrown into powerful simultaneous vibration, and a loud sound is emitted. Quality, or timbre, depends on the form of the sounding body, and the nature of the material composing it. Differences such as are perceived between the same note when produced by a flute, a trumpet, or a violin, are due to this variety. Successive impulses following each other rapidly at irregular intervals, constitute a noise or continued sound, like the rumbling of carriages in the street, or the rattle of machinery; but when they follow at regular intervals, with a velocity exceeding 16 vibrations in a second, they produce a musical note. The pitch of the note depends on the frequency of these vibrations; the more rapid the vibrations, the sharper does the sound become. The connexion of pitch with the frequency of vibration may be readily verified by pressing a card against the edge of a toothed wheel, which is made to revolve slowly; the distinct strokes of the card against each tooth are heard at first; but by increasing the rapidity of rotation, a low humming note is given out, and as the velocity increases the sound becomes more acute.

Musical notes all have a fixed numerical relation to each other, each octave as the scale ascends having twice as many vibrations in equal intervals of time as the corresponding note of the octave immediately below it. The ratios are exhibited in the annexed table :

In this table, a tuning-fork is considered to have made one complete vibration whilst the prong is passing from a to b and back again (fig. 65): the motion from a to b or from b to a is called an oscillation. The further consideration of this subject would, however, be irrelevant in a work on chemistry, as it belongs to the physical science of acoustics.

It rarely happens that all the particles of a sounding body are simultaneously vibrating. A sounding body generally divides itself into portions vibrating in opposite directions; the intermediate lines or points are quiescent, and these quiescent portions are termed nodal lines or points. If a flat plate of glass be held horizontally by the point of the finger and thumb near its centre, and its surface be sprinkled with sand, on eliciting a musical note by drawing a violin bow across its edge, the sand will accumulate on the stationary parts, and show clearly the position of the nodal lines. By altering the points at which the glass is held, the nodal lines, and the note elicited, may be made to undergo a variety of interesting changes.

The stress exerted by the accumulation of these minute molecular motions is extraordinary. A feat occasionally performed by a powerful singer is to crack a glass by swelling his voice upon the note to which the glass responds. Savart has made some important experiments in relation to this subject. (Ann. Chim. Phys. 1837 [2], lxv. 384.) He found that a copper band, 3 metres long, 7 or 8mm. wide, and I' mm. thick, to which 30 or 40 kilogrammes was attached, when made to vibrate longitudinally, became lengthened 15 or 20 centimetres. In the same way a cylinder of brass 1407m. in length, and 34'95mm. in diameter, became lengthened during its longitudinal vibration to an extent that would have required the application of a tension equal to the weight of 1700 kilogrammes. It is needless to insist on the important practical bearing of these facts on the construction of metallic machinery liable to regular partial oscillation, however slight or apparently trivial such vibrations may be.

The experiments just detailed will show in what way it has been clearly ascertained that it is by successive regularly recurring motions, or undulations, that sound is propagated. A similar principle has been with great success applied, with certain modifications, to trace the yet more interesting and complicated phenomena exhibited by light.

(93) Mechanism of Undulation.-There are two modes in which waves may be propagated: 1, in highly elastic media like