THE VELOCITY OF LIGHT. According to the principles of the wave theory, the dispersion of refraction can only be explained as due to a variation of velocity with wave-length or period. In aerial vibrations, and in those propagated through an elastic solid, there is no such variation ; and so the existence of dispersion was at one time considered to be a serious objection to the wave theory. Dispersion in, mem would indeed present some difficulty, or at least force upon us views which at present seem unlikely as to the constitution of free tether. The weight of the evidence is, however, against the existence of dispersion in Vaal°. "Were there a difference of one hour in the times of the blue and red rays reaching us from Algol, this star would show a well-marked coloration in its phases of increase and decrease. No trace of coloration having been noticed, the difference of times cannot exceed a fraction of an hour, It is not at all probable that the parallax of this star amounts to one-tenth of a second, so that its distance, probably, exceeds two million radii of the earth's orbit, and the time which is required for its light to reach us probably exceeds thirty years, or a quarter of a million hours. It is therefore difficult to see how there can be a difference as great as four parts in a million between the velocities of light coming from near the two ends of the bright part of the spectrum."4 For the velocity of light in yam°, as determined in kilometres per second by terrestrial methods (LiGnT, vol xiv. p. 585), Newcomb gives the following tabular statement ; - Newcomb concludes, as the most probable result - Velocity of light in sumo= 299,860±30 kilometres.

It should be mentioned. that Young and Forbes inferred from their observations a difference of velocities of blue and red light amounting to about 2 per cent., but that neither Michelson nor Newcomb, using Foucault's method, could detect any trace of such a difference.

When we come to consider the propagation of light through ponderable media, there seems to be little reason for expecting to find the velocity independent of wave-length. The interaction of matter and (ether may well give rise to such a degree of complication that the differential equation expressing the vibrations shall contain more than one constant. The law of constant velocity is a special property of certain very simple media. Even in the case of a stretched string, vibrating transversely, the velocity becomes a function of wave-length as soon as we admit the existence of finite stiffness.

As regards the law of dispersion, a formula, derived by Cauchy from theoretical considerations, was at one time generally accepted. According to this, /2 - A+BA.-2+CA-4+ (1); and there is no doubt that even the first two terms give a good representation of the truth in media not very dispersive, and over the more luminous portion of the spectrum. A formula of this kind treats dispersion as due to the smallness of wave-lengths, giving a definite limit to refraction (A) when the wave-length is very large. Recent investigations by Langley on the law of dispersion for rock-salt in the ultra red region of the spectrum are not very favourable to this idea. The phenomena of abnormal dispersion indicate a close connexion between refraction and absorption, and Helmholtz has formulated a general theory of dispersion based upon the hypothesis that it may be connected. with an absorbing influence operative upon invisible portions of the spectrum. Upon this subject, which is as yet little understood, the reader may consult Glazebrook's " Report on Optical Theories."' The limits of this article do not permit the consideration of the more speculative parts of our subject. We will conclude by calling attention to two recent experimental researches by Michelson, the results of which cannot fail to give valuable guidance to optical theorists. The first of these3 was a repetition under improved conditions of a remarkable experiment of Fizeau, by which it is proved that when light is propagated through water, itself in rapid movement in the direction of the ray, the velocity is indeed influenced, but not to the full extent of the velocity of the water (v). Within the limits of experimental error, the velocity agrees with a formula suggested by Fizeau on the basis of certain views of Fresnel, viz., v_vo±y2 v (2), Vo being the velocity when the medium is stationary. In the case of water, (p.2 - 1)/y,2= -437. Conformably with (2), a similar experiment upon air, moving at a velocity of 25 metres per second, gave no distinct effect.

From the result of the experiments upon water we should be tempted to infer that at the surface of the earth, moving through space, the ether still retains what must be coarsely called relative motion. Nevertheless, the second research above alluded to3 appears to negative this conclusion, and to prove that, at any rate within the walls of a building, the ether must be regarded as fully partaking in the motion of material bodies.

WAX is a solid fatty substance of animal and vegetable origin, allied both in sources and constitution to the fixed oils and fats. From fats or solid oils wax differs principally in its greater hardness and higher melting-point; but in the strictly chemical sense, while oils and fats are glycerides, a true wax contains no glycerin, but is a combination of fatty acids with certain solid monatomic alcohols. Of wax from animal sources there are in commerce beeswax, which forms wax par excellence, Chinese insect wax, and spermaceti. The more important vegetable waxes are Japanese wax, myrtle-berry wax, carnauba wax, and palm wax.

Beeswax is secreted by all honey bees, and by them formed into the cell walls, &c., of their comb. It is separated by draining the honey, melting the drained comb in boiling water, and collecting the wax which solidifies on the top as the water cools. In this state it is formed into cakes of raw or yellow wax, good examples of which are of a light yellow colour, translucent, with a faint pleasant odour of honey. At ordinary temperatures it breaks with a granular fracture ; and in thin flakes or pellets it softens with the heat of the hand, and can be kneaded between the fingers. Its specific gravity is 0.960 ; it melts at about 62' C., and solidifies just under its melting-point without evolution of heat. It is soluble in hot ether, essential and fixed oils, benzol, bisulphide of carbon, and chloroform, and to some extent in boiling alcohol, but it is unaffected by water and cold alcohol. In chemical constitution it contains 10 per cent. of cerin, an ether of cerotic acid and ceryl alcohol, C271-153° 0, and 90 per Co711„ cent. of myricin - ether of palmitic acid and myricyl alcohol, 1,11,0 O. Yellow wax, on account of the colouring matter and other contaminations it contains, is unfit for many uses. The chief of these is candle-making. To remove soluble matter it is first melted over boilina° water; and for bleaching it is formed into thin shreds and. strips so as to expose the greatest possible surface. So prepared, it is spread out and frequently watered and turned in the direct sunlight, a slow but effective process. To hasten the bleaching action the wax may be mixed with about one-sixth of pure spirit of turpentine; and this preparation, on exposure, by its copious production of ozone, effects in four or five days a bleaching which otherwise would occupy three or four weeks. When the bleaching is complete all trace of turpentine oil will have disappeared. Bleaching may also be effected by chlorine, permanganate of potash, and other chemicals, but these injuriously affect the wax, in some cases forming substitution products which cannot be removed, and which iu burning give of deleterious fumes. Wax is obtained in all parts of the world where there is vegetation sufficient to support bees ; but it is most largely forthcoming from tropical and subtropical regions. It is subject to extensive adulteration from powdered mineral substances, flour, cheaper waxes, paraffin, &c. Its uses are multifarious ; but it is most largely consumed in making candles for the religious services of Roman Catholic and Orthodox Greek Christians, and for wax figures and models (see WAX FIGURES).

Chinese Insect Wax, or Pe La, is a secretion deposited by an insect, Coccus ceriferus, Fabr., in the twigs of a species of ash, Fraxinus chinensis, Roxb. The wax is, in its origin and the functions it performs in the insect economy, closely related to the lac produced by the allied species of Coccus (see Lie, vol. xiv. p. 181). When separated from the twigs which it encrusts, and purified, it is a hard translucent white crystalline body, similar to spermaceti. It melts at from 82° to 86° C., and in composition consists of cerin, one of the constituents of beeswax. It is little known in European commerce, but forms a highly important article of trade in China and Japan, where it is largely used for candle-making and for medicinal purposes.

For SPErtmAcErf, see vol. xxii. p, 395.

Japanese IVax is a hard wax-like fat which now forms an important export from Japan, principally to London. It is obtained from the small stone fruits of several species of Rhus cultivated in Japan. For the extraction of the wax, which is present to the extent of about 20 per cent., the fruits are ground and treated by either of three methods - (1) heating and pressure, (2) boiling in water, and (3) maceration with ether or bisulphide of carbon. The wax is subsequently bleached, and as it comes into the market consists of yellowish hard cakes, covered often with a fine white powdery efflorescence. It has a resinous unpleasant rancid odour. It molts at about 54° C., and solidifies from melting at 41° C. with evolution of heat which raises the temperature about 5° C. Japanese wax becomes translucent about 12° C. under its melting-point, and when it has newly solidified from melting it can again be liquefied at 42° C., from which the melting-point rises by slow degrees with the lapse of time till it reaches the normal. It is not a true wax, but consists principally of the glyceride palmitin with small proportions of stearin and disseminated crystals of free palmitic acid. It is largely mixed with and used as a substitute for beeswax, excepting for uses where its rancidity renders it objectionable.

Myrtle-Berry Wax is obtained from the fruit of several species of Myrica in the United States, New Granada, Venezuela, the Cape of Good Hope, and other regions. It is a hard greenish substance, with a pleasant balsamic odour. Its melting-point is about 45° C., and it consists principally of free palmitic acid with a little stearic acid and myristic acid; - a very small proportion of these being combined as glycerides. It is consumed principally in the United States in combination with beeswax for candles; and it is said the Hottentots eat it like cheese.

Carnauba IVax is an exudation on the surface of the growing leaves of the carnauba palm, Corypha ccri fora, L., which flourishes in tropical South America. The wax is obtained by cutting off and drying the young leaves, from which it is then shaken as line dust, and caked by inciting either over an open fire or in boiling water. It is a true wax, consisting of ethers of myricyl alcohol and ceryl alcohol with cerotic acid, and its melting-point ranges from 85° to 90° C. Carnauba wax is a substance of considerable commercial importance in Brazil, whence large quantities are sent to Europe for use in the candle-trade and otherwise as a substitute for beeswax.

Palm-Tree Wax is an exudation formed on the stems of two South American palms, Ceroxillon andicola, H. and K., and Klopstockia cerifera, Kars. As scraped from the trees and compacted by melting it is a mixture of resin and wax, having a melting-point as high as 102° to 105° C. The pure wax may be separated by digesting with boiling spirit, when it is obtained with a melting-point of 72° C. and a composition analogous to carnauba wax, like which it is used for candles. Palm-tree wax is little seen in European commerce.

For the compound called SEALING WAX, see vol. xxi. p. 586,