A cyclopaedia of physical sciences : comprising acoustics, astronomy, dynamics, electricity, heat, hydrodynamics, magnetism, philosophy of mathematics, meteorology, optics, pneumatics, statics &c. &c / by J.P. Nichol with the assistance of eminent scientific men.

  • Nichol, J. P. (John Pringle), 1804-1859.
Date:
1860
Catalogue details

Licence: Public Domain Mark

Credit: A cyclopaedia of physical sciences : comprising acoustics, astronomy, dynamics, electricity, heat, hydrodynamics, magnetism, philosophy of mathematics, meteorology, optics, pneumatics, statics &c. &c / by J.P. Nichol with the assistance of eminent scientific men. Source: Wellcome Collection.

    806/952 (page 772)
    that it is filled with the same Ether which gives rise to Light and Colours; only vibrates more rapidly, and diffuses itself by means of shorter waves. In an interesting memoir in Poggendorfs Annalen, part xcviii., Esselbach thinks he has established that the undular length of the ex- tremest fluorescent rays yet known to us is 0-0003 millimeters.—Our next inquiry, however, is at leugth freed from all uncertainty and vagueness. II. Under articles Radiant Heat, Diather- mancy, &c., an account has been given of the memorable researches of Melloni, confirmed, and in some cases anticipated by Professor Forbes. The closest similarity between what were then termed the Heating rays and the Light-giving rays, in all their affections, was placed beyond doubt by that brilliant series of discoveries. The rays of Heat, whether in the Solar Beam or rising from some obscure heated source, can be reflected, refracted, dispersed, and polarized pre- cisely as a ray of Light. But a vital question still remained at issue. Is the ray of Heat an independent although analogous ray? Is the spectrum merely a compound spectrum, consisting of a Light-giving spectrum and a Thermal spec- trum, distinct although partially superposed ? or is it one single spectrum, merely varying at dif- ferent paits of the space over which it is dis- persed, by the fact of a slower or more rapid vibration of one and the same Ether ? In short, are we entitled to lay down the general proposi- tion. All effects, Actinic, Light-giving, Thermal, flow from one and the same physical agency— viz., from the propagation of vibrations having different velocities or periods, through the same elastic Ether f If the afiBrm ative be established, the following proposition may be laid down:— The Light-giving effect is confined within limits in the dispersed ray, because—as with the ear the eye can be affected only by vibrations within certain limits of rapidity: some vibrations do not affect the eye, but they affect our other senses through their heating power; while vibra. tions of the higher range appear chiefly through their chemical influences. Melloni, in the earlier stages of his inquiries, was disposed to accept the theory of two superposed spectra; but he .subsequently gave a modified assent to tlie doc- trine now 'almost universally received. The truth of the doctrine seems indeed to be placed beyond reach of question by the recent elaborate researches of Masson and Jamin—these eminent physicists having demonstrated that, if on any part of the visible spectrum any physical change be impressed affecting its ligiit-OIVLNG power, pre- cisely the same change will be found to have passed over its heating power. For instance, take the method of absorption. Destroy by the interven- tion of coloured media, the liglit of any of the colours of the spectrum; the heat belonging to that colour is destroyed also. Or inversely, tal<e any coloured medium—such as red glass, certain green glasses, a solution of sulphate or bichro- mate of copper in ammonia—which can be tra- versed by one colour only, every such medium permits the heat belonging to the special colour to pass, and it extirpates or intercepts all other heat. Again, employing a glass of blue cobalt, which is known to divide the visible spectrum into bright and obscure strips or bands—the same bands exist as to its heating effects. The same Inquirers have however gone further, and occu- pied themselves with still more delicate re- searches. They polarized the coloured ray on its issuing from the prism; they caused it to pass through quartz of different thicknesses and through solutions of sugar, and they found inva- riably that the planes of polarization of Light and Heat deviate in the same direction, and by the same amount. Also, three thin plates of ^, ^, and 1, the length of a wave, presented the same interferences for both classes of phenomena; the first giving a circular polarization, the second a plane polarization in a plane at right angles, and the third leaving the emerged ray polarized as the radiant ray.—Repeating the foregoing general proposition in terms more specific, Mas- son and Jamin conclude thus:— In all pheno- mena produced by a radiation of definite and distinctive refrangibility at once Thermal and Luminous—the relations of the quantities of Heat and of Light, before and after the action, are identical. All vibratory modifications estab- lished in regard to Light, hold with the same in- tensity, and the same numerical value in the case of Heat. And this invariable correspondency of Efi'ects, compels us to admit an identity of Causes. 2.—The inquiry next demanding attention re- lates to the dispersion or longitudinal distribu- tion of this heating influence through the spectral space. In Poggendorfs Annalen, part ci., the student will find a clear resume of all the inves- tigations undertaken in relation to this interest- ing subject. The most recent and the most complete are by J. Miiller, and to an account of these we shall here confine ourselves. Miiller first proceeded by aid of absorption; but he sub- sequently adopted the much more accurate me- thod of estimating the heat in successive strips or portions of the visible spectrum, and in the space beyond the red, by means of a delicate and powerful lineal thermo-multiplier. He also took the essential precaution to compare, previous to making his experiments, the various deflexions of tlio needle, with the force of the currents causing these deflections. He began by using for dispersion a fluid glass prism, and in tiie spectrum so produced he detected the following thermal effects:— Boundarj' of Indigo and violet, 2 Middle of tlie blue, - * Middle of the yellow ~ — ' lliddle of the red '0 1' beyond body of red,..- - J* 2' do. do 4' do. do 6' da do. .11 7