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PHOTOGRAPHIC PHENOMENA.

SECT. XXIV.

marking only, that I have other, and I am disposed to think decisive, evidence of the existence of an absorptive solar atmosphere extending beyond the luminous one.” Several circumstances concur in showing that there are influences also concerned in the transmission of the photographic action which have not yet been explained, as for example the influence which the time of the day exercises on the rapidity with which photographic impressions are made, the sun being much less effective two hours after passing the meridian than two hours before. There is also reason to suspect that the effect in some way depends on the latitude, since a much longer time is required to obtain an image under the bright skies of the tropics than in England, and it is even probable that there is a difference in the sun's light in high and low latitudes, because an image of the solar spectrum obtained on a Daguerreotype plate in Virginia by Dr. Draper, differed from a spectral image obtained by Mr. Hunt on a similar plate in England. The inactive spaces discovered in the photographic spectrum by M. E. Becquerel similar to those in the luminous spectrum, and coinciding with them, is also a phenomenon of which no explanation has yet been given. Although chemical action extends over the whole luminous spectrum and much beyond it in gradations of more or less intensity, it is found by careful investigation to be by no means continuous; numerous inactive lines cross it coinciding with those in the luminous image as far as it extends: besides, a very great number exist in the portions that are obscure, and which overlap the visible part. There are three extra-spectral lines beyond the red, and some strongly marked groups on the obscure part beyond the violet; but the whole number of those inactive lines, especially in the dark spaces, is so great that it is impossible to count them.

Notwithstanding this coincidence in the inactive lines of the two spectra, photographic energy is independent of both light and heat, since it exerts the most powerful influence in those rays where they are least, and also in spaces where neither sensibly exist; but the transmission of the sun's light through colored media makes that independence quite evident. Heat and light pass

abundantly through yellow glass, or a solution of chromate of potash; but the greater part of the chemical rays are excluded, and chlorine gas diluted with common air, though highly pervious to the luminous and calorific principles, has the same effect. Sir John Herschel found that a slight degree of yellow London fog had a similar effect with that of pale yellow media: he also remarked that a weak solution of azolitmine in potash, which admits a great quantity of green light, excludes chemical action; and some years ago, the author, while making experiments on the transmission of chemical rays, observed that green glass, colored by oxyde of copper, about the 20th of an inck thick, excludes the photographic rays, and as M. Melloni has shown that substance to be impervious to the most refrangible calorific rays, it has the property of excluding the whole of the most refrangible part of the solar spectrum, visible and invisible. Green mica, if not too thin, has also the same effect, whereas amethyst, deep blue and violet-colored glasses, though they transmit a very little light, allow the chemical rays to pass freely. Thus light and photographic energy may be regarded as distinct and independent properties of the solar beam.

It is not known whether photographic energy be absorbed by material substances or not, neither is it known whether it be concerned in crystalization, and in producing those changes in the internal structure of crystals when exposed to the sun, already mentioned; but the power is universal wherever the solar beam falls, though the effect only becomes evident in cases of unstable molecular equilibrium. The composition and decomposition of those solids, liquids, and aëriform fluids hitherto attributed to light, are chiefly owing to this energy; and as similar chemical changes may be produced by currents of electricity, an occult connection between these two imponderable influences is shadowed out. S

SECTION XXV.

Heat-Calorific Rays of the Solar Spectrum-Experiments of MM. De Laroche and Melloni on the Transmission of Heat-The Point of greatest Heat in the Solar Spectrum varies with the Substance of the PrismPolarization of Heat-Circular Polarization of Heat-Transmission of the Chemical Rays-Absorption of Heat-Radiation of Heat-Dew-Hoar Frost-Rain-Hail-Combustion-Dilatation of Bodies by Heat-Propagation of Heat-Latent Heat-Heat presumed to consist of the Undulations of an Elastic Medium-Parathermic Rays-Moser's Discoveries.

It is not by vision alone that a knowledge of the sun's rays is acquired, — touch proves that they have the power of raising the temperature of substances exposed to their action. Sir William Herschel discovered that rays of caloric which produce the sensation of heat, exist in the solar spectrum independently of those of light; when he used a prism of flint-glass, he found the warm rays most abundant in the dark space a little beyond the red extremity of the spectrum-that from thence they decrease toward the violet, beyond which they are insensible. It may therefore be concluded, that the calorific rays vary in refrangibility, and that those beyond the extreme red are less refrangible than any rays of light. Since Sir William Herschel's time it has been discovered that the calorific spectrum exceeds the luminous one in length in the ratio of 42 to 25, but the most singular phenomenon of the calorific spectrum is its want of continuity. Sir John Herschel blackened the under side of a sheet of very thin white paper by the smoke of a lamp, and having exposed the white side to the solar spectrum, he drew a brush dipped in spirit of wine over it, by which the paper assumed a black hue when sufficiently saturated. The heat in the spectrum evaporated the spirit first on those parts of the paper where it fell with greatest intensity, thereby restoring their white color, and thus he discovered that the caloric is not distributed uniformly, but in spots of greater or less intensity-a circumstance probably owing to the absorbing action of the atmospheres of the sun and earth. "The effect of the former," says Sir John, "is beyond our control, unless we could carry our experiments to such a point of delicacy as to operate separately

on rays emanating from the center and borders of the sun's disc; that of the earth's, though it cannot be eliminated any more than in the case of the sun's, may yet be varied to a considerable extent by experiments made at great elevations and under a vertical sun, and compared with others where the sun is more oblique, the situation lower, and the atmospheric pressure of a temporarily high amount. Should it be found that this cause is in reality concerned in the production of the spots, we should see reason to believe that a large portion of solar heat never reaches the earth's surface, and that what is incident on the summits of lofty mountains differs not only in quantity, but also in quality, from what the plains receive.'

Thus the solar spectrum is proved to consist of five superposed spectra, only three of which are visiblethe red, yellow, and blue; each of the five varies in refrangibility and intensity throughout the whole extent, the visible part being overlapped at one extremity by the chemical, and at the other by the calorific rays; but the two latter exceed the visible part so much, that the linear dimensions of the three, the luminous, calorific, and photographic, are in the proportion of the numbers 25, 42, 10, and 55.10, so that the whole solar spectrum is more than twice as long as its visible part.

That the heat-producing rays exist independently of light, is a matter of constant experience in the abundant emission of them from boiling water. Yet there is every reason to believe that both the calorific and chemical rays are modifications of the same agent which produces the sensation of light. Rays of heat dart in diverging straight lines from flame, and from each point in the surfaces of hot bodies, in the same manner as diverging rays of light proceed from every point of the surfaces of such as are luminous. According to the experiments of Sir John Leslie, radiation proceeds not only from the surfaces of substances, but also from the particles at a minute depth below it. He found that the emission is most abundant in a direction perpendicular to the radiating surface, and that it is more rapid from a rough than from a polished surface: radiation, however, can only take place in air and in

vacuo; it is altogether imperceptible when the hot body is inclosed in a solid or liquid. Heated substances, when exposed to the open air, continue to radiate caloric till they become nearly of the temperature of the surrounding medium. The radiation is very rapid at first, but diminishes according to a known law with the temperature of the heated body. It appears, also, that the radiating power of a surface is inversely as its reflecting power; and bodies that are most impermeable to heat radiate least.

Rays of heat, whether they proceed from the sun, from flame, or other terrestrial sources, luminous or non-luminous, are instantaneously transmitted through solid and liquid substances, there being no appreciable difference in the time they take to pass through layers of any nature or thickness whatever. They pass also with the same facility whether the media be agitated or at rest; and in these respects the analogy between light and heat is perfect. Radiant heat passes through the gases with the same facility as light; but a remarkable difference obtains in the transmission of light and heat through most solid and liquid substances, the same body being often perfectly permeable to the luminous and altogether impermeable to the calorific rays. For example, thin and perfectly transparent plates of alum and citric acid sensibly transmit all the rays of light from an argand lamp, but stop eight or nine tenths of the concomitant heat; while a large piece of brown rock crystal gives a free passage to the radiant heat, but intercepts almost all the light. M. Melloni has established the general law in uncrystalized substances such as glass and liquids, that the property of instantaneously transmitting heat is in proportion to their refractive powers. The law, however, is entirely at fault in bodies of a crystaline texture. Carbonate of lead, for instance, which is colorless, and possesses a very high refractive power with regard to light, transmits less radiant heat than Iceland spar or rock-crystal, which are very inferior to it in the order of refrangibility; while rock-salt, which has the same transparency and refractive power with alum and citric acid, transmits six or eight times as much caloric. This

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