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PHOTOGRAPHIC TRANSPARENCY.

Diactinic bases, when united with diactinic acids, usually furnish diactinic salts; but such a result is not uniform : the silicates are none of them as transparent as silica itself in the form of rock crystal. Again, hydrogen is eminently diactinic, and iodine vapour, notwithstanding its deep violet colour, is also largely diactinic; but hydriodic acid gas is greatly inferior to either of them.

The same substance, however, whatever may be its physical form, whether solid, liquid, or gaseous, preserves its character; no chemically opaque solid, though transparent to light, becoming transparent photographically by liquefaction or volatilization; and no transparent solid being rendered chemically opaque by change of form. Hence it is obvious that this opacity or transparency is intimately connected with the atomic or chemical character of the body, and not merely with its state of aggregation. Although the absorption of the chemical rays varies greatly in the different gases, which therefore in their action display an analogy to their effects upon radiant heat, yet those gases which absorb the rays of heat most powerfully are often highly transparent to the chemical rays, as is seen in the case of aqueous vapour, of carbonic acid, cyanogen, and olefiant gas, all of which are compound substances, not chemical elements. Compounds as compounds do not appear to act more energetically as absorbents than simple bodies.

The abrupt termination of the chemical spectrum in coal-gas is remarkable in that case the absorption appears to be due, not to the permanent gases, but to the vapours of benzol, and other heavy hydrocarbons that it contains.

In the case of reflection from polished surfaces, the metals were found to vary in the quality of the rays reflected; gold and lead, although not the most brilliant, reflecting the chemical rays more uniformly than the brilliant white surfaces of silver and speculum metal.

Stokes (Phil. Trans. 1862, p. 606) has pursued this investigation in a different manner: instead of photographing the spectra,

that lenses of quartz, or of water enclosed in quartz, would be far superior to those of glass in ordinary use by the photographer. This, however, is not the fact. Glass is very transparent to the less refrangible portion of the chemical rays, extending beyond the violet end of the visible spectrum to a distance as much beyond the line H as the red end of the spectrum is below it; and these rays are precisely the most abundant and powerful chemical rays in the solar spectrum, which contains but few rays of a refrangibility much beyond this point, whereas in the electric arc these highly refrangible rays predo

minate.

FLUORESCENT ABSORPTION.

903

he submitted them to ocular inspection, by receiving the invisible rays upon a fluorescent screen.

He found that the vegetable alkaloids and the glucosides are, almost without exception, intensely opaque for a portion of the invisible rays, absorbing them with an energy comparable, for the

904

FLUORESCENT ABSORPTION.

most part, to that with which colouring matters, such as indigo or madder, absorb the visible rays. The mode of absorption is also generally highly characteristic of each compound, and frequently very different in the same body, according as it is examined in an acid or an alkaline solution. In the examination, a small cell, with parallel faces of quartz, or sometimes a wedge-shaped vessel with its inclined faces also of quartz, was employed. The cell being filled with the solvent, water, dilute acid, dilute ammonia, or alcohol, &c., a minute quantity of the substance under trial is introduced, and the absorptive effects exerted are watched as the substance gradually undergoes solution. Fig. 369, on the foregoing page, is taken from Professor Stokes's paper. The bold lines of aluminum, zinc, and cadmium, are given as points of reference; the border on the left is the limit of the red light visible on the screen. The dotted line in the figure for æsculin denotes the commencement of the fluorescence, which is situated near the line G of the solar spectrum, the luminous portion of which does not extend beyond the termination of the first portion of the spectrum of piperine. Professor Stokes remarks that in the figure the shading merely represents the general effect, the gradation of illumination not having been registered; and although the central parts of the maxima of transparency are left white, in reality there is almost always some absorption. The effect of acids and alkalies on the glucosides presents one uniform feature: when a previously neutral solution is rendered alkaline, the absorption begins somewhat carlier, when rendered acid somewhat later, than in a neutral solution.

(1026) Photographic Spectra of the Elements.-Equally interesting are the results obtained by examining the spectra produced by varying the nature of the metallic electrodes employed as terminals to the secondary wires of the induction-coil. Professor Wheatstone showed many years ago that the visible spectrum of each metal is perfectly characteristic when electro-magnetic sparks are transmitted between two surfaces of the metal; and I have found that the same thing is equally true of the invisible portion of the spectrum.

Even the various gaseous media become so intensely heated by the passage of the electric spark, that they furnish photographic spectra, each of which is characteristic of the body which occasions it; and when the electric discharge of the secondary coil becomes intensified by use of the Leyden jar, the sparks not only produce the spectra due to the metals, but to the gaseous medium in which the electrodes are immersed; so that a mixed spectrum is the re

PHOTOGRAPHIC SPECTRA OF THE ELEMENTS.

905

sult. The spectra produced by the metals are characterized by bands of which the extremities only are visible; whilst the gaseous spectra yield continuous lines which traverse the whole width of the spectrum. When a compound gas is made the medium of the electric discharge, the spectra produced are those of the elementary components of the gas. It seems as though at these intense temperatures chemical combinations were impossible; and at temperatures as intense as those obtained in the voltaic arc, oxygen and hydrogen, chlorine and the metals probably all coexist in a separate form, though mechanically intermingled.

The spectrum produced by the ignition of a solid or a liquid always yields a continuous band of light, containing rays of all degrees of refrangibility; but the same body, when converted into vapour, produces a spectrum consisting of a series of bright bands of particular colours, separated from each other by intervals more or less completely dark, coloured gaseous bodies emitting rays of certain definite refrangibilities only.

From the striped character of the photographic spectra, it is obvious that the vibrations are emitted from the different metals in the form of vapour, and not merely in that of detached particles projected from the electrodes by disruptive discharge.

This observation may give some idea of the intensely high temperature attained by the spark; since it is observed that the higher the temperature, the more refrangible are the vibrations. We are, indeed, furnished in this case with a rude, but still, under the circumstances, with a valuable pyrometric means of estimating these exalted temperatures.*

To give an illustration of the mode of applying the observation :-The hottest wind-furnace of ordinary construction yields a temperature probably not much exceeding 4500° F. By calculations founded upon the amount of heat ascertained by Andrews and others to be emitted during the combustion of a given weight of hydrogen, and the experiments of Regnault upon the specific heats of oxygen, hydrogen, and steam, it has been shown by Bunsen that the temperature of the oxyhydrogen flame cannot exceed 14,580° F. If lime or sulphate of magnesia be introduced into the oxyhydrogen jet, these incombustible materials cannot be heated by the burning gases to a higher point than 14,600° F., but the spectra obtained from those incandescent bodies I found to coincide in their photographic lengths with that of the solar spectrum. Hence, the temperature of the sun may be approximatively estimated to be not higher than that of the oxyhydrogen flame. It certainly appears to be far below that of the electric spark. Magnesium in the electric spark gives a remarkably strong band just beyond the limits of the solar spectrum. Now magnesium is as clearly proved to exist in the solar atmosphere as any element, if we be admitted to have any such proof at all. But unless a very long column of air be supposed to exert an absorbent action not exhibited in shorter ones, it is difficult to avoid the conclusion that the temperature of the solar atmosphere is below that generated by the electric

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PHOTOGRAPHIC SPECTRA OF THE ELEMENTS.

Fig. 370 exhibits a few of the spectra of the metals obtained by the secondary spark, contrasted with the solar spectrum trans

spark, inasmuch as the special band which characterizes magnesium at a high temperature in the electric spark is wanting in the solar spectrum.