274 FLUORESCENT ABSORPTION. [1278. 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 anhydride, 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 surface of silver and speculum metal. Stokes (Phil. Trans. 1862, 606) has pursued this investigation in a different manner; instead of photographing the spectra, 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 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 wedgeshaped 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 127 8.] FLUORESCENT ABSORPTION. 275 as the substance gradually undergoes solution. Fig. 111, below, is taken from Stokes's paper. The bold lines of alu minium, zinc, and cadmium, are given as points of reference; the border on the left is the limit of the red light visible on the 276 PHOTOGRAPHIC SPECTRA OF THE ELEMENTS. [127 8. screen. The dotted line in the figure for æsculin denotes the commencement of the fluorescence, which is situated near the line of the solar spectrum, the luminous portion of which does not extend beyond the termination of the first portion of the spectrum of piperine. 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 earlier, when rendered acid somewhat later, than in a neutral solution. (127 ) 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. Wheatstone showed many years ago that the visible spectrum of cach 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 result. 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 127 t.] PHOTOGRAPHIC SPECTRA OF THE ELEMENTS. 277 into vapour, produces a spectrum consisting of a series of bright bands of particular colours, separated from each other by in tervals more or less completely dark, incandescent gaseous bodies emitting rays of certain definite refrangibilities only (109). From the striped character of the photographic spectra, it is obvious that the vibrations are emitted from the different metals 278 EXTINCTION OF CHEMICAL RAYS. [127 t. 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. Fig. 112 exhibits a few of the spectra of the metals obtained by the secondary spark, contrasted with the solar spectrum transmitted through the same lens and prism under similar circumstances the spectra of oxygen, nitrogen, chlorine, and carbonic anhydride, produced by sparks between platinum points, are also given in fig. 113. Platinum has but a feeble spectrum of its own; and it does not appear in the spectra given in this figure. (127 u) Extinction of Chemical Rays.-Bunsen and Roscoe (Phil. Trans. 1857, 601) have made experiments on the absorbent power of chlorine upon the chemical rays which affect the combination of a mixture of chlorine and hydrogen. They find that when light passes through a medium in which it excites chemical action, a quantity of light is absorbed proportional to the chemical effect produced. For instance, the chemical power of the light of a coal-gas flame of a certain intensity, measured by its activity in |