discovery that the radiations of space can produce enduring images by the chemical alterations of matter, it was thought that the marvels of light were exhausted; but, twenty years after photography, came spectrum analysis— the most brilliant and startling of all modern discoveries. It has endowed the chemist with a power of research of hitherto unapproachable delicacy, by which new elements have been discovered, and our knowledge of the composition of matter greatly extended and refined. And, what is much more astonishing, it has revealed the chemical elements in the atmospheres of the sun and the stars; and thus made chemistry a cosmical instead of a terrestrial science. In spectrum analysis, chemistry and physics become most intimately united, so that an account of it becomes necessary before closing the subject of Chemical Physics.

§ 1. The Luminous Spectrum.

177. Newton's Experiment.-The analysis of the solar beam into its elemental colors by the prism has been referred to in speaking of the general properties of light (150): we have now to study the spectrum more carefully.

Spectral phenomena, as seen in rainbow-tints, in the sparkle of jewels, the chromatic flashes of cut-glass, and the gleam

ing hues of clouds at sunset, have ever been familiar; but they were first explained by Newton in his treatise on optics, presented to the Royal Society in 1675, exactly two hundred years ago. He showed that white light, in passing through the prism (Fig. 69), is resolved into its elements, forming a splendid colored image, such as is shown in the plate at the beginning of the volume. This is proved by reversing the process. If the separated colored rays are recombined by a lens, as illustrated in Fig. 94, they reproduce the spot of white light.

FIG. 95.

178. The Solar Spectrum.-The colors produced by prismatic analysis are ultimate elements. Those seen in the perfect solar spectrum are in the highest degree brilliant and pure. They blend with each other in imperceptible gradations, so that their number and limits are indeterminate. For convenience they are designated as forming seven principal groups; but, as Baden Powell remarked, "the fact is, the number of primary rays is not really seven but infinite.” That the colors produced are incapable of further decomposition may be shown by passing a beam of the spectrum through a second. prism, as represented in Fig. 95. The white ray refracted by the prism, S,

Effect of Second Prism.

B

gives the spectrum on the screen, A B. If, now, an aperture is made in the screen, and a colored pencil passed through a second prism, P, it will not be further analyzed, but only diverted in its course.

179. Spectrum of the Electric Light.-Any source of light may be employed to produce a spectrum; but next to the sun, which is by far the most brilliant, the spectrum of the electric light is most powerful, and is gen

FIG. 96.

crally used where intense effects are required. If two carbon cylinders (Fig. 96) are brought near cach other, and the current of a powerful voltaic battery be sent through them, the stream of discharge takes the form. of a brilliant arc of fire between the points with the emission of a dazzling light. This has to be inclosed in a box or lantern, one of the forms of which is shown in Fig. 97. As the carbon-points gradually waste away, the distance between them would become too great, and they are kept in the proper position by the machinery of the lamp. The intensity of the light from the voltaic are depends

Electric Arc

chiefly upon the amount of electricity generated, and the purity of the carbon-points. Measured by its chemical effects, it has been found that the electric light from a Bunsen battery of forty-six elements has nearly onefourth the intensity of sunlight at noon in August. The electric light is convenient for displaying the spec

tra produced by various substances. Fig. 96 shows a piece of sodium placed for volatilization on the lower carbon.

180. Dispersion.-Although in prismatic analysis the colors are always refracted in the same order, yet different substances have very unequal refractive power, and separate the rays unequally. The degree of separation is called dispersion. The dispersive power of water is low. A hollow prism filled with water gives a short spectrum, as shown in Fig. 98 (the lettered lines of which will be presently explained). The dispersion is seen to be much greater with

a prism of crown-glass. Again, the dispersive power of a prism of flint-glass is twice as great as one of crown-glass: and a hollow prism filled with bisulphide of carbon gives a spectrum twice as long as that of flint-glass. The denser the glass the greater the dispersion; and the greater the angle in prisms of all materials, the greater also is the dispersion. But the spectrum loses in sharpness and brilliancy, in proportion as it is extended.

181. Combination of Prisms.--Dispersion may be increased by adding one prism to another in such a way that the refracted light of the first shall pass on through the second. Fig. 99 shows how this may be effected. The electric light emerging through a slit, is directed by the double-convex lens upon a flint-glass prism, and having

undergone one refraction, falls upon a second prism P filled with bisulphide of carbon, which then forms the image VR upon the screen. The image is seen to be diverted more than 90° to one side. With a spectrum thus

produced, eight feet long, the colors would be distinguishable, but will have lost much of their brilliancy.

182. Trains of Prisms. For the usual purposes of examination, a single prism suffices; but, in delicate researches, it is often desirable to increase the dispersion to a high degree. This is especially the case in working with the feeble light from the stars, comets, nebula, and the aurora. Three, four, and sometimes a dozen prisms, are therefore combined when such delicate observations are required; and the whole effect may be doubled by reflecting the light back through the same train, as will be shown when we come to speak of the applications of the spectroscope to celestial bodies (204).