CHAPTER VI.

SPECTRUM ANALYSIS.

If a metal or the salt of a metal be burned in a flame it imparts to the flame a distinctive color; table salt thrown into the fire burns with a yellowish flame, denoting the presence of sodium, and a greenish tint, indicating the combustion of chlorine. Violet flames accompany the burning of the salts of potassium, and barium burns green. Lithium and strontium give a red hue. But to be ordinarily perceptible, the salts require for the most part to be present in considerable quantities. By the use of the spectroscope, however, extremely small proportions of these metals and salts can be readily detected and classified.

If a beam of light be transmitted through a prism of glass the rays are decomposed, and what is known as a spectrum is formed (Fig. 40). The most generally observed spectrum is the rainbow. When the light from a flame in which is burning some suitable substance be transmitted through the prism, the color which predominates in the flame will predominate in its spectrum. The combination of a prism and tubes for observing these effects is a spectroscope (Fig. 41). The short fat spark from the Ruhmkorff coil is most useful in this work. The electrodes are provided with a portion of the substance to be examined, and the spark is passed and viewed through the spectroscope.

The spectroscope is shown in connection with the coil in Fig. 41. _A_ is the aperture in the screen through which the rays from the metal burning at the discharger balls _D D_ passes. The lens at _L_ is used to view these rays after they have been decomposed by the prism _P_, which, as well as the lens, can be rotated. _I_ is the coil, _P P_ the primary and _S S_ the secondary wires, _C_ being a condenser bridged across the circuit.

The screen should be pierced by a very narrow aperture, _A_, and be placed at a considerable distance from the prism _P_, that the rays issuing through the aperture may not strike the prism until they have widely diverged and become separated from each other. The aperture is practically formed of perfectly parallel knife edges, forming a slit not exceeding one hundredth of an inch in width.

The colored spaces in the solar spectrum do not occupy an equal extent of area; the violet is the most extended, the orange the least. The proportion is in three hundred parts: Violet, 80; green, 60; yellow, 48; red, 45; indigo, 40; orange, 27.

The solar rays exhibit on careful examination dark lines crossing the spectrum at right angles to the order of the colors, and always occupying the same relative positions. These are called Fraunhofer's lines.

If, however, the spectra of metals, gases, and other elements be examined they will be found to present certain characteristic _bright_ lines, the body of the spectrum being often feeble or entirely dark. The spectrum of hydrogen gives two very bright lines of red and orange.

An extremely minute quantity of an element is necessary to give distinct lines. Sodium gives a single or double line of yellow light in a position agreeing with that of the orange rays in the solar spectrum.

Potassium gives a red line in the red end and a violet line in the violet end of the solar spectrum. Strontium presents eight bright lines; calcium gives mainly one broad green band and one bright orange band.

In practical work with the spectroscope a solar spectrum is often arranged that it can be used as a comparison with the spectrum being investigated, one spectrum being formed above the other, and the observation made as to which lines coincide. Iron gives nearly sixty bright lines coinciding with the same number of dark lines of the solar spectrum.

The violet rays of the solar spectrum are the rays which possess the maximum chemical action, the yellow the maximum light effect, the red the maximum heating effect. Beyond the violet band of the spectrum exist certain rays termed the invisible rays or ultra-violet rays, which in themselves are not luminous. Their vibratory rate is higher and their wave length shorter than the violet rays, according to the most generally accepted theory of light. These rays, when passed through certain substances, suffer a change and become visible in a luminous state of the substance, which luminosity is termed fluorescence.

The bright yellow line of sodium in the orange rays is found in nearly all spectra, owing to its extensive diffusion in the atmosphere.

Tesla has succeeded in producing electric waves of length approximating to those of white light, which appear to have very little heat. The ideal light is that which shows no heat and does not liberate noxious gases in the air, and were it not for its feeble luminosity, the light of the electric spark passing through a carbonic acid vacuum would approximate this most nearly.

The present mode of obtaining light—that of raising to a high temperature some substance or collection of particles—seems certainly somewhat antiquated. The following notes may be of interest and assistance in researches bearing on the lighting question.

Solid bodies, when heated, show a red glow in daylight at an elevation of temperature corresponding to 1000° Fahr.

Temperature, Color of degrees F. Substance.

1000 Red. 1200 Orange. 1300 Yellow. 1500 Blue. 1700 Indigo. 2000 Violet. 2130 All colors—_i.e._, white.

The number of vibrations per second necessary for the production of light, and the velocity of light being determined, the calculation of the wave lengths of the colored rays becomes possible.

The following table (Sprague) shows this in ten-millionths of a millimetre (a millimetre = .039 inch) measured in the dark lines of the solar spectrum, from red to violet:

Orange = 6.88 Orange, Higher = 6.56 Yellow = 5.89 Green = 5.26 Blue = 4.84 Blue, Higher = 4.29 Violet = 3.93