Fraunhofer drew the important conclusion that the cause of the dark lines in the solar spectrum exists in the sun, although what that cause could be seemed an impenetrable mystery. 189. Dr. Draper's Investigations.-The next most important step, after Fraunhofer, was taken by Dr. Draper, of New York. He was the first to use Fraunhofer's spectroscope in this country, more than thirty years ago. He modified it in 1842, in such a manner as to cast the fixed lines upon the sensitive surface of photographic plates, and published a map of the results, showing four great groups of these lines beyond the limit of the violet ray, and probably doubling the number of lines up to that time known. But, what is more important, he passed to the examination of spectra formed by incandescent terrestrial bodies, and discovered a principle which is fundamental in the philosophy of the subject. He determined the temperature at which a solid body begins to give off light, showed that it is the same for all solids; that, as the temperature increases, the colored rays are emitted in the order of their refrangibility, from red up to violet; and that the spectra cf all incandescent solids are continuous, or without lines or breaks.' 190. Spectra of Gaseous Bodies. But when a solid body is volatilized, its spectrum is changed, becoming discontinuous, or broken up into separate lines; and these are not dark, but bright, and of various colors. If a little sodium is introduced into the gas-flame (Fig. 102), and the 1 As but very imperfect justice has been done to the work of Draper abroad, I am glad to notice the following admission from a recent and English work of high character: "It has been found that all solid and liquid substances act in the same way with regard to the increase of heat; they all begin to be visibly hot at the same temperature, and the spectrum is in every case a continuous one. This law. was discovered by Draper (Philosophical Magazine, 1847). The only known exception to this law is glowing solid Erbia, whose spectrum exhibits bright lines."(Roscoe's "Spectrum Analysis," third edition, p. 51.) spectrum be then observed through the telescope, a brightyellow line of light will appear, always in the same position; and, if a higher dispersive power is applied, this yellow line will be resolved into two, forming the double line which is the distinguishing spectral mark of sodium. If, now, potassium be submitted to the light, three lines appear, two red at one extremity of the spectrum, and a purple line at the other, all else being darkness. If electric currents are sent through pure hydrogen, oxygen, or nitrogen gas, each produces a spectrum of different lines, as shown in the colored frontispiece. The colors of the lines are as variable as the tints of the spectrum, and they vary in numbers through an immense range: while sodium gives but two lines, iron yields several hundreds. 191. What the Lines indicate.—The spectral lines indicate first, chemical identity, and serve as tests of chemical substances. Each element gives a peculiar spectrum, distinguishable from all others in the number, color, breadth, and grouping of its lines. So distinct are they, that when a compound is vaporized all its elements are at once disclosed. If several substances are volatilized together, all the spectra can be identified. Most of the lines are mere films, like the finest spider's web, so that they really occupy but a very small portion of the spectrum space. In some cases, however, several of the bright lines of different bodies seem to coincide; but upon narrower scrutiny these have been generally found to show real though slight differences of refrangibility. Such coincidences as are still unsolved will probably disappear under higher instrumental power. 192. Physical Indications.-The spectral lines are also, to some extent, indices of physical states. With increasing temperature there is increasing brilliancy of the lines, and, with some metals, lines come out under intense heat that do not appear at lower degrees. Pressure or density also affects the spectrum. If the particles of a gas are forced together so as to approach the solid state, the spectrumlines are widened into bands, so as to approach the continuous spectrum. The spectrum of hydrogen may bé thus made continuous by great pressure. But this in no way interferes with the fixity of the bright lines, or their value as chemical tests. § 4. Theory of Absorption. 193. What are the Spectrum Lines?-The optical answer to this question is, that they are images of the slit. A slit of say the fiftieth of an inch would of course give on a screen a very fine white line, which would be simply an image of the aperture. Now, if that filmy ribbon of white light is passed through a prism, the spectrum formed will be a succession of colored lines into which the white line has been resolved, and the whole spectrum will be but a series of images of the slit, either continuous or broken. It is easy to recognize that the bright-colored lines are images, but it may be asked, what are the dark solar lines images of? Darkness is absence of light, and the dark lines of the spectrum simply indicate the absence of luminous rays. It is sometimes supposed that there are dark lines of gossamer delicacy in the sunlight, but this is a misconception. There are rays wanting in the sunlight, and in the spectrum these vacancies come out as lines of darkness. If the slit is changed to a cross, then, as the mark of sodium, we have a yellow cross, instead of a line, and black crosses in the spectrum of sunlight. 194. Coincidence of Bright and Dark Lines.—We thus reach the vital question of Spectrum Analysis, What has become of the missing rays of sunlight? and what is the relation between the dark solar lines and the bright lines produced by burning terrestrial substances? That there is some close relation was suspected by Fraunhofer, and maintained by others after him. The exact coincidence in position of the double dark line D of the solar spectrum and the double bright line of sodium attracted frequent attention, and it was thought it could not be accidental. This conclusion was at length reënforced by overwhelming evidence. The solution of the problem was given by Kirch hoff in 1859. In order to map the positions of the bright lines of various metals, he employed the dark lines of the solar spectrum as his guide. Upon placing one spectrum over the other, he was astonished to find that whole systems of lines in the two spectra were coincident in position and gradation. The coincidence of more than sixty bright lines of vaporized iron with the same number of dark solar lines, the brightest corresponding to the darkest, was shown as represented in Fig. 107, and Kirchhoff proved mathematically that the chances are more than 1,000,000,000,000,000,000 to 1 that this could not happen without some causal connection. Angström has since identified 470 bright iron lines with the dark solar lines, and it has been established that 75 lines of calcium, 57 of manganese, 33 of nickel, and 170 of titanium, exactly correspond in grouping, breadth, and degree of shade, with the same number of dark lines of the solar spectrum. 195. Absorption Lines.-It was thus proved that both orders of lines belong together, and must have a common cause; but what is that cause? A step toward the answer was taken by producing the dark lines experimentally. When light is transmitted through certain vapors, and then passed through the prism, the spectra exhibit dark lines, which vary in the different cases. Fig. 108 represents the spectrum thus formed by the vapor of iodine. The dark lines, in the lunar band, show the rays that have been intercepted, or absorbed, on their passage through the vapor, and they are hence called lines of absorption. 196. What Lines are absorbed?-Vapors absorb the kind of light that they emit, and let all other rays pass. Sodium-vapor gives out yellow light, and so it stops yellow light. This principle is so important that we must show how it may be proved. In Fig. 109 suppose the part N G removed, we shall then have the oil-lamp L giving light which produces a continuous spectrum, which is observed by the direct-vision spectroscope S, the light entering at the slit 8. If now the glass tube N is interposed (which is filled with hydrogen, instead of air, to prevent combustion), and a little sodium is placed in it and heated by the gas-burner G, the tube becomes filled with sodium-vapor. Upon now observing the spectrum, it will be found that the red, orange, green, blue, and violet, have passed through |