Page:The New International Encyclopædia 1st ed. v. 18.djvu/497

* SPECTROSCOPY. 427 SPECTROSCOPY. It has been found as a result of careful obser- vation that, with one or two doubtful exceptions, the spectrum of a solid or liquid is always con- tinuous; and within recent years attempts have been made successfully to express in mathemati- cal form the connection between the temperature of the solid and the distribution of energy in its spectrum as a function of its wave-lengtli. These laws have been deduced theoretically and verified by experiment. It has been found further that the spectrum of a gas when rendered luminous is in all cases discontinuous, although occasionally there is a faint continuous background. This fact in regard to gaseous spectra is what would be expected from the kinetic theory. The exact ori- gin of the spectrum is in general inside the atom ; but the connection between the parts of the atom and the ether in which waves are produced is not known. The spectra of compounds when ren- dered luminous at a temperature not sufficient to decompose definitely the substance have been studied with care, and many interesting facts have been discovered. It has been sliown that all gases will produce under varying conditions dif- ferent spectra, but the reason for this is by no means clear. There are at least three different spectra of hydrogen, many of oxygen, many of argon, etc. The influence of pressure, of tempera- ture, and of the electrical conditions is marked; and these subjects form at present one of the most important fields of research in spectroscopy. The spectrum of a gas is modified if the source of light is either approaching or receding from the spectroscope, as, for instance, in the ease of a star with a motion toward or away from the earth. It is owing to this fact that one is able, by a comparison of the spectra of certain stars with spectra produced here on the earth, to calculate the motion of the stars in the line of sight. See Doppler's Principle. A careful comparison of the lines in the spec- trum of any one gas or vapor, and of the spectra of different vapors, has led to the discovery of several simple mathematical laws connecting them. Thus the lines in the ordinary hydrogen spectrum have such wave-lengths that they can be expressed in a mathematical formula which is known as Balmer's law. This can be expressed as follows : ^h -5 — J- where X represents the wave-lengths, m has in succession the values 3, 4, 5, etc., and h is a constant whose value is ap- proximately 3647.20. A relation similar to this of Balmer's has been shown by Kayser and Runge to apply to most of the lines in the spectra of the alkalies and the alkaline earths. Another law has been found to express most accurately the distribution of the lines in the well-known bands which are pro- duced by carbon, nitrogen, and other substances. Laws have been found also connecting the spec- tra produced by different substances, in those cases where these substances are related chemi- cally. In the year 1806 Zeeman discovered that a source of light if placed in a magnetic field and viewed either along the lines of force or at right angles to them had its spectrum changed by the resolution of its lines into several components. This fact has a most important bearing upon theories of matter and serves to prove that the Vol. XVIII._28. ibrations in the ether are produced by the vi- brations inside the atom of minute electrical charges which have been called electrons. A re- cent investigation of the Zeeman eliect by Runge has shown that the components of these spectrum lines produced by the magnetic field also obey certain mathematical laws. A most important branch of spectroscopy is the study of the solar spectrum as we observe it on the earth. A few of the absorption lines arc due to the fact that the waves coming from the sun pass through the atmosplicre of the earth, and, therefore, suffer absorption owing to tlio water vapor and oxygen in it. The 'rain band' is due to the presence of the former. The other lines are, as explained above, caused by the absorption in the atmosphere of the sun itself. The interior portion of the sun, which is at a high tempera- ture, emits a continuous spectrum, but, owing to the presence in the atmosphere of the sun of me- tallic vapors at a temperature less t!an that of the interior, there is absorption, and thus the solar spectrum is a continuous one crossed by dark lines. There are radiations also coming to us from the outer portions of the sun, the so- called chromosphere and corona ; but these are not easily observed, except at times of solar eclipses. Most of the solar lines can be identified with the spectra of known substances on the earth ; for instance, sodium, iron, carbon, etc., are known to be in the sun. It may be stated in general that if the earth were raised to a tem- perature as high as that of the sun its spectrum as seen at a distance w-oukl be practically identi- cal with that of the sun as we see it. A careful study has been made of the spectra of the various stars, and attempts have been made with more or less success to group the stars in certain classes according to their spectra, the idea being that some knowledge might be ob- tained in regard to the evolution of the stars and their present stage in this progress. Consult Schuster, "The Evolution of Solar Stars," A»tro- j)h;isical ■Journiil, April, 1893. The wave-lengths of a few of the important Fraunhofer lines in the solar spectrum, as meas- ured by Professor Rowland, are as follows: —8 —8 B 6870.186X10 Cm. F 4801.627X10 Cm. C 656.3.064 ■• p f4308.081 Dj 5896.35" ■• ^ tlSOT-S"' " D, 5890. 1S6 ■• H 3968.625 " E, 6270.495 •■ K 3933.825 Ej 6269.723 Bibliography. There is a complete discussion of the methods, results, and theories of spectro- scopy in Kayser, Lehrhuch der Spektroskopie (Leipzig, 1900). A briefer account is given in Landauer, Hpectrum Annh/sis (New York, 1897). Fraunhofer's original papers are reprinted in Prismatic and Diffraction F!pectra, f^cientific Me- moir faeries, vol. ii. (Mew York, 1898) ; and those of Stewart and KirchhofT in Radiation and Ab- xorption, same series, vol. xv. (New York, 1901). Rowland papers On Concnre Gratinrja for Op- tical Purposes (1883) ; On the Relative Wave Lenfifhs at the Lines of the l?o1ar 'Spectrum (1886); together with other papers contained in his collected physical papers (Baltimore, 1002). should be consulted by the student. Jlore popular books dealing with spectroscopy are: Lockyer. Contrihutinns to f^oJnr Phi/sics (London, 1874) ; id., Chemis-try of the Sun (Lon-