6 A STUDY OF THE ABSORPTION SPECTRA. 



which is found to be wider the smaller the wave-length of the line. Assum- 

 ing that there are particles of varying velocities in the canal-rays, Stark 

 concludes that a certain velocity is necessary before a particle begins to 

 radiate light appreciably. Stark thus considers velocity of translation as 

 one cause of radiation. The other cause of continuous radiation of energy 

 by an atom is frequent collision with electrons or other charged atoms. 



So far no satisfactory model has been devised that will act as a source 

 of spectrum lines. No system that includes electrical charges in orbital 

 motion is permanent on account of their radiation of energy. The favorite 

 and best model consists of systems of coaxial circular rings of equidistant 

 electrons, but even in this case Schott x has shown that such a model (in 

 particular that of Nagaoka), although giving a large number of spectrum 

 lines, is too unstable to produce wave-trains of sufficient length. 



Stark has recently suggested the possibility of explaining a positive 

 charge as due to negative electrons revolving in circular orbits, the centers 

 of these circular orbits being themselves on a circle. By means of such a 

 device a positive charge can easily be explained. He thinks that line- 

 spectra originate from a system of this kind. The general trend 2 of opinion 

 seems to favor the view that spectrum lines are due to some special mechan- 

 isms in the atom which are set in operation during ionization and operate 

 for only a short time. At any one time the atom may be radiating light 

 of but one frequency. 



(b) SPECTRA OF LIQUIDS AND SOLIDS. 



The optical phenomena of gases are so much better understood on 

 account of our more perfect knowledge of gases that considerable space has 

 been given to their discussion. On the other hand, the conditions in liquids 

 and solids are so exceedingly complex that at present our theories are in 

 the main very crude. Considerable advances in our experimental knowl- 

 edge of these phenomena have recently been made and a short summary 

 of these will be given. 



The spectra of liquids and solids can be roughly divided into the con- 

 tinuous spectra emitted by very hot liquids or solids, secondary X-ray 

 radiations, phosphorescent or fluorescent spectra, and the absorption or 

 emission of a banded spectra. 



According to the present theory, X-rays, and possibly ?--rays, and the 

 continuous spectra from hot liquids and solids are due to a rapid and irregu- 

 lar succession of sharp electromagnetic pulses, each of which is due to the 

 change of velocity of electrons. Recent work on X- and ^-rays indicates 

 that in most bodies a certain homogeneous secondary radiation is excited 

 when the body is exposed to the X- and ^--rays. This secondary radiation 

 seems very similar to the phosphorescent bands of the compounds investi- 

 gated by Lenard and Klatt 3 and others. Lenard and Klatt 3 consider that 

 electrons can exist in three different states in a "free" state, as in the metals 



'Phil. Mag., 15,438(1908). 



8 J. J. Thomson: "The Corpuscular Theory of Matter" (1907). Ladenburg and Loria: 



Nature, 79, 7 (1908). Eagle: Ibid., 79, 68 (1908). 

 3 Ann. Phys., 15, 451 (1904). 



