2 THE ABSORPTION SPECTRA OF SOLUTIONS. 



ferred to and from matter is one of the most fundamental problems of science. 

 A striking example of the fact that a few atoms under peculiar conditions have 

 the power of absorbing an enormous amount of energy is exhibited by the iron 

 absorption lines in the solar spectrum. An arc of carbon electrodes containing 

 iron as an impurity emits enough iron vapor to absorb as much as the iron 

 vapor in the sun. It is thus seen that an infinitesimal amount of iron in the 

 very great atmosphere of the sun is sufficient to absorb a large part of the 

 energy emitted by the photosphere. 



Our present theory of the mechanism of the absorption and emission of 

 radiations is very simple. Light and heat are electromagnetic radiations, and 

 hence the emission and absorption centers must contain either or both electric 

 charges and magnetic poles. As free magnetic poles are unknown to us, while 

 free electric charges are known, this theory makes the electric charge the origin 

 of electromagnetic phenomena. At present no positive electric charges are 

 known to be associated with portions of matter smaller than the hydrogen 

 atom. On the other hand, negative electrons are known to be associated with 

 masses only about one two-thousandth that of the hydrogen atom. As far as 

 experiment shows, these electrons always have the same properties and the 

 same charge (the charge is invariably considered as constant when e/m varies), 

 no matter from what element they may come. It is for these reasons that the 

 electron is made the fundamental unit in the electromagnetic theory. 1 



Electromagnetic radiations, then, have their origin in electric charges. 

 Continuous spectra (as from hot metals) are due to free electrons, and these 

 apparently have very little connection with the chemical constitution of the 

 metal molecules. Fine line and band spectra are apparently due to different 

 systems of electrons within the atom, and are greatly affected in intensity 

 by the presence of neighboring atoms. The electrons of this type vibrate in 

 definite frequencies that can be changed onty very slightly by changing the 

 external conditions. 



THE IONIZATION THEORY AND ABSORPTION AND EMISSION CENTERS. 



Several investigators have considered that band spectra are due to vibra- 

 tions in some way peculiar to a condition existing during the dissociation of mole- 

 cules or the recombination of the dissociated parts. According to Koenigs- 

 berger and Kiipferer 2 the band spectra of iodine, bromine, nitrogen dioxide, 

 sulphur, iodine trichloride, and probably nitrogen and the other gases are due 

 to a dissociation of this kind. Iodine is a typical example. At 60 C. and 

 about 4 mm. pressure the reaction, as expressed by them, is as follows: 



I 2 <=I+I 



At about 800 C. this reaction is practically complete. According to them, 

 iodine possesses a continuous absorption which has a maximum in the green, 

 and this continuous absorption decreases with rising temperature. At 600 

 and above, the absorption is almost entirely in the violet and ultra-violet and 

 is due to the iodine atom. The banded absorption, consisting of thousands of 

 fine lines, results whenever chemical reactions represented by the above equa- 



'Phys. Zeit., 8, 72!) (1907). 2 Ibid., 11, 568 (1910). 



