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ON THREEFOLD EMISSION-SPECTRA OF SOLID COMPOUNDS. 129 
On Threefold Emission-Spectra of Solid Aromatic Compounds. 
By Professor E. GOLDSTEIN. 
[Ordered by the General Committee to be printed in extenso.] 
SOME years ago I observed ' that bright, fluorescent, and phosphorescent 
light is emitted by a number of aromatic solid compounds—for example, 
naphthalene, xanthone, anthracene, &c.—if cathode-rays strike on 
these substances, cooled by liquid air for preventing their evaporation 
and decomposition. In this way I was also able to obtain bright-light 
emission from a great many substances, which at an ordinary tempera- 
ture are liquid bodies—for example, benzene, the three xylenes, benzo- 
nitrile, the chinolines, acetophenone, &c. The light emitted by these 
substances gave bright discontinuous spectra of a great variety, all 
consisting of bands of various width and intensity. 
Since that time I have extended this research on nearly all aromatic 
substances which I could obtain in any way, and have thus obtained 
about two thousand emission-spectra of aromatic substances and of 
mixtures of such substances with other bodies. 
Of course, time does not allow me to give a complete report of this 
work. Here I just want to speak about one result of my experiments. 
In the beginning I was satisfied to observe just a single spectrum 
for each substance, because it was thought that every substance could 
emit only one single spectrum. But soon I found that the complexity 
of phenomena is much greater than it seemed at first sight. For each 
substance does not show only one spectrum, but, according to the 
conditions of the experiment, there appear three spectra, which are 
quite different from each other and have no coincident maximum. I 
call these three kinds of spectra respectively the initial-spectrum, the 
chief-spectrum, and the solution-spectrum of the substance. 
At the first moment, when cathode-rays fall upon the substances, 
there appears quite alone and bright the spectrum which I call the 
initial-spectrum. Then the brightness of the initial-spectrum diminishes 
and gets fainter and fainter till its intensity becomes very small, but it 
never entirely disappears. When the initial-spectrum gets fainter, the 
chief-spectrum at the same time appears and grows brighter and 
brighter. The chief-spectrum is for a great number of substances so 
characteristic that it is possible to recognise the substance in this way 
at a glance and without measuring the wave-lengths, just as you can 
recognise nitrogen by its well-known bands, or hydrogen, mercury, and 
helium by their line-spectra. This is even the case with isomeric sub- 
stances, for one is able to distinguish at a glance, for instance, the 
three isometric xylenes or other isometric aromatic hydro-carbons. The 
third kind of spectra, which is quite different from the two others, 
appears if an aromatic substance is dissolved in any other liquid or 
melted compound and the solidified solution is exposed to cathode-rays. 
Now let me just say a few words on the properties of each of the 
three kinds of spectra. 
The chief-spectra always begin from the infra-red, never reach the 
violet end of the visible spectrum, but end about the middle part of 
' Verhandl. d. Deutsch. Physik. Ges., vi. 156 and vi. 185 (1904). 
1909. K 
