ON THREEFOLD EMISSION-SPECTRA OF SOLID COMPOUNDS. 131] 
Beyond this last strip the illuminated ground cannot be separated 
distinctly into strips. 
‘One cannot, however, speak of a single solution-spectrum of a body, 
as the solution-spectrum of the same substance varies greatly with the 
solvent. 
The solution-spectrum of naphthalene, for example, shows 
differences, if the naphthalene has been dissolved in metaxylene or in 
orthoxylene or in paraxylene. Therefore, if one substance shows 
remarkable differences in isomeric solvents, one cannot wonder that the 
solution-spectra of the same substance show even much greater 
differences if more different solvents are used; for instance, if we 
compare the solution-spectra of the same substance when dissolved 
either in a xylene or in aniline, pyridine, ethyl-alcohol, and ethyl-ether. 
On the other hand, each condensed compound and its derivates, 
even in the case of isomers, show an individual solution-spectrum. 
The chief spectrum of the §-bromonaphthalene presents a similar aspect 
to the chief spectrum of the a-bromonaphthalene. But the solution- 
spectra of the two substances, for example, in monochlorobenzene, are 
very different. The solution-spectrum of the a-substance is of a similar 
type to the solution-spectrum of naphthalene itself, presenting only an 
appearance of a certain regularity by the occurrence of some doublets, 
while the solution-spectrum of the 6-form is of a quite different type, 
and shows a most regular structure. It consists of four bands, of quite 
equal aspect, extending from the red into the blue. Each of the four 
bands is formed by five narrow strips, the relative distance and intensity 
of which is quite corresponding in all bands. 
The light of the chief-spectra is fluorescent, and disappears at the 
moment when the cathode-rays stop. 
The light of the solution-spectra is phosphorescent, and very often 
one can see it for some minutes after the discharge which produces the 
cathode-rays is interrupted. 
Only very small quantities of a substance are necessary to produce 
a solution-spectrum bright enough to be remarked and to be measured. 
For example, one can detect in this way less than the hundred thou- 
sandth part of naphthalene dissolved in monochlorobenzene or in 
methylbenzoic ester. 
Of course these phosphorescent solution-spectra are, on the other 
hand, a very sensitive test for the purity of aromatic substances, or, 
what is the same, a very sensitive means of detecting very small 
quantities of admixed foreign aromatic substances. And I am sorry to 
say that, among many hundreds of preparations of the best obtainable 
‘purity,’ the specimens which did not show very marked signs of im- 
purities could be counted on the fingers of one hand, if there were any 
at all. 
I spent much time and money in getting even very small quan- 
tities of certain substances really pure; for example, diphenyle, 
indene, carbazole, fluorene, and other condensed compounds, and some 
of the most famous chemists helped me kindly by the best known 
methods ; but at last I had to give up the hope of getting any of these 
substances in pure condition. Until now they have never been pro- 
duced in a really pure state, and I fear that the same holds true for all 
other aromatic bodies. 
K 2 
