622 Emission-Spectra of Solid Aromatic Compounds. 



On the other hand, each condensed compound and its 

 derivates, even in the case of isomers, shows an individual 

 solution-spectrum. The chief-spectrum of the /3-bromo- 

 naphthalene 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 /3-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 thousandth part of naphthalene 

 dissolved in monochlorobenzene or in methylbenzoicester. 



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 

 impurities could be counted on the fingers of one hand, if 

 there are any at all. 



I spent much time and money in getting even only very 

 small quantities of certain substances really pure, for 

 example, di phenyl, indene, carbazol, fluorene, and other con- 

 densed 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. 



