Polytechnic Association. 



919 



bands, they are, in fact, seen by a different method of observation. 

 In this case (see Fig. 2) the port lumiere A, and lens B, and tank of 

 ammonio-sulphate of copper, C, are arranged as before, but the object, 

 D (a crystal or crystalline film of the salt, or a solution of the same), 



is placed in such a 

 position that the 

 light which passes 



jjj-" — mu ' ' " N *^*sg through may be ex- 



• r \H ^^ amined with the 



Fls- 3- spectroscope, E. 



Except in their similarity of arrangement, no connection has been 

 found between these bands of absorption and the luminous bands of 

 fluorescence. 



Indeed, while solution almost entirely destroys the fluorescence of 

 these salts, I have found no change in the absorption bands of the 

 solution compared with those of the solid, except in the case of the 

 oxalate, where a slight displacement downward in the spectrum occurs, 

 and, perhaps, a still smaller change in a few other salts of uranium. 



The platino-cyanides give spectra which are, in a certain degree, 

 monochromatic, that is to say, are confined to a certain tint, and those 

 above and below it for a moderate distance, but do not present a series 

 of bands, as do the uranium salts. 



Passing next to the substance which has been so useful to us as a 

 means of investigation, I will ask your attention for a few moments 

 while I point out the relations of its spectra, under different conditions, 

 to each other, and to an allied body, commercial anthracene. 



When treated in the manner already described for the observation 

 of fluorescent spectra in the uranium salts (see Fig. 2), the body, which 

 we have already described under the name thallene, shows the spec- 

 trum seen as No. 1, of Plate III, the bright bands at 7 and 8.5 of 

 the scale being of light and dark green color, and the broad one, from 

 4 to 5.5, being orange and yellow. 



Commercial anthracene, even when purified with the greatest care, 

 by repeated distillation and recrystallization, shows the spectrum given 

 at No. 2 of the same plate. 



It was this resemblance, together with its wonderful power of 

 fluorescence, which first drew my attention to this substance in a 

 specimen of petroleum distillate sent me by Prof. E. Ii. Horsford, of 

 Cambridge, Mass. ; though the material with which I have chiefly 

 worked was obtained through the kindness of my friend Dr. G. F. 

 Barker from Mr. John Truax, of Pittsburg. 



