M. E. Hagenbach's Experiments on Fluorescence. 59 



light of fluorescence, by a mirror properly arranged for the pur- 

 pose, on the slit of a large spectroscope, with the aid of which it 

 was analyzed. 



I studied, in all, thirty-six substances. I shall mention here 

 only some which are perhaps less known. 



Solution of Morine Alum. — This liquid, which gives a fine 

 fluorescence, is obtained (as M. Goppelsroder has shown) by 

 dissolving precipitated morine alum (Cuba lac) in alcohol acidu- 

 lated with chlorhydric acid. 



Naphthaline-rose (called in England Magdala red) dissolved 

 in alcohol. — This substance, one of the colouring-matters ob- 

 tained from tar, was discovered by M. Schiendl, of Vienna, and 

 was afterwards studied by M. A. W. Hofmann. It, too, exhibits 

 a remarkable fluorescence, intense and beautiful, of yellow light. 



Thiomelic Acid. —Whoever has made any elementary studies 

 in chemistry knows that after heating for some time a mixture 

 of alcohol and sulphuric acid, as is done in the preparation of 

 olefiant gas, we obtain a thick greenish black residuum. M. 

 Erdmann has given to this substance the name of thiomelic 

 acid. M. Goppelsroder drew my attention to the remarkable 

 fluorescence of this liquid. 



Amide of Phthalic and Amide of Terephthalic Acids. — Under 

 this double denomination I designate two substances, the so- 

 lutions of which in alcohol and ether give a beautiful green 

 and blue fluorescence, and which M. Hugo Miiller, of London, 

 has described as amides obtained by the reduction of nitro- 

 phthalic and nitroterephthalic acicls. 



Phthaleine of Resorcine or Fluoresceins — This substance, 

 which gives a magnificent green fluorescence, was sent me by M. 

 Ad. Bseyer. 



I shall not here give in detail the results of my observations ; 

 they will be found in full in my complete memoir in Poggen- 

 dorfFs Annalen ; but I will come at once to the general conclu- 

 sions set forth by my labours. And first I must remark that 

 the property in question presents itself under the most varied 

 forms. There can be no doubt that here we have to do with a 

 phenomenon which, conformably to what takes place with most 

 physical effects, depends essentially on molecular constitution 

 and chemical composition — each substance appearing as an iso- 

 lated individual with special characteristic properties, in such 

 manner that it is very difficult to lay down general laws suffici- 

 ently exact. In the sequel I shall endeavour to demonstrate 

 this view. 



Fluorescence in the Spectrum or Fluorescent Spectrum. — To 

 the question whether all the radiations of the spectrum are 

 capable of exciting fluorescence, we can answer in the affirma- 



