348 On the Fluorescent Relations of certain Hydrocarhons. 



is very characteristic and may be recognized in all but the 

 white forms (5 and 6). 



This spectrum consists of a dark band about and above the 

 Fraunhofer-line F, of another not quite so well defined and 

 broader between F and G^ and lastly a band about G, which 

 continues into the absokite absorption commencing at 14 of 

 scale (see fig. 6). 



Fig. 6. 



S S /O II J2 3 14 15 



This absorption=spectrum has its bands displaced upwards by 

 solution in the same manner as the fluorescent one j and the 

 same is true of the new body obtained from petroleum above de- 

 scribed^. This absorption (figs. 6 and 7) is undoubtedly due to 

 ehrysogen ; for^ among other facts^ it rapidly disappears from a 

 solution on exposure to sunlight. To see it clearly^ the brown 

 matter soluble in alcohol should be removed; for this exerts a 

 general absorption of the entire spectrum above 10 of scale. 



Maxima and Minima. — ^Then a pure spectrum is thrown on 

 a screen coated with either of these hydrocarbons^ or on a tank 

 filled with their solutions^ a series of maxima and minima are 

 observed corresponding exactly vdth the absorption-spectra of 

 the same substances, a maximum of fluorescence coinciding with 

 a band of absorption. Such a condition as this was entirely to be 

 expected^ and was observed by Stokes in solution of leaf-green 

 (Phil. Trans. 1852^ Pt. II. p. 491), in canaiy-glass, and in nitrate 

 of ui-anium (lb. pp. 497 and 517). Hagenbach has likewise re- 

 marked the same thing in many instances ; but the complete- 

 ness of this relation, changing with the change due to solution, 

 makes these examples specially interesting. 



I would also here remark that the analogy between anthra- 



* I have observed also a displacement downwards of the absorption- 

 bands of oxalate of uranium bv sokition. 



