M. E. Hageiibach's Experiments on Fluorescence. 63 



nearer to the violet. The colour of the fluorescence and the 

 position of the maxima in the fluorescence-spectrum also change 

 in some cases with the nature of the solvent ; this is seen, for 

 instance, with the amide of phthalic acid and with solutions of 

 lamp-black. On the other hand, there are cases in which the 

 solvent appears to exert no influence, either on the spectrum 

 thrown on the surface of the liquid or on the fluorescence- 

 spectrum. 



Influence of the State of Aggregation. 



When a substance is fluorescent in the solid state, does it ne- 

 cessarily follow that it must also be so in the liquid state, and 

 vice versa ? The answer to this question differs according to the 

 different substances. 



There are substances, as the double cyanide of platintyn and 

 barium, which are fluorescent in the solid and not at all in the 

 liquid state. There are others, such as nitrate of uranium, 

 which as solids give a very intense fluorescence, and as solu- 

 tions give only a very faint one. There are also substances 

 which are very fluorescent in both states of aggregation ; it is 

 so with photene, sugar of malt, and tincture of curcuma. More- 

 over some substances, feebly fluorescent in the solid state, are 

 very strongly so in solution — e.g. sesculine, sulphate of quinine, 

 chlorophyl, and the amides of phthalic and terephthalic acids. 

 There are, lastly, substances which are not at all fluorescent in 

 the solid state, while they are so in the liquid condition — for ex- 

 ample, naphthaline-rose. 



Relation between Phosphorescence and Fluorescence. 



It is at present impossible to decide whether phosphores- 

 cence and fluorescence are two altogether distinct phenomena, 

 or whether there is an insensible transition from one to the 

 other ; the presumptions, however, are in favour of the latter 

 view. 



This question can only be satisfactorily answered when we 

 have succeeded in ascertaining in some substances the persist- 

 ence of fluorescence, even if for only a very short time. I have 

 tried to obtain this result, but have not succeeded ; nevertheless 

 I do not pretend to give this fact as decisive, because my appa- 

 ratus could only make perceptible a persistence of y^Vo- of a 

 second or more, while it would be indispensable, for this class 

 of experiments, to operate with apparatus very much more de- 

 licate. 



I remark further that the fluorescence-spectra, with inter- 

 mission which we have demonstrated for a great number of sub- 

 stances are evidently analogous to the spectra of phosphorescent 

 substances studied and described by M. Edm. Becquerel. This 

 fact would also tend to connect the two classes of phenomena. 



