160 Intelligence and Miscellaneous Articles. 



presence of certain substances may be detected by fluorescence in a 

 mixture of different substances, but not in the opposite case. 



(9) There are substances which become strongly fluorescent by 

 the addition of acids, and others by the addition of alkalies : in these 

 cases it is immaterial which acid or which alkali is used ; the cha- 

 racter of the fluorescence is always the same*. (Hydrochloric and 

 hydriodic acid form an exception, as they destroy fluorescence.) 



(10) The light of artificial sources, or such as has passed through 

 coloured media, occasionally produces changes in the fluorescence 

 compared with that of the sunlight, inasmuch as the commencement 

 and maximum fluorescence may fall at other parts of the spectrum. 



Connexion between Fluorescence and Phosphorescence . 



It was of some interest to include in my investigations the phe- 

 nomena of phosphorescence which are so markedly developed by the 

 sulphides of the different earth-metals ; and I used for this purpose 

 preparations partly obtained from Albert in Frankfort, and partly 

 from Lenoir in Vienna. 



If the prismatic spectrum is projected on one of these phospho- 

 rescent bodies, the phenomena are the same as with fluorescent 

 bodies. Phosphorescence begins sometimes in the visible, some- 

 times in the ultra-violet rays — -in short, in different preparations, in 

 different parts of the same spectrum ; it has also a maximum (in 

 some substances I found two maxima), and the colour of the light 

 of phosphorescence is the same in the entire extent of the part of the 

 spectrum which excites phosphorescence. If a linear spectrum pro- 

 jected upon such a body be analyzed by a prism, a derived spectrum 

 is also obtained, which in its principal features has the same appear- 

 ance as that of a fluorescent substance ; but the uneven rough sur- 

 face of this body permits no pure spectrum ; a good deal of light is 

 always irregularly scattered, owing to which it was impossible to 

 ascertain with certainty whether there is not here something analo- 

 gous with compound fluorescence ; I imagine that in some of the 

 substances examined something of that kind is the case. The 

 entire appearance which a phosphorescent body offers in the pris- 

 matic spectrum is so completely the same as in a fluorescent one, 

 that from this appearance alone it could not at all be decided 

 whether it was a case of fluorescence or of phosphorescence ; the 

 difference of the two phenomena consists only in the fact that fluo- 

 rescence immediately disappears if the incident light is cut off, 

 while phosphorescence continues in this case, though it quickly 

 diminishes in intensity ; the phenomena occur just as with a fluores- 

 cent liquid in which the concentration is altered to the disappear- 

 ance of fluorescence. The phenomena is first imperceptible at the 

 side of the commencement and in the ultra-violet, and is finally 

 only perceptible at the positions of maximum. Thus I agree with 

 the statement of Becquerel, that fluorescence and phosphorescence 

 are only distinguished by their duration, inasmuch as the former is 

 at once extinguished with the cessation of the exciting rays, while 

 the latter continues. — Wiener Berichte, May 11, 1866. 



* This deportment is, in a certain sense, analogous to that of coloured 

 transparent media, which change their colour by the addition of acids or 

 alkalies. In this also the resulting mixture is independent of the nature of 

 the acid or base added. 



