Fluorescence and Photo-Chemistry. 759 



nearly symmetrical, sloping gradually in both directions. 

 It is to this peculiarity that the red colour is due. 



Continued illumination eventually bleaches the photo- 

 compound. 



Eosine behaves in a similar manner — the action of the 

 light being much more rapid, however. 



Rhodamine behaves in a very curious manner. A solution 

 in water is gradually bleached by the action of the light 

 without the formation of an intermediate coloured body, 

 the absorption band fading away without change of form 

 or position. In solution in methyl or ethyl alcohol, how- 

 ever, a strongly fluorescent intermediate bodv is formed, 

 which emits a green fluorescence and exhibits an absorption 

 band of totally different form from that shown by the 

 rhodamine (which shows an orange-red fluorescence). Tho 

 rhodamine absorption band has a double maximum, while 

 the band of photo-rhodamine is single and shifted towards the 

 region of shorter wave-lengths (see PL XVI.). 



If the alcoholic solution of photo-rhodamine, which shows 

 the green fluorescence, is evaporated, and the residue dis- 

 solved in water, the solution shows the same green fluor- 

 escence — a circumstance which is interesting in connexion 

 with the fact that the photo-rhodamine cannot be formed by 

 the action of light on an aqueous solution. It seems probable 

 that in this case the photo-compound may be formed also as 

 an intermediate product, but its rate of decomposition is 

 as great as that of the rhodamine. It is clear that to obtain 

 a recognizable amount of an intermediate body it must be 

 more stable under the action of the light than the original 

 substance ; otherwise it will disappear as rapidly as it is 

 formed. 



It was thought that the rhodamine might be a mixture 

 of two substances — one having a single absorption band 

 at 5500, the other with a band at 5200, the double band 

 actually photographed being the superposition of the two. 

 If this were the case the first substance would probably 

 yield a red fluorescence, the second a green fluorescence ; 

 and if the former were decomposed more rapidly by the 

 action of the light we should have observed change in 

 the colour of the fluorescent light without the intervention 

 of an intermediate body : in other words, the green 

 fluorescence would be present initially, but masked by the 

 orange-red fluorescence. This point was tested by ex- 

 amining the fluorescent light of two solutions simultaneously 

 with a grating — one a solution of rhodamine, the other 



