30 



yellow fluorescence of acridine orange gives place to an orange 

 phosphorescence. The same happens with acridine yellow. As we 

 go below 0°C the yellow fluorescence disappears and all light 

 emission vanishes. On further cooling an orange light emission 

 appears and becomes intense around — 50°C. In acridine yellow 

 this light emission comes from a long-lived excitation since it is 

 very intense even in the slow moving phosphoroscope. If the test 

 tube is held in the light of the UV lamp and then the illumination 

 is suddenly disconnected, the frozen solution shows a strong after- 

 glow lasting for a second or more.* Pyronine B, a dye related to 

 rhodamin B, which shows in liquid solution an orange-yellow 

 fluorescence, loses (like rhodamin) all light emission as soon as 

 the water freezes. No light emission appears even at the tempera- 

 ture of dry ice. However, if 1% glucose is present, below — 40°C 

 a strong red light emission appears which comes from a long- 

 lasting excitation as shown by the strong afterglow. Acridine red, 

 eosin Y, tetrabromo fluoresceine, rose bengal, titan yellow, all 

 show similar sudden changes in their fluorescent light emission on 

 freezing. These changes are not limited to dyes. A saturated watery 

 solution of acridine, which has a blue fluorescence at room tem- 

 perature, emits a strong orange phosphorescence if cooled in dry 

 ice. The blue fluorescence of folic acid disappears on freezing. 

 Atebrin (0.0005 M), which has an intense yellowish-green fluo- 

 rescence in water shows, in the frozen state, an orange-green light 

 emission which can be shown to be composed of a yellow fluores- 

 cence and a red phosphorescence by adding 1% glucose to the 

 solution prior to freezing. On direct observation the light emission 

 of the frozen system is found to be green-yellow, while in the 

 phosphoroscope the system shows an intense red phosphorescence, 

 coming from a long-lived excitation. In quinine and quinidine the 

 intense blue fluorescence fades out on freezing to give room, at 

 lower temperatures, to a whitish-blue phosphorescence. "Whitish" 



'If afterglow had to be observed, or if the light emission was studied in 

 the phosphoroscope, no light filters were shunted between the lamp and the 

 material. 



