THE ACTION OF LIGHT 557 



It is very common to find that substances which absorb light of a particular 

 wave length radiate it again, either at an increased wave length, or of the same 

 wave length as that absorbed. The cases in which ultra-violet light is absorbed 

 and given out again as visible light are the most striking. Such are : solutions 

 of quinine salts, solid anthracene, and so on. In the cases mentioned, part of the 

 light, as we have seen, is used for chemical change. A solution of eosin, which 

 has an absorption band in the blue-green, gives a green fluorescence. 



When we are dealing with colloidal solutions it is sometimes more difficult 

 to state whether the phenomena observed are properly to be called fluorescence. 

 For example, a colloidal solution of the acid of Congo-red gives an orange-red 

 "fluorescence." The light transmitted is blue, and it seems that the particles 

 really reflect orange red light in the same way as the dry solid itself does, like 

 other solids of the same colour. The light transmitted would naturally be the 

 complementary to this colour. 



In examining colloidal solutions by the Faraday-Tyndall beam, confusion may sometimes 

 be caused by fluorescence. When this is present, the path of the beam will be illuminated, 

 whether colloidal substances are there or not. The distinction can usually be made by 

 interposing screens of various colours between the light and the solution, in order to absorb 

 that part of the light, usually the ultra-violet, causing the fluorescence. The colloidal 

 phenomenon is, of course, found with any wave length of light, provided it is powerful enough. 

 An interesting solution to examine is fresh urine, filtered to remove any coarse particles. 

 Examined in white light, the beam is not extinguished in any position of the Nicol prism 

 used to observe it. But that this is due to the fluorescence of the pigment is shown by the 

 interposition of a yellow screen to absorb the violet end of the spectrum. The Faraday- 

 Tyndall beam is still present, and can be abolished by rotation of the Nicol prism, showing 

 the presence of colloids. 



It seems probable that both fluorescence and phosphorescence are really cases 

 of photo-chemical reactions with storage of light energy. Phosphorescence is the 

 giving off of light, not only during illumination, but for a longer or shorter time 

 afterwards. Weigert (1911, p. 26) suggests that a substance A is changed 

 to a substance B, with storage of light energy. The spontaneous return of B to A 

 is associated with the giving out of this energy, again in the form of light. The 

 view is supported by the fact that fluorescence can be changed into phosphorescence 

 at the temperature of liquid air, owing to the reverse reaction being retarded. 

 Phosphorescence itself may be abolished at this temperature, but appears on 

 warming. 



The use of fluorescence in the observation of living tissues under the microscope, 

 has been described above (page 9). 



CHEMI-LUMHSTESCENCE 



When bodies are heated gradually they may be seen to begin to give off light 

 when a certain temperature is reached. This light consists, when it first appears, 

 at a temperature of somewhere about 1,000, only of the longer wave lengths. 

 As the temperature rises, shorter and shorter wave lengths are progressively added. 

 The temperature called " white-heat," as is well known, is very high. Thus light 

 of a particular wave length is associated with a particular temperature in the case 

 of this form of radiation. But light is given of by many chemical reactions at 

 a temperature much below that corresponding to the wave length of the light 

 emitted, supposing it to have been produced by rise of temperature only. This 

 phenomenon is known as chemi-luminescence and is not uncommon. It may be 

 seen by taking a mixture of 10 c.c. of 10 per cent, pyrogallol, 20 c.c. of potassium 

 carbonate, and 10 c.c. of commercial formalin. Add, in the dark, 30 c.c. of 

 30 per cent, hydrogen peroxide. An orange-red glow, accompanied by considerable 

 foaming, will be seen. A list of the reactions in which similar emission of light 

 can be seen, will be found in the paper by Trautz (1905). The reactions in which 

 it occurs are themselves sensitive to light, and the wave length of this light is 

 the same as that which is emitted in the reaction. Thus the system is set into 

 vibration of its own particular rate by the chemical reaction itself (see Nernst's 

 book, 1913, p. 815). 



We may also speak of such reactions as being cases of direct conversion of 



