at the Royal Institution, 1908-1916 769 



light at low temperatures. The phosphorescence of ozone was also 

 brought under investigation. 



These inquiries have reached their culmination during the sep- 

 tenate under discussion. In this period a furtlier study has been 

 made of the phosphorescent phenomena attending the production 

 of ozone ; in addition photogenic bacteria have been thoroughly in- 

 vestigated. 



The phenomena of phosphorescence are familiar ; for example, 

 the delicate haze of blue light. seen when paper brushed over with a 

 solution of quinine sulphate is exposed to invisible ultra-violet radia- 

 tions. A number of dye-stuffs fluoresce in a most wonderful 

 manner — the cosines in particular. In all these cases, as in that of 

 quinine, rays of higher refrangibility are absorbed and rays of lower 

 refrangibility emitted. 



In the discussion of the phenomena of colour, in my previous 

 essay, in the section on the properties and structure of carbon, atten- 

 tion is called to the close connexion of colour with constitution. The 

 eosin dye-stuffs are peculiar as existing in a coloured form and in a 

 colourless form of different but closely allied structure : it is prob- 

 able that, in solution, the one form constantly passes into the other. 

 It may well be that fluorescence is a consequence of this oscillatory 

 change and it is not improbable that the phenomena of phosphores- 

 cence generally are of a similar order (cp. R. Soc. Proc, pp. 70, 99). 



In the remarkable series of demonstrations which he gave in 1895, 

 Sir James Dewar was able to show that a great number of ordinary 

 materials become brilliantly phosphorescent when exposed to light at 

 a low temperature. Further illustrations were given, in again alluding 

 to these phenomena, in his lecture on January 21, 1910. Thus, an 

 ivory paper-knife, cooled by liquid air, after exposure to light from 

 an arc lamp, phosphoresces brilliantly ; a slip of gelatin, even an 

 ordinary paraffin candle, behaves in a similar manner. The so-called 

 luminous paint, calcium sulphide, on the other hand, which shines 

 after exposure to light at ordinary temperatures, when cooled in hquid 

 air ceases to phosphoresce ; but if it be cooled while in the non- 

 luminous state and then exposed to ultra-violet light it emits light 

 when allowed to become warm. Such cases may be explained as due 

 to changes of molecular structure conditioned by the action of light 

 and followed by an oscillatory change from one form to another when 

 the temperature conditions are favourable. 



An extreme case of the action of ultra-violet light being causative 

 of disruptive action is the altogether remarkable illustration given by 

 Sir James Dewar, in the lecture under consideration, showing that not 

 only is ozone produced if a beam from an electric arc be focused upon 

 liquid oxygen by means of a quartz lens (through the walls of a 

 quartz vacuum vessel) but that even solid oxygen is so affected. 

 Vol. XXL (Xo. 110) 3 e 



