FLUORESCENCE. 381 



similar vegetable growths, are blanched and colourless, or nearly 

 so, because the light does not reach the growing parts ; whereas 

 green coloured leaves and stalks, &c., are produced instead of 

 white ones when the plant grows in such a form that the light 

 freely reaches all parts of it. The evolution of free oxygen gas 

 (Expt. 195) from growing vegetation by the decomposition of carbon 

 dioxide appears generally to require the presence of chlorophyll, 

 in order that the action may go on freely (mushrooms and 

 naturally white vegetation forming exceptions in certain cases). 

 Green plants that freely evolve oxygen in the daytime when exposed 

 to sunlight cease to do so at night or in artificial darkness ; on the 

 other hand, by artificially illuminating such plants at night time 

 (by means of powerful electric lights) continuous growth and evolu- 

 tion of oxygen may be kept up for an unlimited period, if the 

 plant be sufficiently hardy to retain vigorous vitality under such 

 altered conditions. 



Expt. 402. To Illustrate Fluorescence. The phenomenon of 

 fluorescence, as already stated (Chapter XXII.), consists in the 

 absorption of rays of such high refrangibility as to be invisible to 

 the human eye, and the re-emitting of them with a lowered degree 

 of refrangibility, so as to be visible, and of colour dependent on 

 the particular degree of refrangibility of the emitted rays. Sub- 

 stances possessing this property in a high degree will often exhibit 

 a peculiar coloured " sheen " or luminous appearance when viewed 

 in certain positions in ordinary daylight ; thus a bottle half full of 

 solution of sulphate of quinine held level with the eye will 

 generally exhibit a pale blue luminous appearance near the upper 

 surface owing to the fluorescent emission of blue rays ; similarly, 

 many other substances glow in analogous fashion with other colours 

 even in daylight. One of the most effective ways of showing this 

 is to dissolve in a large bulk of water an appropriate fluorescent 

 agent, and by suitable pumping machinery cause the water to 

 form a fountain. The effect, however, is far more marked if 

 visible rays of light other than those emanating from the fluor- 

 escent body examined are entirely excluded ; this is most easily 

 effected by passing a brilliant beam of arc light or sunlight through 

 a prism (Expts. 349, 377) and cutting off by means of opaque 

 screens all the less refrangible and visible part of the spectrum. 

 On placing a piece of glass tinted with uranium, or a solution of 

 quinine,* of the organic compound termed fluorescein (from its high 



* It is curious that if quinine be dissolved in the watery solution of an 

 acid not containing oxygen (such as hydrochloric acid), the liquid does not 

 exhibit fluorescent action ; whilst solutions in nitric or sulphuric acid, or 

 other acids containing oxygen, fluoresce readily. 



