SECT. IT. PHOSPHORESCENCE. 67 



the secondary terminals of an induction coil in connec- 

 tion with the coatings of a Ley den jar a and refracted by 

 quartz prisms. 



Mr. Stokes has employed fluorescence as a means of 

 tracing substances in impure chemical solutions. When 

 a pure fluorescent substance is examined in a pure 

 spectrum it is found that on passing from the extreme 

 red to the violet and beyond, the fluorescence commences 

 at a certain point of the spectrum, varying from one 

 substance to another, and continues from thence onwards 

 more or less strongly in one part or another according 

 to the particular substance. The colour of the fluores- 

 cent light is found to be nearly constant throughout the 

 spectrum. ( Hence when in a solution examined in a 

 pure spectrum we notice the fluorescence taking as it 

 were a fresh start with a different colour, we may be 

 pretty sure that we have to deal with a mixture of two 

 fluorescent substances.' 



Experience as well as theory shows that rapid ab- 

 sorption is accompanied by copious fluorescence. But 

 experience has hitherto also shown what could not have 

 been predicted, and may not be universally true, that 

 conversely absorption is accompanied in the case of a 

 fluorescent substance by fluorescence. 



The phosphorescent light of insects, fish, and plants 

 is owing to chemical action, which produces many 

 luminous phenomena ; but a great number of inorganic 

 and organic substances shine in the dark with a phos- 

 phorescence which is nearly allied to fluorescence. It 

 is produced by exposure to the sun, by heat, electricity, 

 insulation, cleavage, friction, and motion. For if a 

 bottle containing nitrate of uranium be shaken, it 

 shines spontaneously with a vivid light ; even the hand 

 shows phosphorescence in the dark after being exposed 

 to the sun. 



The essential difference between fluorescence and 



F2 



