68 POLARIZATION OF PART i. 



phosphorescence consists in the time during which the 

 light lasts. Fluorescence ceases almost immediately 

 after the exciting cause is withdrawn, while a phospho- 

 rescent body whether excited by heat, solar light, or 

 electricity, lasts a much longer time; besides, the fluores- 

 cent rays are generally of lower refrangibility. Light 

 and heat 'are temporarily absorbed and given out again 

 by every body on the surface of the earth, more or less, 

 that are exposed to the sun's light. The nights would 

 be much darker even when illuminated by the stars were 

 it not for earth light, for the molecules restore to the 

 ether, in the form of phosphorescence, the undulations 

 they have received from the sun's light during the day. 

 The snow and ice blink of the sailors is a striking 

 instance; generally, however, it is of much shorter 

 duration. The phosphorescent property is nearly allied 

 to electricity, for bodies that are bad conductors are apt 

 to become phosphorescent, while "good conductors of 

 electricity rarely if ever show it. Ozone must be phos- 

 phorescent, for oxygen exhibits persistent light when 

 electric discharges are sent through it, and Mr. Faraday 

 saw a flash of lightning leave a luminous trace on a 

 cloud which lasted for a short time. 



In the solar spectrum the chemical or actinic rays 

 produce phosphorescence, which the red rays have the 

 power to extinguish. M. Mepce de St-Victor found 

 that solar light impresses its vibrations so strongly 011 

 substances exposed for a short time to its influence that 

 they not only shine in the dark, but that the phospho- 

 rescent light they radiate has chemical energy enough 

 to decompose substances in unstable equilibrium, and 

 leave daguerreotype impressions of great delicacy and 

 beauty. 



The polarization of light and heat affords a remark- 

 able instance of the elective power of matter. Light 

 and heat are said to be polarized, which, having been 



