Phosphorescence 351 



ultraviolet light. He repeated many of the French experiments and 

 studied some forty transparent materials to determine whether they 

 were " permeable " for the phosphorogenic emanation. He noted 

 the phosphoresence of such materials as sulphate of potash and chalk 

 and considered that "it is not improbable that the chalk cliffs of 

 England are sometimes rendered phosphorescent by flashes of light- 

 ning during a thunderstorm." He later demonstrated to his students 

 how the light from an aurora borealis would excite the fluorescence 

 of quinine, 



Henry also studied the ability of the " emanation " to be polarized, 

 using a pile of mica plates instead of a Nicol prism, Avhich was not 

 very permeable to the " emanation." He found that it could be 

 polarized, that it " is an emanation possessing the mechanical proper- 

 ties of light, and yet so different in other respects as to prove the 

 want of identity. That the same ' emanation ' from a spark also dif- 

 fers from heat is " manifest from the fact, that the lime [sulphide] 

 becomes as luminous under a plate of alum as under a plate of 

 rock salt. . . ." He found no excitation of phosphorescence by 

 electromagnetic induction, and any electrical origin of the light was 

 ruled out by the fact that the material could be covered with water 

 and would still phosphoresce. Henry concluded that the excitation 

 resulted from a wave motion perhaps " differing in length and ampli- 

 tude, and possibly also slightly differing in the direction of vibra- 

 tion " from visible light. He was observing the effect of ultraviolet 

 light. 



E. Becquerel's contributions may be grouped in three series, the 

 first between 1839 and 1848, particularly papers published in 1843 

 and 1848; the second, consisting of four memoirs to the French 

 Academy, three in 1857, 1858, and 1859, collected in a booklet, 

 Recherches sur Divers Effets Lumineux qui Resultent de I' Action 

 de la Lumiere sur les Corps (Paris, 1859) and a fourth in 1860; 

 the third period included the occasional papers beginning after 

 1860 and lasting until 1888, three years before his death in 1891. 

 In 1867 his two-volume work, La Lumiere, ses Causes et ses Effets, 

 Paris, appeared. It recorded additional experiments carried out with 

 more elaborate and precise techniques, and was illustrated with fine 

 colored plates of luminescence spectra. 



Becquerel's work had to do with many aspects of the light emis- 

 sion: (I) with the composition of the light and the nature of the 

 agencies which cause phosphorescence, i. e. new methods of excita- 

 tion; (2) the duration of phosphorescence; (3) the intensity of 

 phosphorescence and the law of decay; (4) the spectrum of phos- 

 phorescence; (5) temperature effects. It was reviewed by M. Fara- 



