Phosphorescence 349 



become phosphorescent in such dry solids as gelatin, sugar, or 

 phthalic acid. The solid often affects the color of the light. Fuchsin 

 in gelatin phosphoresces yellow but in phthalic acid is green. If 

 the gelatin is dissolved in water a short phosphorescence detectable 

 with the phosphoroscope is to be observed. In recent times a boric 

 acid glass containing fluorescent dyes " has been particularly useful 

 in developing theories of fluorescence and phosphorescence. The 

 combination exhibits a remarkably bright and long lasting phos- 

 phorescence after exposure to light. However, the new Lenard and 

 Klatt (1904) phosphors, ^^ prepared in the first years of the twentieth 

 century, have become the more useful variety for commercial 

 purposes. 



Edmond Becquerel and the Recent Period 



Far more important than the preparation of new phosphors is a 

 quantitative study of the physical characteristics of the light which 

 phosphors emit, in relation to various modes of excitation and under 

 various conditions— temperature, pressure, previous treatment, etc. 

 Such investigation requires adequate measuring instruments and 

 brings study of phosphors to a more modern period, beginning 

 about the middle of the nineteenth century. 



In 1839 a paper appeared by A. C. Becquerel (I788T878) , J. B. 

 Biot (1774-1862) , and E. Becquerel (1820-1891) , " Memoire sur la 

 Phosphorescence Produit par la Lumiere Electrique," a summary 

 of previous papers in the Comptes Rendus, in which an attempt was 

 made to find out exactly which radiations in the electric spark " 

 were active in exciting phosphorescence. Various colored films and 

 glasses were used for filters, as well as clear glass, quartz, calcite, and 

 water in a quartz container. The last three allowed as much exciting 

 light to pass as does air, but clear glass and gelatin weakened the 

 exciting rays considerably. The sodium flame was used for the first 

 time as an exciting light and found to be non-effective. The conclu- 

 sions were very definite, the same as those of previous workers at the 

 time the ultraviolet region of the solar spectrum was discovered. 

 The ultraviolet plus the violet and blue regions were most active. 



" Introduced by E. Tiede and P. Wulf {Ber. d. d. Chem. Ges. 55: 588-597, 1922. 

 See also 53: 2206-2216, 1920) . 



^° See E. N. da C. Andrade, Lenard's researches in phosphorescence, in Science 

 Progress 8: 54-71, 1913. 



^~ Excitation by light of the electric spark and by other light sources (flames of 

 phosphorus, sodium, and potassium) was also studied by Carlo Matteucci (1811-1868) 

 the Italian physicist, whose interest in biochemical and biophysical problems led to 

 his memoir (1844, 1847) on luminous animals. Matteucci's work on the phosphors, 

 published in 1842, merely confirmed that of the Becquerel-Biot group. 



