20 THE CHEAPEST FORM OF LIGHT. 



No attempt is here made at a complete bibliography, but the follow- 

 ing papers may be consulted by the reader : 



C. Verhoeff ; Zur Biologie von Phosphcenus Jiemiplerus unci Verwandten. Verb. Ver, 



Rheinland, 1894, 208. 

 W. L. Distant; (Luminosity of Fulgorid;e.) Trans. Entonaol. Soc. London, 1895, 



429. 

 A. S.Packard; Pliosphorescence of Organs of Insects. J. N. Y. Entoniol. Society, 



1896, 61. 

 R. Dubois ; Les leufs bmiineuses et leurs larves. Leyons de pbysiologie generales 



et conip. xii, Paris, 1898, p. 301. 



Besides these biologic sources of light, there have been discovered 

 several kinds of mineral matter possessing the extraordinary power of 

 emitting visible, actinic, and skiagra})hic ra^^s. Becquerel, in 1890, an- 

 nounced that all salts of uranium, whether fluorescent or not, yield 

 invisible radiations capable of discharging electrified bodies and of j)ro- 

 ducing skiagraj)hic images on photographic plates ; and this property 

 persisted even after long confinement in a double leaden box. 



Two years later Mme. Sklodowska Curie, of Paris, having examined a 

 series of uranium minerals, found in pitchblende a substance emitting 

 Becquerel rays, so called, 4,000 times stronger than uranium compounds, 

 and named the element believed to be contained therein " polonium." 

 A few months later M. and Mme. Curie announced a second l)ody hav- 

 ing similar })roperties ; this they called " radium." Not long after, a 

 third element, " actinium," was added to the list. Radium, which is the 

 best known, is analogous to barium and accompanies it in chemical 

 reactions so closely that the two have not been satisfactorily separated. 



I have, with Dr. H. C. Bolton, verified the fact that the rays given out 

 by small amounts of radium compounds not previously insolated yield, 

 under favoring circumstances, photographic transparencies very clear in 

 details ; they also exert chemical action, and they excite fluorescence in 

 barium-platino-cyanide screens even through plates of vulcanite, alu- 

 minium, etc., like Rontgen raj^s. These radio-active bodies have been 

 studied by ])hysicists and chemists for several years, but without suc- 

 cess, in isolating the elements or in demonstrating the true source of the 

 energy manifested. 



