Makch 16, 1906.] 



SCIENCE. 



419 



fluorescence spectrum and enables us to 

 secure data for this distribution in the case 

 of spectra the intensity of which is far too 

 weak to admit of direct measurements of 

 the energy. In the present paper such 

 curves for four typical fluorescent solu- 

 tions, sulphate of quinine in water, fluo- 

 rescin, rhodamin and chlorophyll in alco- 

 hol (together with the energy curves of the 

 absorption spectra of these substances), are 

 presented. These curves and a discussion 

 of their properties will be published in the 

 Physical Review. 

 Conditions Which Change the Resistance 



of the Selenium Cell: P. C. Brown, Uni- 

 versity of Illinois. 



The paper reviews the conditions that 

 produce change in the electrical resistance 

 of the selenium cell— those that produce a 

 remarkable change such as is not found in 

 any other element : 



Light, which changes the resistance as 

 much as ten times. 



Heat, which changes the resistance al- 

 most as much as does light. 



Hydrogen peroxide decreases the resist- 

 ance thirty per cent, when the selenium is 

 placed three cm. from the surface of the 

 liquid. 



Increase of B.M.P. in the circuit may de- 

 crease the resistance as much as 1,000 times. 



Hydraulic pressure decreases the resist- 

 ance even more than sixty per cent. The 

 accompanying curves show how uniform is 

 the effect of pressure. The cells used 

 were, in general, patterned after those of 

 Bidwell. 



In five of the curves which were shown, 

 for three different cells, the pressure coeffi- 

 cient is quite constant, as is shown by the 

 following approximate values: 



.00103 ohms per gram pressure. 

 .00105 ohms per gram pressure. 

 .00120 ohms per gram pressure. 

 .00109 ohms per gram pressure. 

 .00107 ohms per gram pressure. 



Other conditions which produce minor 

 changes of resistance according to different 

 investigators are: X-rays, radium rays, 

 Hertzian waves, ozone treated caoutchouc. 



The following data for one of the selen- 

 ium cells at low temperatures seem to 

 show that the resistance is not much dif- 

 ferent from ordinary room temperatures, 

 and that the sensitiveness to light is about 

 three times as great : 



Resistance — ohms 



86,000. 



82,000. 



07,000. 

 110,000. 

 110,000. 

 114,000. 

 114,000. 



21,000. 



17,000. 



17,000. 



15,800. 



15,700. 



15,600. 



25,600. 



26,800 . 



28,400. 



32,000. 



33,000. 



45,000. 



75,000. 



— 61 



— 60 



— 60.S 



■ 58 to - 

 . —58 

 . —55 

 . —53 

 . —57 

 . —61 

 . —61 

 . —61 



-58 



— 28 



— 10 



off 



off 



off, room darkened 



off 



off 



off 



off 



on 



on after 2 min. 



on after 4 min. 



on after 6 min. 



on after 8 rain. 



119,000 roomtem. 



after 1 min. 

 after 5 min. 

 after 7 min. 

 after 17 min. 

 after 19 min. 

 after 45 min. 

 after 55 min. 

 after 5 hrs. 



Those conditions which decrease the re- 

 sistance very much probably do so for the 

 same reason. 



The theory that has been most generally 

 accepted as to the cause of decrease of re- 

 sistance, is that of Bidwell. He said that 

 it was due to a selenide which was found 

 more or less in every selenium cell, and 

 which made the cell a better conductor 

 when the light fell upon it. Up to the 

 present time, we are not at all assured that 

 a selenide plays any important part. 



Another theory has been given, namely, 

 that light produces crystallization, and 

 since some kinds of crystals conduct better 



