Action of Light on Selenium. 157 



that the action through media which absorb all the more chemically 

 active rays is very nearly as great as when they are not interposed, 

 so that the chemical rays produce very little effect. 



Experiments with the lime light, with rock-salt, alum, and 

 quartz, and their combinations, two together, show that the resist- 

 ance diminishes at the same rate as the illumination increases. 

 This seems to show that the action is almost entirely due to the illu- 

 minating power of the light falling on the selenium. 



Experiments with the electric light, with smoked rock-salt, alum, 

 and a solution of iodine in bisulphide of carbon show that the ob- 

 scure heat-rays do not act powerfully on the selenium. 



In one series of experiments an attempt was made to separate 

 the instantaneous effect from the gradual effect of the light. 



This was done by first balancing the resistance of the selenium 

 before exposure by a resistance E of the coils, then diminishing E 

 by 300, 400, or 500 ohms, according to the brightness of the 

 light, so as to get no sudden deflection when the current is made 

 at the first instant of exposure. 



It was difficult to determine beforehand by estimation what 

 diminution of E should be made ; but after several trials it was 

 quite possible to make the sudden deflection very small, either on 

 one side of the zero or the other, and to keep the needle near the 

 zero by continuing to diminish the value of E as long as the ex- 

 posure lasted. 



In this way the effects of exposure in successive equal intervals 

 f time can be measured. 



The light allowed to pass through the coloured glasses and 

 ther absorbing media was examined by a spectroscope, and it was 

 Pound that the yellowish-green rays were among the most active 

 in altering the electrical state of the selenium. 



A series of experiments was made to determine the effect of light 

 trom different sources. 



A Bunsen burner was employed, and chloride of barium, chlo- 

 ride of strontium, thallium, and sal-ammoniac were introduced into 

 he flame. 



The effect with barium seemed to be less than with strontium. 



"With sal-ammoniac in the flame the effect was as great as with 

 strontium and more lasting. 



With thallium the effect was considerably greater, more gradual, 

 is well as far more lasting than with strontium. 



The effect on repeating an experiment is very much less than the 

 effect of the first exposure with each new source of light. 



Experiments were made with the Bunsen burner alone in its 

 ordinary state and when it is rendered luminous by stopping the 

 ir-holes. 



Exposure to the ordinary Bunsen flame for several seconds only 

 caused a slight deflection of about 10 divisions of the scale. 

 Aiter this slight diminution of resistance the needle gradually re- 

 turned to zero, and was deflected to the other side, as the heat 

 radiated from the Bunsen burner was absorbed by the selenium. 



