

Electrical Properties of Selenium. 239 



did not all possess exactly the same degree of sensitiveness : 

 some were better than others, but the sensitiveness of the 

 best was probably not more than 8 or 10 per cent, greater 

 than that of the worst. Lately, however, I have been sup- 

 plied commercially with a batch of selenium which, under the 

 usual treatment, is almost absolutely insensitive to light. 

 This Se differs a little in appearance from the ordinary 

 article. In the vitreous condition its colour is of a lighter 

 shade, and its surface is less lustrous than usual. When 

 melted (above 217°) it forms a tenacious semi-fluid mass, 

 which adheres like treacle to the mica and spatula and is 

 difficult to spread ; it does not exhibit the capillary effects 

 previously referred to, and at a suitable temperature it crys- 

 tallizes with great rapidity. It has not been analysed, but 

 such impurities as it may contain probably do not include 

 the metallic selenides commonly met with*. 



Exp. 4. — Three copper-wire cells of the usual size were 

 made with this Se and annealed for several hours. Their 

 resistances in the dark had the abnormally high values of 

 170,000, 290,000, and 520,000 ohms respectively, the last 

 named being very thinly coated. When illuminated by the 

 incandescent lamp at 12 inches, the decrease of the resistance 

 amounted in the case of the first two to only a fraction of 

 1 per cent, (instead of the customary 40 or 50 per cent.), and 

 in the thinly-coated cell to about 6 per cent. 



Exp. 5. — Two perfectly similar copper-wire cells of smaller 

 size were coated, one with the insensitive Se and the other 

 with ordinary Se, and annealed side by side for 5J hours. 

 On subsequently testing, the resistance of the ordinary cell 

 was found to be 20,500 ohms in the dark and 40 per cent. 

 less when illuminated, while that of the other was 780,000 in 

 the dark and only 2*5 per cent, less under illumination. 



Exp. 6. — A cell was constructed having exactly the same 

 dimensions as those last mentioned, and was coated with 

 insensitive Se with which 3 per cent, of cuprous selenide had 

 been previously incorporated. It w T as not annealed, but cooled 

 gradually during about fifteen minutes. Next day its resist- 

 ance in the dark was 190,000 ohms, and when illuminated 

 by the lamp, 140,000, the decrease being 26 per cent.; in 

 sunlight it fell to 43,000 ohms, or 77 per cent, less than in 

 darkness. 



Exp. 7. — Another cell of the same size was made in which 



* Two other samples of insensitive Se have since been supplied to me 

 from different sources. — June 28th. 



