Action of Selenium Cells. 221 



That by radiation the allotropic modification of selenium 

 may be changed, but that such changes do not occur in the 

 selenium cells, was proved most conclusively by one piece of 

 selenium, which had the anomalous property of showing in- 

 creased resistance on being illuminated. 



Till now I had prepared all selenium cells and all pieces of 

 selenium as I have described above, in the open air or in an 

 air-bath. I never succeeded in obtaining selenium pieces of 

 such low resistance as Messrs. Adams and Day got in three 

 of their pieces, the resistances of which were 55, 58, Q8 ohms. 

 These three pieces differ, it is true, strongly from most of 

 the other pieces in their resistances, which were as high as 

 7,600,000 ohms. I therefore followed their method almost 

 literally. I laid several pieces of amorphous selenium, fur- 

 nished with platinum electrodes and wrapped in paper, for 

 twenty-four hours in sand which had been warmed before 

 by a red-hot iron ball, — a process in which the influence of 

 the aqueous vapour evolved from the paper is not excluded. 

 In this way I got, indeed, pieces of the comparatively low re- 

 sistance of 700 and 2000 ohms; whilst that of the other pieces 

 prepared at the same time amounted still to 350,000 ohms. 



The selenium piece of lowest resistance (700 ohms) showed 

 at first so variable a resistance, that this could scarcely be 

 measured. Then, when it became more constant, a determi- 

 nation of 700 ohms was possible. But on exposure to light, 

 the resistance increased. This behaviour is just the opposite 

 of what all my other cells or pieces of selenium show. And 

 it is equally in contradiction to the diminishing of resist- 

 ance found in all other cases by all other observers ; only 

 Messrs. Adams and Day mention at the end of their nume- 

 rous observations, that one single piece out of the great series 

 behaved like this of mine, increasing its resistance on being- 

 illuminated. This piece of mine in which I observed this ano- 

 malous quality, exhibited in general the tendency to rise 

 steadily in resistance; so that there occurred, on illuminating 

 it, a continuous increase of resistance, and, on darkening, a 

 feeble but never a complete return to the original value. 

 Every exposure to light again renewed this increase; and 

 even a slight shaking of the table changed the resistance. 

 This tendency to increase the resistance was manifested so 

 strongly, that on the following day I found the resistance had 

 risen from 700 to 5600 ohms. 



But this piece of selenium showed still another anomaly, con- 

 tributing, however, to the explanation of the first. I intended 

 to examine the polarization produced in the selenium by the 

 primary battery-current when separated from the battery and 



Phil. Mag. S. 5, Vol. 12. No. 74. Sept. 1881. R 



