250 Mr. Shelford Bidwell on the 



by a flexible wire to the negative terminal. When this black 

 pencil is drawn over the paper, it makes a bright red mark 

 which is immediately broken off if the current is interrupted 

 or reversed. If a copper plate be used, the marks will be 

 brown instead of red. 



The red stuff separated in the foregoing experiments turned 

 black when heated, and fused and crystallized like ordinary 

 Se. The resistance of a small cell made from a portion of it 

 was 65 per cent, lower than the average ; the cell was very 

 sensitive to light, the diminution of its resistance under 

 illumination being 53 per cent. 



The question naturally presented itself, whether the separa- 

 tion of the red Se might not in some not easily imagined 

 manner be due to the hydrogen liberated from the water, 

 and not primarily to the current. 



Exp. 25. — To test this a zinc wire 1*7 millim. in diameter 

 was coated with a fused mixture of Se and Cu 2 Se and immersed 

 in dilute sulphuric acid. The acid penetrated the pores of 

 the coating, and soon small bubbles of hydrogen began to 

 rise from the surface, continuing for about 5 hours. The 

 coating was then broken up, and it was found that all the 

 zinc inside it had been dissolved. A microscopic examination 

 of the fragments failed to disclose the smallest trace of any 

 red Se. 



The experiment was repeated with the substitution of 

 ordinary commercial Se, and the same negative result was 

 arrived at. It was ascertained that sulphuric acid did not 

 dissolve the red stuff. 



Exp. 26. — An experiment was arranged with the view of 

 determining whether the current exerted any action, real or 

 apparent, upon Se itself when unmixed with any metallic 

 selenide except such as it might contain as an accidental 

 impurity. 



A well-annealed plate of crystalline Se having a Pt elec- 

 trode fused into one edge was supported in water, the elec- 

 trode being about 5 millim. above the surface. A slip of Pt 

 foil was also immersed, and the two were joined respectively 

 to the terminals of a storage-battery of 26 cells. Until the 

 current had been passing for more than 24 hours, there was 

 no perceptible discoloration of the water. Shortly afterwards 

 it was found that the plate was surrounded by a slight red 

 deposit just above the water-line, and that the lower part of 

 the water had become pink. On examination, it appeared 

 that the red stuff proceeded entirely from a small hole in the 

 plate above the water-line : the Se was therefore broken up 

 and the course of the hole, which was full of water, was 



