180 Mr. S. Bidvvell on the Sensitiveness 



the selenium is therefore covered with a good electrical con- 

 ductor, yet is practically bare to the light, which passes through 

 the conductor to the selenium underneath." The sensitive- 

 ness to light of cells constructed in this manner seems to be 

 far in excess of anything that has been previously obtained ; 

 and the " photoelectric " currents which (like the selenium 

 bars of Messrs. Adams and Day) they are capable of origina- 

 ting, are said to be strong enough to be actually useful in 

 practical work. 



It is impossible to read Mr. Fritts's paper without being 

 impressed by the resemblance of some of the phenomena 

 which he describes to those of electrolysis. The mere arrange- 

 ment of the apparatus — two metallic plates with a third sub- 

 stance between them — is in itself strongly suggestive ; while 

 the unequal resistance offered by the two surfaces, and the 

 generation of an independent electromotive force, in conjunc- 

 tion with the polarization-effects above referred to*, make it 

 hard to believe that the conduction of selenium (in the form 

 used in experiments) is not truly and literally electrolytic. 



The only considerable difficulty in the way of this hypothesis 

 arises from the fact that selenium is not an electrolyte. Ever 

 since its discovery in 1817, selenium has been regarded as an 

 element, and very strong evidence indeed would be necessary 

 to deprive it of its elementary character ; this is perhaps the 

 reason why the electrolytic theory has not previously been 

 proposed. But there is a possible way out of the difficulty, 

 which was suggested to me by the first words in the above 

 quotation from Mr. Fritts's paper. He spreads the selenium 

 upon a plate of metal with which it will form a chemical 

 combination. Now selenium will, I believe, combine more 

 or less easily with all metals, forming selenides; and in ex- 

 periments upon the conductivity of selenium, it has been 

 usual to submit the substance to prolonged heating in contact 

 with metallic electrodes. This prolonged heating (generally 

 followed by slow cooling) has hitherto been called " annealing;" 

 and the undoubted fact that it diminishes the specific resist- 

 ance of the selenium and increases its sensitiveness to light, 

 has been explained by supposing that the process is favourable 

 to perfect crystallization. 



I venture to suggest, as the true explanation of the effect, 

 that heating is favourable to a chemical combination between 

 the selenium and the metal forming the electrodes, that a 

 selenide is thus formed which completely surrounds the elec- 



* " The existence of polarization," says Clerk Maxwell, " may be 

 regarded as conclusive evidence of electrolvsis." i Electricity,' vol. i. 

 p. 363. 



