of Selenium and Sulphur to Light. 187 



the resistance, it may easily happen that the effect of gradu- 

 ally rising temperature escapes notice, a balance not being 

 obtained until the temperature has become constant ; and thus, 

 probably, is to be explained the fact that different observers 

 have attributed the most powerful action upon selenium to 

 different parts of the spectrum, ranging from infra-red to 

 greenish yellow. 



The resistance of the sulphur cell which has been de- 

 scribed, unlike that of most selenium cells, was diminished 

 by a rise of temperature"^. When in circuit with a Le- 

 clanche cell and a galvanometer, the effect of holding a nearly 

 red-hot brass rod at a distance of 3 centim. from its surface, 

 was a gradual fall of resistance, which in 15 seconds was 

 indicated by 23 scale-divisions. When the rod was removed, 

 the spot of light slowly returned to its original position, 

 occupying several seconds in doing so. It is certain that the 

 temperature of the sulphur must in this experiment have 

 been much higher than when it was exposed to burning 

 magnesium, with a solution of alum interposed, yet the effect 

 was very much smaller; moreover, it was gradual instead of 

 instantaneous. 



Another experiment seems to prove conclusively that the 

 resistance of the cell is diminished by the direct action of 

 radiation, quite apart from any effect which may be produced 

 by an incidental rise of temperature. On a cloudy day the 

 cell, with the alum trough before it, was placed at a distance of 

 16 feet from a small window, all the other windows in the 

 room being darkened. With the same battery and galva- 

 nometer as before, it w 7 as found that closing the window-shutter 

 caused an instantaneous swing of the spot of light through 90 

 scale-divisions in the direction indicating increased resistance; 

 and when the shutter was again opened, there was immediately 

 an equal swing in the opposite direction. A delicate thermo- 

 pile of 54 pairs, connected with an astatic reflecting- galva- 

 nometer of low resistance, was then put in the place of the 

 sulphur cell, and the alum trough placed before the open end 

 of the conical reflector attached to it. On opening the window- 

 shutter, a deflection occurred indicating a current which was 

 found by trial to be equal to that produced by the radiation 

 of the human body at a distance of 10 ft. 6 in. It is needless 

 to say that such a minute change of temperature as this 

 implies was without sensible effect upon the resistance of the 

 sulphur cell. There can then be no doubt whatever that the 

 whole of the observed effect of the light upon the sulphur 



* This was not so with all the cells subsequently made. S«e descrip- 

 tion of cell no. 3 below. 



