630 Mr. Shelford Bidwell [March 11, 



produced by orange glass, the smallest by green. It was, too, 

 observed as a remarkable fact that the light transmitted by a dark- 

 blue glass produced a greater effect than that which had been passed 

 through a blue glass of much lighter tint. But on a spectroscopic 

 examination the darker one was found to transmit a certain portion of 

 red light. I also tried the effect of radiation from a black-hot poker 

 held at a distance of about 6 inches from the selenium, and upon the 

 first trial found that the resistance, instead of being diminished, was 

 increased by several thousand ohms. I imagined this to be due to a 

 rise of temperature in the selenium, and was thus led to experiment 

 upon the effect of temperature. In this matter, too, there is a remark- 

 able discrepancy between the authorities. Professor Adams says that 

 an increase of temperature increases the resistance of selenium, and 

 even suggests that a selenium bar should be used for the construction 

 of a very delicate thermometer. Dr. Guthrie, Messrs. Draper and 

 Moss, and others, make the directly opposite assertion that the 

 resistance of selenium diminishes with heat. I repeated my poker 

 experiment, which had in the former case apparently corroborated 

 Professor Adams, and now to my utter astonishment I found that the 

 resistance was greatly diminished. This second experiment, there- 

 fore, seemed to support Dr. Guthrie's statement. A great number 

 of experiments were now undertaken for the purpose of arriving 

 at the truth of the matter, with the details of which I will not 

 weary you. Solutions of alum in water, of iodine in bisulphide of 

 carbon, plates of glass and of ebonite were interposed between the 

 selenium and the sources of light and heat. The selenium was now 

 fried, now frozen ; and the most contradictory results were obtained. 

 At one moment I felt convinced that Professor Adams was right, at 

 the next there appeared to be no shadow of doubt that Dr. Guthrie's 

 was the true theory. In fact, it seemed as if the selenium was pos- 

 sessed by a demon which produced the variations in accordance with 

 the caprices of its own unaccountable will. At length, when the 

 confusion was at its height and the demon most bewildering, the true 

 explanation was suddenly revealed, and so exceedingly simple is it 

 that now the only marvel is that it should have so long eluded dis- 

 covery. The secret of the matter is this : and it discloses one of the 

 most remarkable properties of this most remarkable substance. There 

 is a certain degree of temperature at which a piece of crystalline 

 selenium has a maximum resistance. If a piece of selenium at this 

 temperature is exposed to either heat or cold — it matters not which — 

 its resistance will at once be diminished ; and extremes of either pro- 

 duce a far greater variation than is ever effected by the action of light. 

 A selenium cell which at the ordinary temperature measured in a dim 

 light 110,000 ohms, was reduced by immersing it in oil at 115° C. to 

 18,000 ohms. The resistance of the same cell was reduced by im- 

 mersing it in turpentine at — 6° C. to 49,000 ohms. In the case of 

 the single cell with which I have hitherto made the experiment, the 

 temperature on each side of which the resistance is diminished is 



