ELASTIC PROPERTIES OF SELENIUM. 137 



centre or end of the thread being measured with the aid of 

 a microscope mounted on a vertical screw. 



It was found that the behaviour of the selenium resembled 

 that of such substances as pitch and sealing-wax, the 

 application of a load resulting in an immediate depression, 

 which increased continuously as long as the load was 

 applied; upon removal of the load there was an immediate 

 partial recovery and a further gradual rise lasting for a 

 time, depending on the magnitude and duration of the load, 

 the total recovery never equalling the total depression. 

 Gradual and continuous bending took place without any 

 load other than the weight of the thread itself. 



Observations made at various times for a week on one 

 thread, showed that the magnitude of the viscosity effect, 

 that is the continuous movement under the application of 

 a steady load, depended on the illumination. This thread 

 was 9*5 cm, long, about 0*04 cm. in diameter, and was 

 supported on two brass cylinders 0*7 cm. in diameter, 

 placed 8*5 cm. apart, no external load being applied. It 

 was noticed that, while the fall of the central point was 

 continuous, the rate of movement was much less in the 

 night than during the day when the selenium was illumin- 

 ated by diffused daylight. That this effect was not due 

 directly to temperature changes alone, was shown by 

 darkening the room during the daytime when the rate of 

 fall was found to decrease. At the end of the week the 

 rate of fall was practically the same as at the beginning 

 under like conditions; in this time the originally straight 

 thread had been bent to form an arc, the central point of 

 which was 0*9 cm. below the straight line joining the ends. 



Experiments with several other pieces of selenium gave 

 similar results; below are given some of the readings 

 obtained in the case of a short thread of average diameter 

 0*041 cm., one end of which was fixed while a rider of mass 



