140 O. U. VONWILLER. 



these measurements were made a new electric light instal- 

 lation was being erected in the Physical Laboratory, and 

 it was not possible to secure satisfactory artificial illumin- 

 ation of constant intensity for any prolonged period. 



Some trials made with threads shortly after their pre- 

 paration showed that the viscous movement was much 

 greater than with older specimens; for example, a thread 

 held at one end without any load was first examined 15 

 minutes after being drawn; the free end was observed to 

 fall as follows: 



in 10 minutes 0*104 cm. 

 20 „ '184 „ 



30 „ -246 „ 



60 „ '376 „ 



A fortnight later it was found that in 4 \ hours the fall 

 was only 0*058 cm. 



Owing to the viscosity effect it was not possible to obtain 

 a satisfactory value of Young's modulus by bending methods 

 but the readings obtained indicated that it was of the 

 order of 3 X 10 10 dynes per sq. cm. Jt seemed probable 

 that more trustworthy results would be obtained by a 

 method of direct extension, and an attempt was made with 

 a thread of vitreous selenium about 22 cm. long and of 

 average diameter 0'0430 cm., the upper end of which was 

 held in a clamp while a light scale pan was attached by 

 thin wires and sealing-wax to the lower end. By means 

 of two microscopes the movements of two points, one near 

 the upper and one near the lower end, were observed upon 

 the application of a load. The movement of the upper 

 point was always very small and was read with the aid of 

 a scale in the eye-piece of the upper microscope while the 

 lower microscope was moved on a vertical screw. With 

 the exception of a few millimetres at each end the whole 



