ELASTIC PROPERTIES OF SELENIUM. 14.3 



Summary of Results. 

 Thin threads of vitreous selenium tested by methods of 

 bending and of direct extension, show mechanical properties 

 similar to those of viscous solids, such as pitch and sealing- 

 wax, the first application of a load causing an immediate 

 elastic strain, while the continued application results in a 

 continually increasing permanent deformation. 



Similar effects are observed with crystalline selenium, 

 but the viscosity effect is relatively very much smaller. 



In the case of preparations of vitreous selenium the 

 viscosity effect, the gradual movement under continued 

 application of a load, is found to depend upon the illumin- 

 ation, the rate of movement being greater when the 

 selenium is exposed to light than when it is in the dark; 

 the response to change of illumination is not instantaneous, 

 the rate decreasing for several hours, when a change is 

 made from diffused daylight to darkness. 



No such light effect has been detected with preparations 

 of crystalline selenium. The observations made on these 

 are, however, less satisfactory than those on threads of 

 vitreous selenium. 



With threads both of vitreous and of crystalline selenium 

 the viscous movement is much greater shortly after pre- 

 paration than a few hours later. 



A test by direct extension of a thread of vitreous selenium 

 gives a value for Young's modulus of 4*89 x 10 10 dynes 

 per sq. cm. at 20°'8 0. 



