ELASTIC PROPERTIES OF SELENIUM. 141 



leogth of the thread could be shielded from light by a brass 

 cylinder blackened on the inside. A narrow longitudinal 

 slit and radial slits in the closed ends enabled the cylinder 

 to be removed without disturbing the thread or its attach- 

 ments, so that an exposure to light could be made when 

 desired. 



Readings with loads of from to 50 gm. showed that 

 Hooke's law was followed, and that with the load applied 

 for several minutes the reading did not change appreciably. 

 A number of readings made with a load of 50 gm. altern- 

 ately applied and removed, with the thread in darkness 

 and exposed to light, gave a mean extension of 0*0143 cm. 

 in a length of 20*7 cm., and any difference on illumination 

 was certainly less than the probable error of this extension, 

 about 0*0003 cm. The value of Young's modulus calculated 

 from these observations is 4*89 x 10 10 dynes per sq. cm., 

 but it must be noted that only one thread was examined 

 in this way so that no great weight should be attached to 

 the numerical result. The mean temperature of these 

 observations was 20°*8 O. 



When the load was allowed to remain for a prolonged 

 period it was found that the extension gradually and con- 

 tinuously increased, and observations made when a load of 

 50 gm. was applied continuously for a week showed that 

 the movement was more rapid when the selenium was 

 illuminated than when it was in darkness. Although the 

 rate of movement was very small and some uncertainty 

 exists as to the coefficient of expansion of the material, 

 the light effect was unmistakeable, and the ratio of move- 

 ment in darkness to that in light seems to be of the same 

 order as in the earlier experiments. The total increase in 

 length in the week was 0*06 cm. 



In order to ascertain whether crystalline selenium 

 possessed similar mechanical properties, endeavours were 



