2 Thomson, Notes on Arsenic. 



evaporate some of the arsenic, which condensed on the 

 top of the bulb as a white film, and rapidly became black 

 as it did when sublimed in an atmosphere of hydrogen 

 or of carbonic anhydride. 



It occurred to me that this blackening action in the 

 dark may be due to some obscure radiations which are 

 everywhere present, and some experiments were made 

 by enclosing these films in thick lead tubes, but they 

 appeared to blacken when so enclosed just as rapidly as 

 when enclosed in an ordinary wooden pencil case. Never- 

 theless these white films may prove of interest to the 

 physicist, as some means may be found for preserving 

 their whiteness, and so making it possible to determine 

 the presence of certain radiations as yet unknown. 



On placing tubes containing these mirrors in liquid 

 air they remained white for 5 hours, during which time 

 they were immersed. The test mirror which was momen- 

 tarily withdrawn from the liquid air from time to time 

 became blackened, whilst the others which were immersed 

 and in the dark, remained white. 



I find that this form of arsenic has been studied by 

 several observers,* but more especially by Hugo Erdmann 

 and Max von Unruh who find that it is more volatile than 

 the black form (which our observations confirm). They 

 say — it is soluble in carbon disulphide, and whilst in solu- 

 tion it is not affected by light, but may be recovered as a 

 yellow deposit on evaporation of the solvent, and it is then 

 very sensitive to all kinds of light. On being left in solution 



*Schuller, Math. ti. naturit. Ber. aus Ungarn, 1S89, vol. 6, p. 94. 



Retgers, Zeit. anorg. Chem., vol. 4, p. 403 — 409, and vol. 6, p. 

 397—320. 



McLeod, Chemical Ne^vs, 1894, vol. 70, p. 139. 



Linck, Zeit, Kryst. Min., 1896, vol. 26, p. 280. 



Erdmann & Max von Unruh, Zeit. anorg. Chem., 1902, vol. 32, p. 

 437—452. 



