Manchester Memoirs, Vol. I. (1906), No. \%. 



XII. Notes. — On an allotropic form of Arsenic 



and 



On the Estimation of Arsenic when in Minute 

 Quantities. 



By William Thomson, F.R.S.E., F.I.C. 



Read Alarch ijth, igo6- Received for ptibli cation June 2bth, J gob. 



I wish to call attention to a peculiar allotropic form 

 of arsenic which is produced when that substance is 

 sublimed and rapidly condensed in vacuo or in presence 

 of an inert gas such as hydrogen or carbon dioxide. 

 When in small quantities it condenses as a white film 

 which rapidly becomes black. This blackening is brought 

 about almost instantaneously by the light from a burning 

 magnesium wire, or by bright daylight, but it takes place 

 rather rapidly by ordinary gaslight, and the whiteness is 

 not preserved when the film is kept in the dark. It 

 continues white for a longer time when kept at the 

 ordinary temperature of the atmosphere than when heated 

 to, say, lOO'^C. It becomes black in about 20 minutes 

 when cold and in the dark, and in about five minutes in 

 diffused light. 



A platinum wire covered with a thin layer of glass 

 upon which ordinary arsenic was laid was enclosed in a 

 glass bulb. A vacuum was made in the bulb by a 

 Topler vacuum pump till no more air could be extracted, 

 and left 24 hours over phosphoric anhydride. The pump 

 was again worked and proved that no leakage of air into 

 the bulb had taken place. A current of electricity was 

 then passed through the wire to heat it to redness, and so 



August 1 4th, igo6. 



