6 Thomson, Detection of Arsenic in Beer. 



lustre, is that which is crystalline and firmly adherent to 

 the glass tube ; the second, or black form, is amorphous, 

 and can easily be removed from the tube by gentle 

 rubbing. 



I was led to believe that the amorphous form contained 

 occluded gaseous matter, and I have spent some time 

 in collecting a quantity of it (several grammes) which 

 was placed in a tube from which the air was exhausted 

 by a Topler pump, which is the method employed by 

 Sir William Ramsay for removing helium from various 

 minerals. After the tube was exhausted till no further 

 gas could be drawn from it, the black arsenic was heated 

 and it volatilized and condensed in the crystalline form, 

 but no trace of gaseous matter was liberated from it. 



Internal diameter of the tube upon wJiich the arsenic 

 mirror is deposited. 



As the tubes used by me are all drawn out in the same 

 manner from previously selected tube, they are found to 

 be very closely the same in the internal diameter of the 

 bore, but, as these tubes are slightly conical, I devised a 

 simple measuring arrangement for obtaining the mirrors 

 on exactly the same internal diameter of tube. This 

 consists of an iron wire with the one end thinner than the 

 other ; the thicker end is first put into the drawn-out 

 portion of the tube and then the thinner end, to see 

 that the difference in the distance between which they 

 enter is about \ of an inch, which it generally is, 

 although the distance of the wider portion from the 

 beginning of the drawn-out part varies slightly ; the tissue 

 paper for cooling is then placed so that the edge nearer 

 the flame is exactly at the point where the entrance of the 

 thicker end of the wire is arrested, and the mirrors, being 



