4 Thomson, Detection of Arsenic in Beer. 



covered with one layer of tissue paper from {d) to {e) and 

 a stream of water at 50'' C. was kept flowing over it. It 

 is remarkable that at this temperature the bulk of the 

 arsenic was deposited at the point where the compara- 

 tively hot water flowed over the tube, and it is curious to 

 observe that on the further portion of the tube heated to 

 this temperature no further mirror formed, but when the 

 gas passed along the tube to the part which was not heated 

 (to 50^" C.) a second faint black deposit Avas formed, and, 

 after leaving an interval of the tube free from deposit, a 

 third, still fainter, black deposit made its appearance. 



The tube No. 4 in this series was cooled by placing 

 over it beween the points (/) and {g) a single layer of 

 tissue paper, which was cut into a triangular shape at the 

 bottom, and over which water at 15" C. was kept rapidly 

 dropping, as shown in Fig. i. In this tube only one 

 mirror was formed at the po'nt where it came in contact 

 with the cooled tube. 



It was evident, then, first, that the cooling of the tube 

 was an important condition for obtaining the largest 

 mirrors ; secondly, that when the tube was cooled to about 

 I 5*^ C. only one mirror formed; thirdl}', that that mirror 

 had a metallic lustre, andiio second or third black deposit 

 ever formed on any other [jart of the tube. 



Since making these experiments I find that two other 

 chemists have drawn attention to the importance of 

 cooling the portion of the tube arranged for receiving the 

 mirror, the first being Gabriel Bertrand, who employs a 

 strip of filter paper 4 to 5 mm. wide wrapped two or three 

 times round the tube and fed with water drop by drop ; 

 and the second A. Gautier, whose method {Bull. Soc. Chini., 

 1902, p. 27 (20, 2\)) consists in applying a brass rider, the 

 lower part of which is kept immersed in crushed ice. 



My experience has shewn that the best results have 



