6 Thomson, Detection of Arsenic in Beer. 



are examined by an ordinary pocket lens, there can 

 remain no doubt as to which is the greater, the 

 relative densities of the two becoming then easy to 

 discern. To determine the length of time required to 

 eliminate all the arsenic, when working with my own 

 electrolytic method, and when using respectively currents 

 of 5 and 3 amperes, an apparatus was devised consisting 

 of a three-way stop-cock ; the hydrogen from the electro- 

 lytic apparatus could thus be turned through a drawn out 

 tube, heated and arranged for the deposit of the mirror at 

 the right and at the left of the apparatus at will, without 

 loss of gas from the apparatus. Each of these tubes was 

 provided with an arrangement by which pure hydrogen 

 was kept passing through it till required for the test, and 

 the tube was also kept heated. The tubes were changed 

 every 5 minutes, the arsenic coming off being deposited 

 during 5 minutes in each tube. One-fiftieth of a grain of 

 arsenic trioxide was dissolved in i gallon of liquid, and 

 50 c.c. of that liquid used for the test, and it will be 

 observed by looking at Plate VII., Fig. 2, where photo- 

 graphs of the resulting mirrors are shown, that most of the 

 arsenic is brought off during the first 1 5 minutes, when using 

 5 amperes, the remainder being brought away during the 

 following 15 minutes, for during the seventh and eighth 

 5 minutes no arsenic was deposited. With 3 amperes the 

 result was rather different, the arsenic was brought off in 

 a steadily diminishing quantity till the seventh 5 minutes, 

 no arsenic being shown during the eighth 5 minutes. 

 It may be taken, therefore, that by working the zinc 

 kathode electrolytic apparatus with 3 amperes, all the 

 arsenic from any reasonable quantity, if present, would be 

 removed from the apparatus, and deposited in the narrow 

 tube within 40 minutes after introducing the suspected 

 liquid. It is hardly necessary to mention that in every 



