8 Thomson, Detection of Arsenic in Beer. 



inserted, and the circles further on shew 4 sets of apparatus 

 working in series at the same time, but about a dozen 

 may be so worked if required from the same current. 



The remarkable fact was observed by Dr. T. E. 

 Thorpe that arsenic, when it existed in the form of pen- 

 toxide, was not liberated at all from the platinum kathode 

 of the Committee's apparatus, and it became necessary in 

 using it to reduce any of the arsenic which might exist in 

 that form to the condition of the trioxide by boiling with 

 ^ a gramme of potassium metabisulphite and 5 c.c. of 

 dilute sulphuric acid before introducing it into the appa- 

 ratus ; this complicates the process a little and is also a 

 source of loss of arsenic, because on condensing the steam 

 produced during the boiling process the presence of a 

 minute quantity of arsenic was detected in the distillate, 

 the amount evaporated however does not seriously 

 interfere with the result. 



I have simplified the Committee's electrolytic appa- 

 ratus by doing away with the bell glass and grinding a glass 

 stopper into the top of a small porous jar, 3 inches in 

 height by 1}^ inches internal diameter, with about xV^h 

 inch thickness of wall. The stopper is provided with two 

 openings, through one of which projects the narrowed end 

 of a circular zinc rod which constitutes the kathode, about 

 ;^nds inch in diameter and i ^ inch long, the thicker part 

 being 2 inches long by about ^ inch diameter. The other 

 opening in the stopper is used for the double purpose of 

 providing an exit for the hydrogen and for introducing the 

 liquid. P/ate VII., Fi^: i, shows the apparatus, with a 

 section removed in the drawing, to shew the position of the 

 pure zinc kathode, I have dispensed with the glass stopper 

 used in the Committee's apparatus for introducing the 

 liquid to be tested, and employed the method previously 

 devised by me for the Marsh-Berzelius apparatus, viz., a 



