404 



Dr. Brett on detecting minute quantities 



generators, about the circumference. 

 I use oil of vitriol of commerce, which 

 if not free from arsenic, I purify by 

 diluting it with water and submitting 

 It for some considerable time to the 

 action of sulphuretted hydrogen gas. 

 The diluted sulphuric acid employed 

 has a specific gravity of 1'231 ; and 

 this may be obtained by mixing 1 

 measure of the concentrated acid with 

 4 measures of water: I find these 

 proportions better than any others 

 for obtaining a speedy and liberal supply of hydrogen gas. 



It is generally considered that the zinc of commerce con- 

 tains arsenic ; I may here observe, however, that I have ex- 

 amined many specimens both of English and foreign zinc du- 

 ring the last two or three years, and when the sulphuric acid 

 employed was free from arsenic I could never get any indica- 

 tions of that metal by means of the apparatus just described. 

 It will be presently seen how minute a quantity can be de- 

 tected when really present in zinc. 



The first set of experiments was made on alloys of zinc 

 and arsenic. 



200 grs. of zinc were placed in the hydrogen apparatus. I 

 may just state that all the zinc employed in this set of experi- 

 ments was from the same sample, with the requisite quantity 

 of diluted sulphuric acid ; after a short time the gas issuing at 

 the jet was fired and the flame kept up for ten minutes, or a 

 quarter of an hour : no indications of arsenic could be pro- 

 cured on white porcelain. 



200 grs. of zinc were melted in a crucible, and one grain of 

 metallic arsenic dropped in and stirred about ; a small quan- 

 tity of the latter metal was volatilized ; the alloy, still fluid, was 

 cast on a clean iron plate ; when cold, cut into fragments and 

 introduced into the hydrogen apparatus with the diluted acid; 

 very large and abundant arsenical stains wei'e obtained. In 

 this experiment, therefore, rather less than one part of arsenic 

 was alloyed with 200 parts of zinc. 



200 grs. of zinc were melted in a new crucible and a quarter 

 of a grain of metallic arsenic dropped in and stirred about; a 

 small quantity of the latter in this case too was volatilized ; the 

 alloy was cast, broken up, and placed in the apparatus as be- 

 fore : abundant arsenical stains were obtained on porcelain. 



In this experiment there was rather less than one part of 

 arsenic to 800 parts of zinc. 



400 grs. of zinc were melted and '4 grain of arsenic stirred 



