10 IS "I" Til!' AMERICAN ACAD] 



I in carboy, and we have not observed any in- 



iu the acid on standing, snob as might result i'roin 



bion upon tfa i if the latter contained arsenic. It is better, 



:•, th.it Buch acid should be ship}>e<l, or at Least stored, whether 



■ 1 or dilute, in ccresine bottli 



\\ ■ h r,v noticed that tin' citric acid from the carboy, which gave no 



•, took up traces from the Btorage bottle on Long stand- 



We have therefore stored the nitric arid in cere.sine. A slight 



unimportant amount of paraffine is taken up. 50 c.c. lots of this 



■ i irated with a small quantity of sulphuric acid, gave residues 



which Bhowed no traces of arsenic. It must be borne in mind thai a 



nitric acid residue contains the arsenic as arsenic acid and that the 



procedure must accordingly be modified as explained below. 



The second sample of hydrochloric acid, referred to in the footnote 



was shipped in ceresine, and the diluted acid is also kept in 



■ ine. Two loo c.c. lots of this acid were dropped into nitric acid 



and evaporated with sulphuric acid. The residues were reduced with 



Bulphurous acid free from arsenic and gave color bands equal to <>..** 



and 0.5 mmg. arsenious oxide respectively. This is equivalent to 0.004 



mg. ofarsenious oxide per liter. 



Procedure. At the end of a run, a slight annular sublimate is often 

 observed on the inside of the deposition tube where the color band is 

 in contact with the glass. With very small amounts of arsenic this 

 sublimate is white, but is ordinarily slightly colored. It is probably 

 transference of mercuric chloride, either through volatilization 

 or capillary action, and a slight color reaction may take place on the 

 deposit. The amount is without influence on the result, but the tube 

 Bhould be cleaned with a bit of dry cotton before being used again. 



temperature during reduction should not be allowed to rise 



ih, as the moisture equilibrium in the deposition tube is 



from the excels of moisture carried over. For this reason 



lure of Bird, 25 which consists in heating the Liquid under 



ion to the boiling point, is not adapted to this method. 



have found no advantage in nsing very Large amounts of /ine, 



tended by many, especially in the .Marsh process, nor do we 



think it necessary that the zinc should be entirely dissolved. 



We have long noticed that solutions of the dilu- 

 of 0.01 mg. per cubic centimeter undergo a change on standing, 



25 Loc. tit. 



