310 PROCEEDINGS OF THE AMERICAN ACADEMY. 



This acid was shipped in carhoy, and we have not observed any in- 

 crease of arsenic in the acid on standing, such as might result from 

 the action upon the glass if the latter contained arsenic. It is better, 

 however, that such acid should be shipped, or at least stored, whether 

 concentrated or dilute, in ceresine bottles. 



We have noticed that the nitric acid from the carboy, which gave no 

 test for arsenic, took up traces from the storage bottle on long stand- 

 ing. We have therefore stored the nitric acid in cei'esine. A slight 

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

 acid, evaporated with a small quantity of sulphuric acid, gave residues 

 which showed no traces of arsenic. It must be borne in mind that 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 

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

 ceresine. Two 100 c.c. lots of this acid were dropped into nitric acid 

 and evaporated with sulphuric acid. The residues were reduced with 

 sulphurous acid free from arsenic and gave color bands equal to 0.3 

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

 mg. of arsenious oxide per liter. 



P^'ocedure. 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 

 due to 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 

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



The temperature during reduction should not be allowed to rise 

 very much, as the moisture equilibrium in the deposition tube is 

 disturbed from the excess of moisture carried over. For this reason 

 the procedure of Bird,^^ which consists in heating the liquid under 

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



We have found no advantage in using very large amounts of zinc, 

 as recommended by many, especially in the Marsh process, nor do we 

 think it necessary that the zinc should be entirely dissolved. 



Standard Bands. We have long noticed that solutions of the dilu- 

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



25 Loc. cit. 



