332 PROCEEDINGS OF THE AMERICAN ACADEMY. 



volume of urine taken, the quantity of arsenic per liter is calculated. 

 Should the mirror exceed in size the standard of 0.06 mg., it may be 

 necessary to obtain another mirror from a smaller portion of the solution 

 or from a smaller volume of urine, since the reading of mirrors above 

 0.06 mg. is not accurate. 



The determination of the amount of arsenic in the solution may also 

 be made by the modification of the Gutzeit method described by us in 

 the preceding paper. ^^ In this case, owing to the size of reduction flask 

 used, the volume of the solution should not exceed 20 c.c, of which an 

 aliquot part or all may be taken. This method consists briefly in 

 allowing the arsenical hydrogen to pass through a tube containing a 

 strip of paper saturated with a five per cent solution of mercuric 

 chloride and dried. The resulting band of color is compared with 

 a set of standard bands. 



Reagents. The zinc used, known as Bertha spelter, irom the New 

 Jersey Zinc Company of New York, has been used in this laboratory 

 for many years, and has been exhaustively tested for arsenic with nega- 

 tive results. It contains not over 0.013 per cent of iron and not more 

 than 0.019 per cent of lead. The amount taken is from five to ten 

 grams. We have used it in a rather finely granulated form in the 

 reduction bottle, reserving the larger pieces for the constant generator. 

 As the metal is too pure to generate hydrogen with sufiicient rapidity 

 from sulphuric acid, we place in the reduction bottle a thin disk of 

 platinum foil nearly as large as the bottom of the bottle. With this 

 the evolution of the hydrogen is most regular. That the platinum 

 does not cause arsenic to be held back, we have assured ourselves by 

 obtaining mirrors of equal size and same appearance as those formed 

 without the disk. The deposition of platinum on the zinc by use of 

 platinic chloride is, however, not allowable, as one of us has shown, ^^ 

 and cupric sulphate is equally inadmissible. The formation of a coat- 

 ing of copper on our zinc, after the procedure of Lockemann,!^ does 

 not add to its sensitiveness, nor does the addition of tin or lead salts 

 to the solution during reduction. In the constant generator, the zinc 

 is sensitized, according to the suggestion of Gooch,!^ by treatment 

 with a solution of cupric sulphate, but we take the precaution to pass 

 the hydrogen from the generator through a ten per cent solution of 



" These Proceedings, 43, 297 (1907) ; Jour. Soc. Chem. Ind., Vol. 26 (1907) ; 

 Zeitschr. f. anorg. Chem., Vol. 56 (1907). 

 " Loc. cit., p. 39. 

 " Loc. cit. 

 18 Amer. Jour. Science, [3] 48, 292 (1894). 



