i38 PROCEEDINGS OP THE AMERICAN ACADEMY 



To the same amount of paper 1 mgr. were added, and the char 

 extracted as before, but, in addition, the particles were well triturated. 

 9.19 mgr. were recovered, = 91.9 per cent. 



That this loss was not due to error in estimation of the mirrors was 

 shown by repetition of the first two trials, mirrors being obtained from 

 separate portions of the same extract. The char was triturated in 

 each case : — 



1 mgr. AS2O3 gave 0.9 mgr. and 0.9 mgr. Average, 90%. 

 • 5 " " " 2.92 " " 2.95 " " 58.00%. 



That arsenic would be left in the char was shown as follows : 100 

 sq. cm. of paper as above, after addition of 5 mgr. AS2O3, were charred, 

 and the finely ground mass extracted first with cold water. 3.11 

 mgr. were recovered, — 62.2%. The residue was then extracted with 

 30 c. c. hot water. 1.63 mgr. were recovered, = 32.6%. A third 

 extraction with hot water gave a solution free from arsenic. Total 

 amount recovered, = 4.74 mgr. = 94.8%. 



The quality of the paper has no effect on the loss : 5 mgr. AsoOs 

 were added to 100 sq. cm. filter paper and treated as before. Two 

 readings from the same cold extract gave 3.51 and 3.46 mgr. respect- 

 ively. Average, 69.9%. Finally, to show the practical completeness 

 of extraction with only 25 to 30 c. c. hot water, 100 sq. cm. of filter 

 paper were charred with different amounts of arsenious oxide. 



1 



2 

 3 



10 mgr. of arsenious oxide per 100 sq. cm. of paper would corre- 

 spond to 1,000 mgr. per sq. meter, which is an unusual amount. Hence 

 any smaller amounts would be easily extracted. It should be borne 

 in mind that the error is necessarily great in such large amounts, for 

 the deposit which is compared with the standards is so small a propor- 

 tion of the total amount that a slight difference in reading is propor- 

 tionally increased. 



4. Reagenfs. — Many chemists prefer to use hydrochloric acid in- 

 stead of sulphuric acid, on account of the quicker action of the former 

 on the zinc. Opinions vary considerably as to the error arising from 

 volatizatiou of zincic chloride, and consequent deposition at the heating 



