340 PKOCEEDINGS OF THE AMERICAN ACADEMY. 



smaller than 0.01 mg., although the delicacy of the Marsh and Gutzeit 

 methods permits a fairly exact estimation of much smaller amounts. 

 If the question of the occurrence of arsenic in normal urine is to 

 be investigated, — and we hope that opportunity for such an important 

 study may be found at some future time in this laboratory, — the deli- 

 cacy of the methods is secondary in importance to that of the source 

 and manner of collection of the urine. The absolute delicacy of the 

 Marsh method is claimed by Thomson 2* to be 0.0004 mg. of arsenious 

 oxide, by Lockemann^s and others, 0.0001 mg. arsenic, and we have 

 been able to recognize by our modification of the Gutzeit method as 

 little as 0.00008 mg. of arsenious oxide. But until it is shown that a 

 urine has had absolutely no arsenical contamination, such extreme 

 delicacy is apt to be misleading. 



The use of the method in the analysis of other liquids containing 

 organic matter suggests itself, for example in the more exact determin- 

 ation of arsenic in beer. Although the distillation of small quantities 

 of arsenic from animal tissue with hydrochloric acid has been rejected 

 by Lockemann ^5 and others, either on account of the amount of arsenic 

 in commercial, pure acid, or the difficulty of purifying the acid, yet 

 we believe that the distillation of organic matter with acid of only 

 0.004 mg. arsenic to the liter would not introduce a serious error into 

 an investigation of the normal occurrence of arsenic in the organs 

 of man. 



Hakvard Univeksity, Cambridge, Mass., U. S. A., 

 August, 1907. 



2* Loc. cit. 25 Loc. cit. 



