SANGER AND BLACK. — QUANTITATIVE DETERMINATION OF ARSENIC. 323 



one of us (S.) in 1891, 35 in which a standard set of mirrors was em- 

 ployed for the first time, the absolute limit of delicacy was placed at 

 1 mmg. of arsenious oxide. The most important improvement in pro- 

 cedure which has been made of late years is the cooling of the capil- 

 lary tube, described by Gautier, 36 Thomson, 37 Lockemann, 38 and 

 others. By this means the scattering of the deposit of arsenic is pre- 

 vented and the mirror takes a more compact and hence more easily 

 identifiable form. In spite of this advantage, we have not been able, 

 as yet, to reach the absolute limit of delicacy in the Marsh process 

 which is set by Thomson at 0.4 mmg. of arsenious oxide, by Locke- 

 mann and others at 0. 1 mmg. arsenic. We cannot think that this 

 failure is due to insensitiveness of the zinc, but to other reasons not 

 yet discovered. Sanger and Gibson 39 have shown, for example, that 

 the nature of the antimony mirror depends upon the kind of glass 

 tubing used, and they suggest that a greater or less oxidation of the 

 stibine may take place in the accidental presence of air, if the glass 

 contains a catalyzing agent. If this were true, it is easy to imagine a 

 slight retention of the arsenic from the same cause, since the oxide 

 formed would be fixed by the base of the glass. This point will be 

 soon investigated in this laboratory. 



Not only, as far as our experience goes, has the Gutzeit method 

 proved to be more sensitive than the Marsh, but we think it will be 

 found so by others. In certain lines of work, in which the sample 

 may be tested directly or quickly freed from interfering substances, 

 the Gutzeit in the form proposed by us may be preferable to the 

 Marsh, particularly when the routine analysis of a large number of 

 samples is concerned. In toxicological or legal work it will serve as a 

 valuable adjunct to the Marsh method, since the exhibits from both 

 methods can be presented and will corroborate each other, qualita- 

 tively or quantitatively. Though not convertible, like the Marsh 

 mirror, to a definite and obvious compound of arsenic, yet the color 

 band can be easily differentiated from the effect produced by other 

 substances on mercuric chloride. 



We have not studied the application of the method to the analysis 

 of many products, though we have used it successfully for the deter- 

 mination of arsenic in wall paper, in the urine, and in certain salts. 



35 Loc. cit. 



36 Bull. Soc. Chim., 27, 1030 (1902). 



37 Chem. News, 88, 228 (1903) ; also, 94, 156 and 166 (1906). 



38 Loc. cit. 



39 These Proceedings, 42, 719 (1907) ; Jour. Soc. Chem. Ind., 26, 585 (1907) ; 

 Zeits. f. anorg. Chem., 55, 205 (1907). 



