SANGER AND BLACK. — QUANTITATIVE DETERMINATION OP ARSENIC. 299 



the failure of Thomson to get good results was merely due to unsuit- 

 able conditions. 



Our experience has not only confirmed the conclusion which has been 

 reached by most of those who have investigated the Gutzeit reactions, 

 that the use of mercuric chloride is preferable to that of argentic 

 nitrate from a qualitative standpoint, but it has also shown that the 

 former reagent is the one better suited to the quantitative analysis. 



A careful study of the conditions of the reaction, following the prin- 

 ciple stated above and made for the most part without knowledge of 

 the work of the above-quoted authors, has shown that the reaction 

 can be made the basis of a simple and fairly accurate quantitative 

 method with no more than ordinary analytical precautions. 



The Method. 



Sensitized Mercuric Chloride Paper. For this purpose we used 

 at first a smooth filter paper of close texture, but we have recently em- 

 ployed to greater advantage a cold pressed drawing paper made by 

 Whatman. The latter not only gives better color results, but also, on 

 account of its greater strength, withstands better any subsequent treat- 

 ment for development or identification of the color. A square meter 

 of this paper weighs about 160 grams (4 1-4 ounces per square yard). 

 It is cut into strips having a uniform width of 4 mm., and we use for 

 this purpose a carefully made brass rule of exactly this width. The 

 cutting may be done with a sharp knife, but more accurately and in 

 large quantity by the machine which should be accessible at any print- 

 ing office. 



The strips, which must be clean and free from dust, are sensitized 

 by drawing them repeatedly through a five per cent solution of recrys- 

 tallized mercuric chloride until they are thoroughly soaked. They are 

 then placed to dry on a horizontal rack of glass rods or tubing, and, 

 when dry, are at once cut into short lengths of 7 cm., discarding the 

 ends by which the strips were held during the immersion. A bundle 

 of these strips is placed in a stoppered tube or bottle containing calcic 

 chloride covered by cotton wool, and is kept in the dark until needed. 



The Reduction Apparatus. (See Figure A.) This consists of a 

 glass bottle of 30 c.c. capacity, closed by a pure rubber stopper with 

 two holes. Through one of these holes passes a small thistle tube, 

 about 15 cm. long, reaching to the bottom of the bottle and constricted 

 at its lower end to an opening of about 1 mm. The other hole carries 

 an exit tube bent first at a right angle, then back again in the same 



