OF ARTS AND SCIENCES. 39 



place, when hydrochloric acid is used. I.if\)ig,* very soon after the 

 publication of Marsh's method, called atteniiou to possible error from 

 this source, and several years later Wackeurodert coulirmed Liclii"',s 

 opinion. Beckurts t claims that there is no danger to be feared, but 

 does not show conclusively that this is the case. Brescius § recoof- 

 nizes the chance of error, and recommends passing the gas througli 

 sulphuric acid, if hydrochloric acid is used for generation. The com- 

 mittee of the British Health Association, above referred to, recom- 

 mends the use of hydrochloric acid, without comment on its possible 

 disadvantage. 



While the volatilization of zincic chloride might not interfere with 

 the detection of arsenic in considerable quantity, yet it is of primary 

 importance in this method for the mirror to be of arsenic alone, and 

 hence capable of comparison with standards. We cannot, therefore, 

 run the risk of the small mirror being contaminated by any impu- 

 rity whatever. For this reason, if for no other, the use of hydro- 

 chloric acid is wholly unadvisable. Then the time gained in using 

 hydrochloric acid is not to be considered in this method, as the use 

 of a constant generator reduces the time of analysis so decidedly. 



The same desire to hasten the evolution of hydrogen in the ordi- 

 nary Marsh process has led to the addition of stimulants to the action 

 in the shape of platinic chloride or cupric sulphate. Bernstein || has 

 shown that the use of platinic chloride is inadmissible, because arsenic 

 is thereby held back. At the same time, however, he finds no loss 

 when the zinc is platinized or silvered before being used. Headden 

 and Sadler 1[ agree with Bernstein in the case of platinic chloride, and 

 find that cupric sulphate also causes a loss. They also get low re- 

 sults by using a spiral of platinum wire in contact with the zinc. 

 Here, again, the use of the constant generator precludes the neces- 

 sity for increasing the sensitiveness of the zinc. 



Mohr,** in 1837, called attention to the fact that the residual zinc 

 even after careful washinjr contained arsenic. This would seem to 

 substantiate the statement of Blondlot, quoted above, concerning the 

 solid hydride of arsenic. Fresenius tt ^ilso considers that the effect of 

 nitric acid is due to the formation of a hydride. When we consider 

 the case of the mere detection of arsenic by the Marsh process, where 

 all the arsenic is not necessarily reduced to arseniuretted hydrogen, it 



* Ann. d. Chem. u. Fharm., XXIII. 217. || Inaug. Dissertation, Rostock, 1870. 

 t Archiv f. Pharm., LXX. 14. 1 Loc. cit. 



t Ibid., CCXXII. 653. ** Ann. d. Pharm., XXIII. 217. 



§ Dingl. Polyt. Jour., CLXXXVI. 226. tt Loc. cit. 



