40 PROCEEDINGS OP THE AMERICAN ACADEMY 



is quite possible that part is left ia the ziuc, especially if the extract 

 be not free from nitric acid. But in this method, where all of a very 

 small amount of arsenic is to be reduced, and the action is pushed as 

 far as possible, the probability is that no arsenic is Jeftin the zinc. I 

 have often, after deposition of the mirror, pushed the action as far as 

 the complete solution of the zinc, and have never observed any in- 

 crease of the mirror at the close. It would seem hardly possible for 

 the solid hydride to remain in that state in a reduciug medium for 

 such a length of time (compare also 2./). A large amount of carbon 

 in the zinc is apt to cause too rapid evolution, but I cannot confirm 

 the statement of Headden and Sadler * that zinc containing carbon 

 causes a loss of arsenic, and that the zinc must in consequence be free 

 from carbon. 



5. General Precautions. — a. Means of drying the Hydrogen. — 

 The use of sulphuric a.cid is not allowable. I have found the statement 

 of DragendorfE f to be true, that sulphuric acid absorbs arseniuretted 

 hydrogen. This is assumed byJanowskyJ to be due to decomposi- 

 tion into arsenic and hydrogen, but no proof is given. DragendorfFf 

 quotes the suggestion of Otto,§ that a small amount of fused potassic 

 hydroxide be placed before the fused calcic chloride, in order to absorb 

 any sulphuric acid which might be carried over, it being possible that 

 the acid might act on the calcic chloride, giving hydrochloric acid, 

 which might form arsenious chloride and thus cause loss of arsenic. 

 The potassic hydroxide would also hold back sulphuretted hydrogen. 

 It is well known that potassic hydroxide absorbs antimoniuretted hy- 

 drogen, and it has been recently shown by Kuhn and Saeger,|| as well 

 as by Headden and Sadler,* that arseniuretted hydrogen is also ab- 

 sorbed by it. There is, however, no need of its use as a precaution 

 against either of the contingencies mentioned above. 



LyttkensIF considers sulphuric acid to be a better drying agent than 

 calcic chloride, and Lenz ** agrees with him, but neither shows that 

 there is no loss of arsenic attending ics use. 



As to Headden and Sadler's* statement, unsupported by analysis of' 

 the calcic chloride used, that fused calcic chloride holds back arsenic 

 when moist, I have never met with any indication that there was 

 danger of loss from this source. 



* Loc. cit. II Log. cit. 



t Ermittehing von Giften, 2te Aufl., p. 336. 1 Loc. cit. 



X Ber. d. deutsch. chem. Gesell., VI. 216. ** Loc. cit. 

 § Ausmittelung der Gifte. 



