24 INSECTICIDES AND FUNGICIDES. 



the whole to a 200 cc. flask by means of a jet of hot water. This trans- 

 ference, with care, can be made very complete. Add sodium sulphid 

 and digest on the water bath until all arsenious sulphid has gone into 

 solution, leaving the copper sulphid behind; cool, make to the mark, 

 mix thoroughly, and filter off 100 cc. through a dry filter, rejecting the 

 first few cc. The solution thus obtained is warmed, and made acid 

 with hydrochloric acid, which reprecipitates the arsenious sulphid and 

 a large amount of free sulphur; filter and wash; extract the still 

 moist precipitate on the filter with ammonia; wash the residual sulphur; 

 reprecipitate the filtrate with hydrochloric acid, without heat; filter in 

 a Gooch crucible; wash; dry at 100 C. ; extract the remaining free sul- 

 phur with carbon disulphid; dry at 100 C., and weigh; repeat extrac- 

 tion and drying until constant weight is obtained. From the arsenious 

 sulphid compute arsenious oxid. While this method leaves much to be 

 desired, it is the best we could find to work on mixtures of this kind. 

 The old method, as described in many text-books, of treating the mixed 

 sulphids of copper and arsenic with sodium sulphid and filtering the 

 entire arsenic solution thus obtained from the copper sulphid left 

 behind did not work in the hands of the author, since in most cases 

 the copper sulphid would commence to run through the filter directly 

 after washing was begun. 



Iron. The iron in these compounds is determined by precipitating 

 with ammonia directly in the presence of arsenic and copper. 



Calcium and magnesium. These two substances are determined by 

 first precipitating out the copper and arsenic with hydrogen sulphid, 

 then precipitating the iron with ammonia, and filially determining cal- 

 cium by means of ammonium oxalate in the usual way, and magne- 

 sium in the filtrate by means of sodium phosphate in the ordinary 

 manner. 



Sulphur trioxid. This constituent is determined as barium sulphate 

 by precipitating sulphur trioxid from the hydrochloric acid solution 

 of the substance in the ordinary manner. 



In determining the lead and arsenic oxids in "Swift's Lead Arse- 

 nate," the compound is first dissolved in fuming nitric acid to get rid 

 of all organic matter. It is then evaporated to drive off most of the 

 acid and transferred to a measuring flask, where it is made to a volume, 

 aliquot portions of which are taken for analysis. Sulphuric acid is 

 added to one portion and the sample evaporated on the steam bath till 

 the odor of nitric acid ceases to come off. It is now diluted with water 

 and filtered, washing with water slightly acidified with sulphuric acid, 

 till the precipitate is free of arsenic. This filtrate is saved. The filter 

 and contents are washed with 60 per cent alcohol till free of sulphuric 

 acid, dried, and finally burned and weighed as lead sulphate, using the 

 usual precautions mentioned for this compound. To the filtrate add 

 ammonia till slightly alkaline and magnesia mixture in the usual man- 



