1904] MCCAY — TRISULPHOXYARSENIC ACID. 113 



Prag, established the existence of the disulphoxyarsenic acid, and 

 recently Dr. William Foster, Jr., and I have succeeded in preparing 

 several salts of the trisulphoxyarsenic acid. Ever since Preis dis- 

 covered the disulphoxyarsenic acid I have been convinced that the 

 trisulphoxy-compound existed, and since it was the only acid 

 necessary to complete the series, a great deal of experimental work 

 was undertaken in order to isolate it. 



Dr. Foster^ published recently the results of an elaborate investi- 

 gation of the action of magnesium oxide on a mixture of equivalent 

 amounts of arsenic trisulphide and sulphur suspended in water, it 

 being hoped that during, the reaction the magnesium salt of the acid 

 would be formed in sufficient amounts to make possible its transfor- 

 mation into the corresponding sodium salt, and the separation of 

 this by means of alcohol. His work has served to clear up many 

 matters bearing on the modes of formation of the sulphoxy- 

 compounds; and although no perfectly consistent results were 

 reached, sodium salts were prepared again and again whose compo- 

 sition approached so closely to one corresponding to the formula 

 NagAsOSg -f 11H2O that I felt convinced the substances were 

 really impure tertiary sodium trisulphoxyarseniate. 



A few preliminary experiments having established the fact that 

 freshly precipitated arsenic pentasulphide suspended in water is, 

 decomposed far more readily by^^an excess of magnesium oxide than 

 a mixture of equivalent amounts of arsenic trisulphide and sulphur, 

 I suggested to Dr. Foster that we conjointly make a careful exam- 

 ination of the resulting solution. The proposal was a profitable 

 one, for we find that the solution contains large quantities of mag- 

 nesium trisulphoxyarseniate. 



The magnesium salt of trisulphoxyarsenic acid, then, is formed 

 when magnesium oxide in excess acts upon freshly prepared arsenic 

 pentasulphide suspended in water kept during the entire reaction at 

 0° C. The change is rather slow, when the amount of arsenic 

 pentasulphide is large, but it is generally complete in four to 

 five hours. 



By precipitating the magnesium in solution by means of sodium 

 hydroxide, as magnesium hydroxide, adding an equal volume of 

 alcohol to the filtrate from the magnesium oxide and hydroxide, and 

 keeping the corked flask and its contents in the ice chest, the 



^ Z. anorg. Ckem., 37, 64 (1903). 



PROC. AMER. PHIL03. SOC. XLIII. 176. H. PRINTED MAY 19, 1904. 



