I9I3] 



/. E. Greaves 



II 



and 2 gm. of dry blood, and the water-soluble arsenic determined 

 as above, without incubating. The average of the two sets of 

 results should give a very close approximation of the quantity of 

 water-soluble arsenic existing in the soil during the activity of the 

 organisms. 



The results are given in Table 5, as milligrams of water-soluble 

 arsenic occurring in 100 gm. of the soil (before and after three 

 weeks' incubation), to which 1,120 parts per million of the various 

 forms of arsenic were added. Each is the average of two or more 

 closely agreeing determinations. 



TABLE 5 



Data pertaining to water-soluble arsenic in wo grams of soil to which 1120 

 parts per million of arsenic were added 



Treatment 



Incubated three weeks. Water-soluble arsenic 

 determined (mg.) 



Water-soluble arsenic determined directly 

 (mg.) 



Average (mg.) 



Lead 

 arsenate 



Paris green 



Zinc arse- 

 nite 



Arsenic 

 trisulfide 



14-3 



20.2 

 17-3 



80.0 



82.0 

 81.0 



36.9 



31-7 

 34-3 



50.0 



5.6 

 27.3 



These results show that there are great differences in the quan- 

 tities of water-soluble arsenic in a soil to which various forms of 

 arsenic have been added, and that even soil rieh in iron and calcium 

 to which arsenic has been added in large quantities may have a 

 high water-soluble arsenic content. This content is highest when 

 the arsenic is added in the form of Paris green. Paris green ap- 

 parently becomes less soluble, while arsenic trisulfide becomes much 

 more soluble, as nitrification takes place. Comparing these results 

 with the ammonia and nitrates formed under the several conditions, 

 we note a general relationship between the toxicity of the substance 

 added and the quantity of water-soluble arsenic present. The great- 

 est toxic effect is noted with Paris green and this gives the greatest 

 amount of water-soluble arsenic. 



Part of this toxicity may be due to the copper ion, as it is well 

 known that this substance acts as a strong poison to many of the 

 lower plants. Miss Benchley^ found it to be toxic to higher plants 



sBenchley: Annais of Botany, 1910, xxiv, p. 571. 



