122 INFLUENCE OF ARSENIC ON BACTERIAL ACTIVITY 



protoplasm, causing them to utilize their food more economically 

 in the presence of arsenic than in its absence. This is similar to the 

 influence of the arsenic upon the cells within the horse. 



Other experiments have demonstrated that the addition of arsenic 

 to a soil increases the liberation of the insoluble plant-foods of the 

 soil, especially of the phosphorus. Thus arsenic by various means 

 stimulates all the bacterial activities of the soil, and these increased 

 activities, as experiments have shown, are reflected in greater crops. 

 This increased growth must be looked upon as due to a stimulus 

 and not to the direct nutritive value of the substance added. Soils 

 so treated would produce larger crops and wear out more quickly 

 than would untreated soils. It is interesting and important to know 

 that arsenic has to be applied to a soil in enormous quantities before 

 it retards microscopic plant life, and probably before it retards the 

 growth of higher plants. 



The data available prove conclusively that the arsenical com- 

 pounds, with the single exception of Paris green, stimulate the 

 nitrogen-fixing organisms of the soil and that this influence varies 

 qualitatively but not quantitatively with the various soils. The 

 results also bring out the fact that both the anion and the cation of 

 the compounds have a marked influence upon the growth of the 

 organisms/ With some compounds both the anion and cation act 

 as stimulants, but with other compounds one stimulates and the 

 other retards. It is likely that little or no influence is exerted upon 

 the nitrogen-gathering organisms by the sodium of sodium arsenate 

 and that the stimulating influence noted with dilute solutions and 

 the toxic influence exerted with more concentrated solutions are due 

 entirely to the arsenic. It is rather likely that the stimulating 

 influence which Riviere and Bouilhac have found sodium arsenate 

 to have upon wheat and oats is an indirect effect which is exerted 

 upon the bacterial flora of the soil and which in turn influences the 

 yield of the various grains. 



Both the anion and cation undoubtedly act as stimulants in the 

 lead arsenate. Stoklasa has shown that lead when present in soil 

 stimulates the growth of higher plants. This he ascribes to the 

 catalytic action of these elements on the chlorophyll. The results 

 reported indicate that it is due to the influence of the compounds 

 upon the biological transformation of the nitrogen in the soil. The 

 fact that the lead plays no small part in the stimulating influence 

 is borne out by the work of Lipman and Burgess who found lead to 

 stimulate nitrifying organisms. 



Paris green is toxic to the nitrogen-fixing organism in the lowest 

 concentration tested. This is due to the copper and not to the 

 arsenic, as it is well known that the copper ion is a strong poison 

 to many of the lower plants. Brenchley found it to be toxic to 

 higher plants when present in water to the extent of one part in 



