404 Journal of Agricultural Research voi. vi, no. n 



fixing properties. It is not, however, increased by the addition of 

 arsenic. Hence, it would appear as if the substance which is suppressed 

 by the arsenic is very thermolabile and is easily injured by drying, for it 

 has been repeatedly brought to our attention that the drying of the 

 soil prevents the arsenic from greatly stimulating its nitrogen-fixing 

 properties. Harden and Young (191 1, p. 72; 1906) have shown that 

 the addition of arsenates to a yeast-juice sugar solution greatly accelerates 

 the rate of fermentation of such a mixture. The close analogy existing 

 between the chemical properties of phosphorus and arsenic led to the 

 idea that possibly the arsenic replaced the phosphorus in the reaction 

 characteristic of phosphorus, but they found that this is not the case, 

 for while the arsenic has an optimum concentration, as has the phos- 

 phorus, there was no direct relationship between the amount of arsenate 

 added and the extra amount of fermentation, the arsenic in this way 

 acting more like a catalyzer than does the phosphorus. Furthermore it 

 was shown that fermentation can not proceed in the absence of phos- 

 phorus, even though there be present either arsenates or arsenites. The 

 arsenic acts mainly as a liberator of the phosphorus from the hexosephos- 

 phates and does not of itself enter into the vital reactions of the cell as 

 does the phosphorus. 



These facts make it likely that a similar action may be exerted by the 

 arsenic upon the bacteria. For these reasons a series of experiments 

 was arranged in which the phosphorus had been replaced by arsenic. 

 These were carried on in the nitrogen-free quartz sand. To each 100 gm. 

 of the sand there was added the quantity of carefully tested nutrient 

 without phosphorus found in 100 c. c. of Ashby's solution. To one-half 

 of them was added the phosphorus, while to the other half there was 

 added 0.0728 gm. of lead arsenate. They were each inoculated with 

 I c. c. of a soil extract and then incubated the regular length of time. 

 The nitrogen determinations were made on them and sterile blanks with 

 the following results: When incubated with complete Ashby's solution 

 and 0.0728 gm. of lead arsenate, 100 gm. of sand fixed 11.62 mgm. of 

 nitrogen. Similar samples without phosphorus but with arsenic fixed 

 0.03 mgm., while without phosphorus or arsenic there was fixed o.oi 

 mgm. of nitrogen. The results for the set with the complete nutritive 

 media show that sufficient of the soil extract was taken to get the nitrogen- 

 fixing organism, and the results without phosphorus show that there was 

 not sufficient phosphorus in the i c. c. of soil extract to furnish phosphorus 

 for the organisms. These results show conclusively that arsenic can not 

 replace phosphorus in the vital activities of the nitrogen-fixing organisms 

 of the soil, and establish for this set of organisms what Stoklasa (1897) 

 has established for the higher phanerogams, Molisch (Lafar, 191 1, p. 37) 

 for algae, Giinther (1897) for the molds, and Harden and Young (1906) 

 for the yeasts. 



