HOW DOES THE ARSENIC ACT 



125 



tion. The graphs also bring out the fact that adding arsenic and 

 filtering the soil only shift for the time the equilibrium within the 

 soil, which later tends to regain its old equilibrium. This is a condi- 

 tion that coincides well with what would be expected if the limiting 

 element were some other microscopic forms of life. The filter 

 would not separate them quantitatively, and it is possible that the 

 arsenic has only a selective influence. Later, many of the organ- 

 isms become accustomed to its presence; or, what is more likely, the 

 arsenic becomes fixed within the soil. 



That this limiting factor is a thennolabile body is brought out 

 more clearly by Fig. 19. The quantity of nitrogen fixed by the 

 unheated soil receiving no arsenic has been taken as 100, the heated 

 soil with and without arsenic being compared with this. 



320 f 



3oo,; - 



280 



260 $ 



FIG. 19. Graph showing the effect of the heat on the nitrogen-fixing power of soil 

 treated and not treated with arsenic. 



The heating of the soil extract to 50 C. for fifteen minutes has 

 exactly the same influence measured in terms of nitrogen fixed as 

 does 0.0728 gin. of lead arsenate. The stimulating influence of heat 

 is noted even in the presence of arsenic and reaches its maximum 

 effect in the absence of arsenic at 60, and in the presence of arsenic 

 at 65 C. Above these temperatures there is a decline in the nitrogen 

 fixed. Even soils inoculated with solutions which had been heated 

 to a temperature of 85 fixed nitrogen ; at least there is more nitrogen 

 accumulated in such soil than in that inoculated with the untreated 

 soil solution. The results indicate that many of the organisms which 

 take part in the gathering of nitrogen in soils are very resistant to 

 heat. It is also significant that the greatest stimulating influence 



