E. J. KussEiJi AND H. B. Hutchinson 



131 



and the whole is left for three days in an incubator. The ammonia 

 produced is then determined. In most of our experiments made in 

 this way inoculation with toluened soil caused about 15 per cent, 

 greater production of ammonia than inoculation with untreated soil, 

 whilst heated soil only yielded about half as much : the results however 

 were not always consistent and it sometimes happened that the toluened 

 soil culture gave no more ammonia than the untreated. After numerous 

 trials three modifications were finally introduced. 



(1) Instead of stopping the reaction at any arbitrary moment and 

 expressing the result as a number we have found it better to make a 

 series of determinations at definite intervals and plot the results as 

 a curve expressing the rate at which reaction takes place. In this way 

 the method becomes more sensitive and gives more useful information. 



(2) The cotton-wool plug was replaced by an acid trap to prevent 

 loss of ammonia by volatilisation. 



(3) A stronger inoculation was made, 25 grains of soil being 

 introduced into 15 c.c. of a 3'3 per cent, solution of peptone. 



Of these the third is purely a matter of convenience ; the general 

 type of curve obtained is independent of the strength of inoculation. 

 Two experiments made by the modified method are recorded below : 

 the figures are plotted on Curve 2 (p. 114). 



Experiment 1. Arable soil as used for determinations given in Table 2. 



The number of organisms in the culture was determined by nutrient 

 agar plate culture : the results, expressed as the number per gram of 

 soil present, are : 



