NITRIFICATION 33 



red reaction ") on the addition of acid alone. The reaction 

 can be obtained as early as twelve hours after inoculation, 

 and becomes very marked in twenty -four to forty-eight 

 hours. 



The " indole-reaction " is not necessarily always due 

 to indole ; the writer has shown x that the indole-like 

 . reaction obtained with cultures of the diphtheria and 

 pseudo -diphtheria bacilli is owing to the presence of 

 skatole-carboxylic acid. This substance is distinguished 

 from indole by being non- volatile. To make sure of the 

 presence of indole, the culture should therefore be made 

 alkaline with caustic soda and distilled. The alkalinity 

 should correspond to a P H value of about 9-2 (Zoller). 

 If 100 c.c. of the culture be distilled, the first 20 c.c. of 

 the distillate will contain the bulk of the indole. 



Skatole (methyl indole) seems also to be formed by some 

 organisms. It is volatile like indole, but if a solution containing 

 it be boiled with an acid solution of paradimethylamidobenzalde- 

 hyde (5 per cent, in 10 per cent, sulphuric acid) it yields a blue 

 colour, which gives a blue solution in chloroform. 



Nitrification. Another important series of changes is 

 that included under the term " nitrification." As men- 

 tioned before, protein, albuminoid, and other complex 

 nitrogenous matters and urea, all of which are valuable 

 manures for plant life, cease to be so unless bacteria are 

 present. 



It is necessary, in fact, for the nitrogenous matter to 

 be converted into nitrates, in which form alone is it 

 available for the nutrition of plants. 



Although so important, extremely small quantities of 

 nitrates are present in the soil ; in fertile soils, for example, 

 under some conditions there may be as little as one part 

 of nitrogen in 1,000,000 and there is often less than ten 

 parts. The bodies yielding nitric acid in the soil are : 



1 Trant. Path. Soc. Lond., vol. lii, pt. ii, 1901, p. 113. 

 M.B. 3 



