28 A MANUAL OF BACTERIOLOGY 



presence of indole, the culture should therefore be made 

 alkaline with caustic soda and distilled. 



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 dimethylamidobenzaldehyde (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 avail- 

 able 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) free nitrogen ; (2) small quantities of nitrates in rain- 

 water ; (3) ammonium salts, applied intentionally or 

 carried to the soil by rain or derived from the decay of 

 organic matter ; (4) various nitrogenous organic sub- 

 stances arising from the decay of animal and vegetable 

 matters. 



With regard to the production of nitric acid from 

 nitrogenous organic matters very little was formerly 

 known. In 1877 Schloesing and Miintz by an ingenious 

 experiment showed that nitrification (as the production 

 of nitric acid is termed) of nitrogenous organic matter is 

 brought about by living organisms in the soil. Sewage 

 was passed continuously through a tube containing a 

 mixture of ignited quartz sand and limestone. After 



