56 Biology of Bacteria 



nitrites was already present, but that more have been pro- 

 duced by the growth of the bacteria. 



The presence of ammonia in either fluid is easily deter- 

 mined by the immediate development of a yellow color or 

 precipitate when a few drops of Nessler's solution * are 

 added. 



Failure to determine either ammonia or nitrites may 

 not mean that the nitrates were not reduced, but that they 

 were reduced to N. It is, therefore, necessary to test the 

 solutions for nitrates, which is done by the use of phenol- 

 sulphonic acid and sodium hydroxid, which in the presence 

 of nitrates give a yellow color. 



12. Combination of Nitrogen. Not only do bacteria 

 destroy or reduce nitrogen compounds, but some of them 

 are also able to assimilate nitrogen from the air and com- 

 bine it so as to be useful for the nourishment of vegetable 

 and animal life. The most interesting organisms of this 

 kind are found upon the roots of the leguminous plants, 

 peas, clover, etc., and have been studied by Beyerinck.f 

 It seems to be by the entrance of these bacteria into their 

 roots that the plants are able to assimilate nitrogen from 

 the atmosphere and enrich sterile ground. Kvery agri- 

 culturist knows how sterile soil is improved by turning 

 under one or two crops of clover with the plough. 



13. Peptonization of Milk. Numerous bacteria possess 

 the power of digesting peptonizing the casein of milk. 

 The process varies with different bacteria, some digesting 

 the casein without any apparent change in the milk, some 

 producing coagulation, some gelatinization of the fluid. In 

 some cases the digestion of the casein is so complete as to 

 transform the milk into a transparent watery fluid. 



Milk invariably contains large numbers of bacteria, that 

 enter it from the dust of the dairy, many of them pos- 

 sessing this power and ultimately spoiling the milk. In 

 the process of peptonization the milk may become bitter, 

 but need not change its original reaction. 



The phenomena of coagulation and digestion of milk can 

 be made practical use of to aid in the separation of simi- 

 lar species of bacteria. Thus, the colon bacillus coagulates 

 milk, but the typhoid bacillus does not. 



* Nessler's solution consists of potassium iodid, 5 grams, dissolved 

 in hot water, 5 c.c. Add mercuric chlorid, 2.5 grams, dissolved in 10 

 c.c. of water, then to the mixture add potassium hydrate, 16 grams, 

 dissolved in water, 40 c.c., and dilute the whole to 1000 c.c. 



fSee "Centralbl. f. Bakt.," etc., Bd. vn, p. 338. 



