BIOLOGY OF BACTERIA. 59 



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 phenolsulphonic acid and sodium hydroxid, which in 

 the presence of nitrates give a yellow color. 



11. 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 in association with the roots of the legu- 

 minous plants, peas, clover, etc., and have been studied 

 by Beyerinck. It seems to be by the entrance of these bac- 

 teria into their roots that the plants are able to assimilate 

 nitrogen from the atmosphere and enrich sterile ground. 

 Every agriculturist knows how sterile soil is improved by 

 turning under one or two crops of clover with the plough. 



12. Peptonization of Milk. — Numerous bacteria possess 

 the power of digesting — peptonizing — the casein of milk. 

 The process diners 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 usually contains bacteria, entering it from the 

 dust of the dairy, possessing this power. In the process 

 of peptonization the milk may become bitter, but need 

 not change its original reaction. As the peptonization 

 progresses the milk very often becomes poisonous, espe- 

 cially to individuals under two years of age, and may 

 bring about a fatal enterocolitis or "summer complaint." 

 Vaughan and Novy believe the poisonous change to de- 

 pend upon the formation of tyrotoxicon. Lubbert has 

 shown, 1 however, that the disease not only occurs in 

 consequence of toxic substances formed from the split-up 



1 Zeitschrift fur Hygiene, 1896, xxii., Heft 2, p 1. 



