100 AGRICULTURAL BACTERIOLOGY. 



absolutely pure, and that even the distilled water which he 

 used did contain enough organic matter to furnish the bacteria 

 with a small amount. This seems rather doubtful, but whether 

 it is true or not, it. still remains that these nitrobacteria can live 

 in a solution containing only minerals and such an infinitesimal 

 amount of organic matter as may be contained in the purest 

 distilled water. 



Not only is this true but it appears that the presence of or- 

 ganic matter in the solutions, even in very small quantity, is 

 directly injurious. The bacteria will grow readily enough in 

 these mineral solutions, but if a small quantity of organic mat- 

 ter is added their growth is stopped. So sensitive are they to 

 the presence of organic matter that these compounds act ex- 

 actly like antiseptics. When present in very small quantities 

 they are able to stop the growth of the bacteria, acting as 

 powerfully in this respect as the antiseptic poisons act upon 

 ordinary bacteria. For example : If peptone is present in the 

 proportion of I to 4,000, the nit rate -forming bacteria are 

 checked in their action ; and I part to 2,000 stops their growth 

 entirely. The nitrate-forming germ is absolutely checked by 

 the presence of ammonia 15 parts to 100,000. 



In most of these respects the nitrous bacteria are more re- 

 sistent than the nitric bacteria, requiring a larger amount of or- 

 ganic matter to check their activities. In regard to ammonia, 

 however, the nitrate bacteria are excessively sensitive, as above 

 shown, while the nitrite bacteria are capable of growing in the 

 presence of considerable ammonia. 



Further, it is found that the more highly organized the com- 

 pound, the more decided is its checking action upon the nitro- 

 bacteria ; the more valuable the material for ordinary kinds 

 of bacteria, the greater is its injury upon the nitrobacteria. 

 These bacteria thus grow only under conditions in which other 

 bacteria will not grow, and will not grow under the conditions 

 which other species find most favorable. A more sharp con- 



