Discovery of Bacterial Relation 195 



present in the soil-air. It was assunied, therefore, that 

 the production of nitrate in the soil out of the gases, 

 nitrogen and oxygen, was affected by certain compounds 

 of iron, by ozone, and by sulfate of hme. It was assumed, 

 also, that the electrical discharges in the atmosphere 

 not only caused the formation of small quantities of 

 nitric acid which was washed into the soil by rain, but 

 that electrical discharges led also to the formation of 

 nitric acid in the soil itself. 



The bacteria theory. — The proof furnished in 1877 by 

 Schloesing and Miintz that nitrification is a bacterio- 

 logical process and that it does not take place in sterile 

 soil, disproved the claim that nitrogen and oxygen are 

 directly condensed in the soil to nitric acid. The earlier 

 experiments were recalled that indicated the formation 

 of nitrates only when nitrogenous organic materials 

 were supplied, and the non-formation of nitrates in 

 soils devoid of humus. It was remembered that the 

 nitric acid brought down in the rain scarcely exceeded 

 three or four pounds per acre, annually, and the wonder 

 grew again as to the manner in which the supply of com- 

 bined nitrogen in the world is maintained. 



The existing uncertainty was dispelled in 1886 with 

 the announcement of the German investigator, Hell- 

 riegel, that certain plants are capable of using for their 

 development the nitrogen gas of the air, but that they 

 are enabled to do so only with the aid of bacteria which 

 live in their roots. The power of thus gathering atmos-_ 

 pheric nitrogen was confined, with few exceptions, to 

 the plants of the legume family. A few years later, the 

 Russian bacteriologist, Winogradsky, furnished the 



