L9 



troduce the organism ; indeed, at the date of these observations it was 

 not known that the actiou was brought about by bacteria. The answer 

 is plain ; these organic solutions obtained the organism which reduced 

 nitrates by ordinary contact with atmospheric air. We are now familiar 

 with the fact that the air is the great distributor of bacteria, yeasts, and 

 fungi, and that a sterilised solution can seldom remain a single minute 

 in contact with ordinary air without becoming infected. In Deherain's 

 experiments on denitrification, soils which had been sterilised by heat, 

 and had lost the power of reducing nitrates, were found frequently to re- 

 gain this power by merely transferring them to another vessel. 



Having grasped the facts now before us, we are disposed to smile 

 when we are gravelv informed that straw bears on its surface the organ- 

 ism reducing nitrate to nitrogen gas and that, it is in consequence of this 

 fact dangerous to use it as a manure for soils. It is doubtless quite true 

 as we are informed by Wagner and others, that rye-straw, placed in 

 water containing saltpetre, slowly reduces the nitrate present, But this 

 result is not due to any peculiar property of straw ; the action which 

 occurred would equally have been observed if other forms of organic 

 matter contaminated with atmospheric dust had been made use of. 

 Breal, in fact, experimenting in this way has obtained similar results 

 when using dead leaves ; the straw of wheat, maize, or haricot bean ; 

 lucerne silage (conserve de luzerne), or maize cake. That atmospheric 

 contamination was the source of the organisms reducing nitrates which 

 were present in these experiments is strikingly evident in the case of the 

 maize cake. This cake is prepared on the Continent by crushing the 

 embryo of maize grain, which, like other embryos, is extremely rich in 

 fat and albuminoids. Now, we cannot conceive that this embryo, em- 

 bedded in the substance of the seed, naturally contains the bacterium re- 

 ducing nitrates to gas. The cake must therefore have gained this or- 

 ganism during the process of its manufacture ; or, possibly, the organism 

 was supplied by the air in the course of the experiment. 



It is assumed by the German experimenters that the solid excrement 

 of herbivorous animals, and especially that of the horse, is peculiarly 

 rich in the organisms reducing the nitrogen of a nitrate to gas ; the only 

 foundation for this supposition is, apparently the marked power of re- 

 ducing nitfates possessed by these excrements. Wagner tells us that if 

 100 grams of horse dung are added to 1,000 grams of water, containing 

 5 grams of saltpetre, and the whole allowed to stand in a warm place, the 

 nitrate will disappear in a few days, the escape of gas being shown by a 

 brisk effervesence of the liquid. So many hasty conclusions have been 

 arrived at during the discussion of the question before us that we must 

 just point out that in the case of an action like denitrification, requiring 

 the concurrence of several conditions, each of them equally essential, 

 the activity of this action in any particular case cannot be taken as proof 

 of a special preponderance of one of these conditions, but rather as 

 showing that all are fully present. 



There is, however, one reason which would lead us to expect that 

 animal excrement would contain a relative abundance of the denitrifying 

 organism. If we may assume, as we probably may, that the denitrify- 

 ing bacteria present in the food passed uninjured through the intestines, it 



