nisms to the soil in the form of manure, when the results were really du^ 

 to the supply of an excess of organic matter. We shall see presently 

 that, taking the more correct explanation of the facts, the practical con- 

 clusions become very different from those hitherto assumed. The prin- 

 ciple that denitrification proceeds, other things being equal, in direct 

 proportion to the quantity of combustible organic matter present is, how- 

 ever, so very important that we shall quote a few more experimental 

 illustrations of this fact before discussing the German results from the 

 new. point of view. 



In Gayon and Dupetit's investigations the preponderating influence 

 of the quantity of the organic manner present is fully recognised. The 

 same organism placed in sewage and in chicken broth, reduced in the 

 first instance 0.1 to 0.2 gram of nitre per litre, and in the second instance 

 50 grams per litre. Some experiments of my own are equally clear on 

 the point. Two equal volumes of diluted urine were taken ; one received 

 nitre at the rate of 1 gram per litre ; the other received both nitre and 

 sugar, each at the rate of 5 grams per litre. Both solutions received 0.5 

 gram of soil to supply to the denitrifying organism. The solutions were 

 then covered with a layer of paraffin oil to exclude air, and thus inten- 

 sify the action of the organic matter, and were kept at the temperature 

 of 70 ° Fahr. In the solution containing no sugar a slight evolution 

 of gas occurred, and nitrites appeared in the liquid ; some reduction 

 of nitrate thus took place. The action then stopped, and much 

 of the nitrate remained permanently in the solution. In the 

 second case, in which five times the quantity of nitre was pre- 

 sent, accompanied by its own weight of sugar, the whole of the 

 nitrate disappeared in eleven days. This experiment was afterwards 

 repeated at a higher temperature, 95 ° Fahr. the whole of the nitrate 

 then disappeared in four days. The presence or absence of sugar thus 

 entirely determined the extent to which dentrification would take place. 

 An experiment of Munro's is perhaps still more to the point, as it shows, 

 that the presence or absence of organic matter is sufficient to determine 

 whether nitrification or denitrification will occur. Muuro was experi- 

 menting with a river water. When he added to this water an ammon- 

 ium salt, active nitrification took place, the ammonia disappeared and 

 was replaced by a nitrate. If, however, to this actively nitrifying me- 

 dium he added a small quantity of a soluble tartrate, denitrification set 

 in, and the whole of the nitric acid in the water disappeared. After a 

 time the effect of the tartrate ceased, and the water again became a 

 nitrifying medium. This water like ordinary soil, clearly contained 

 both nitrifying and denitrifying organisms. In the absence of oxidi sable 

 organic matter the nitrifying organisms held command of the situation; 

 but when organic matter was introduced the denitrifying organisms 

 sprang into activity, and the character of the chemical changes taking 

 place was entirely reversed. 



We pass now to consider more particularly the German results. 



