20 



is evident that they will occur in larger proportion in the solid excre- 

 ment than in the original food. If, for instance, a horse is fed on hay, 

 and digests one-halt" of the food consumed, the dung will then contain 

 twice the proportion of the denitrifying bacteria originally present in 

 the food. The proportion of the organisms supplied by different kinds 

 of foo:i must vary greatly. If the bacteria are, as we assume, deposited 

 from the atmosphere, and lie in all cases on the surface of the food, their 

 proportion must depend largely on the extent and character of the sur- 

 face, and on the length of exposure to the atmosphere. A hundred 

 pounds of hay should contain a far greater proportion of bacteria than a 

 hundred pounds of clean mangel or turnips. From this point of view 

 there is thus some ground for assuming that horse excrement, will be 

 Ficher in denitrifying organisms than cattle excrement. Wiithrich and 

 Freudenreich have recently attempted to estimate the relative number 

 of bacteria present in the dung of cows fed on fresh grass, on hay alone, 

 and on hay with potatoes or brewers' grains. They found that when 

 nothing but hay was given the number of bacteria in the dung was far 

 greater than with any other diet amounting indeed, to the extraordinary 

 proportion of 165 millions per gram of excrement. 



It is suggested by some who have written on the question, that the 

 denitrifying bacteria may largely increase in numbers during the pas- 

 sage of the food through the intestines. Of this increase there is as yet 

 no proof, nor does it seem very probable. The increase of an orgaaism 

 can onlv be safely assumed when the circumstances are favourable to the 

 exercise of its particular functions ; now, the denitrifying bacteria be- 

 long to a class of organisms requiring oxygen, free or combined, to ac- 

 complish their special work, and this necessary condition is absent in 

 the animal intestine. The bacteria which certainly flourish in the animal 

 intestine are those which determine the march-gas fermentation of 

 oellulose. It has been said, with great truth that the fermentation of 

 the dung heap is simply a continuation of the fermentive processes 

 which have commenced in the animal intestine. 



Before further discussing the question before us we must turn once 

 more to the results of former investigations, and see what are the pre- 

 cise conditions necessary to induce denitrification. 



Schloesing (1873), in his investigation on the rate of nitrification 

 in soil, kept various portions of the same moist, humous soil, in atmos- 

 pheres in which different proportions of oxygen were constantly main- 

 tained throughout the experiment. He found that the quantity of nitrate 

 produced in the soil steadily diminished as the proportion of oxygen in 

 the air decreased ; when, however, no oxygen was present, denitrification 

 took place, and the nitrates originally contained in the soil entirely dis- 

 appeared. He repeated the experiment, this time adding a known 

 amount of saltpetre to the soil. He found that such a soil placed in a 

 vessel containing very little air, first absorbed all the oxygen present and 

 then evolved a considerable quantity of gas, this gas contained nitrogen 

 equivalent to the whole of that contained in the saltpetre added. The 

 consumption of the oxygen of the air had thus determined denitrification. 



The supply of atmospheric oxygen to a soil is effectually prevented 

 if the soil is kept saturated with water ; this condition alone i> .-urhcient 



