TBANSACTIONS OF SECTION B. 683 



result of the action of an organised ferment, which occurs ahundantly in soils and 

 in most impure waters. The evidence for the ferment theory of nitrification is now 

 very complete. Nitrification in soils and waters is found to he strictly limited to 

 the range of temperature within which the vital activity of living ferments is eon- 

 fined. Thus nitrification proceeds with extreme slowness near the freezing-point, 

 and increases in activity with a rise in temperature till 37° are reached ; the action 

 then diminishes, and ceases altogether at 55°. Nitrification is also dependent on 

 the presence of plant-food suitable for organisms of low character. Recent experi- 

 ments at Rothamsted show that in the absence of phosphates no nitrification will 

 occur. Further proof of the ferment theory is afforded by the fact that antiseptics 

 are fatal to nitrification. In the presence of a small quantity of chloroform, carbon 

 bisulphide, salicylic acid, and apparently also phenol, nitrification entirely ceases. 

 The action of heat is equally confirmatory, liaising sewage to the boiling-point 

 entirely prevents its undergoing nitrification. The heating of soil to the same 

 temperature effectually destroys its nitrifying power. Finally, nitrification can be 

 started in boiled sewage, or in other sterilised liquid of suitable composition, by the- 

 addition of a few particles of fresh surface soil, or a few drops of a solution which 

 has already nitrified ; though without such addition these liquids may be freely 

 exposed to filtered air without nitrification taking place. 



The nitrifying organism has been submitted as yet to but little microscopical 

 study : it is apparently a micrococcus. 



It is difficult to conceive how the evidence for the ferment theory of nitrifica- 

 tion could be further strengthened ; it is apparently complete in every part. 

 Although, however, nearly the whole of this evidence has been before the scientific 

 public for more than seven years, the ferment theory of nitrification can hardly be 

 said to have obtained any general acceptance ; it has not indeed been seriously con- 

 troverted, but neither has it been embraced. It is partly with the view of calling 

 the attention of English and American chemists to the importance of a decision on. 

 this question that I have been induced to bring- this subject before them on the 

 present occasion. 



The Distribution of the Nitrifying Organism in the Soil. — Small quantities of 

 soil were taken, at depths varying - from two inches to eight feet, from freshly-cut 

 surfaces on the sides of pits sunk in the clay soil at Rothamsted. The soil removed 

 was at once transferred to a sterilised solution of diluted urine, which was after- 

 wards examined from time to time to ascertain if nitrification took place. From 

 the results it would appear that in a clay soil the nitrifying organism is confined to- 

 about eighteen inches from the surface ; it is most abundant in the first six inches. 

 It is quite possible, however, that in the channels caused by worms, or by the roots 

 of plants, the organism may occur at greater depths. In a sandy soil we should 

 expect to find the organism at a lower level than in clay, but of this we have as yet 

 no direct evidence. 



Some very practical conclusions may be drawn from the facts now stated. It 

 appears that the oxidation of nitrogenous matter in soil will be confined to matter- 

 near the surface. The nitrates found in the subsoil, and in subsoil drainage waters,, 

 have really been produced in the upper layer of the soil, and have been carried down 

 by diffusion, or by a descending column of water. Again, in arranging a filter-bed 

 for the oxidation of sewage, it is obvious that with a heavy soil lying in its natural 

 state of consolidation very little will be gained by making the filter-bed of con- 

 siderable depth ; while, if an artificial bed is to be constructed, it is clearly the top- 

 soil, rich in oxidising organisms, which should be exclusively employed. 



The Substances susceptible of Nitrification. — The analyses of soils and drainage- 

 waters have taught us that the nitrogenous humic matter resulting from the decay 

 of plants is nitrifiable ; also that the various nitrogenous manures applied to land, as 

 farmyard manure, bones, fish, blood, rape-cake, and ammonium salts, uudergo- 

 nitrification in the soil. In the Rothamsted Laboratory experiments have been 

 made on the nitrification of solutions of various substances. Besides solutions con- 

 taining ammonium salts and urea, I have succeeded in nitrifying solutions of 

 asparagine, milk, and rape-cake. Thus, besides ammonia, two amides, and two 

 forms of albuminoids have been found susceptible of nitrification. In all cases in 



