144 PRODUCTS OF VITAL ACTIVITY. 
A peptonizing ferment for gelatin is produced by a considerable 
number of bacteria, as stated under the heading “ Liquefaction of 
Gelatin.” The jellified albumin in cultures in blood serum is also 
liquefied by a peptonizing ferment produced by certain species of bac- 
teria. 
Some authors also speak of a soluble ferment capable of inverting 
cane sugar or milk sugar. According to Hueppe, such a ferment 
is produced by the Bacillus acidi lactici. A soluble ferment for cel- 
lulose is supposed by Fliigge to be produced by several species— 
among others by Bacillus butyricus and by Vibrio rugula. 
Several bacilli produce a soluble ferment capable of coagulating 
the casein of milk. 
Reduction of Nitrates, and Nitrification.—The researches of 
Gayon, Dupettit, and others show that certain bacteria are able to 
reduce nitrates with liberation of ammonia and free nitrogen. This 
is effected in the absence of oxygen by anaérobic bacteria, and, 
among others, by Bacillus butyricus. Certain aérobic bacteria also 
accomplish the same result. Thus Herzus obtained two species 
from water which reduced nitrates in a very decided manner. On 
the other hand, a number of species are known to oxidize ammonia, 
producing nitric acid. Schlésing and Miinz, as a result of numerous 
experiments, arrived at the conclusion that in the scil nitrification is 
effected by a single species. But it is doubtful whether they worked 
with pure cultures, and more recent researches show that several, 
and probably many, different bacteria possess this power. Accord- 
ing to Heres, the following species, tested by him, oxidize am- 
monia : Bacillus prodigiosus, the cheese spirillum of Deneke, the 
Finkler-Prior spirillum, the typhoid bacillus, the anthrax bacillus, 
the staphylococci of pus. The oxidation does not always go to the 
point of forming nitrates, but nitrites may be formed in the soil 
(Duclaux). Warrington states that certain bacteria which formed 
nitrates in a suitable culture medium produced only nitrites when, 
after an interval of four or five months, some of the culture was 
transferred to a solution containing muriate of ammonia. The same 
author states that the process of nitrification occurs only in the 
dark. 
The researches of Winogradsky, of the Franklands, and of Jor- 
dan show that the failure of earlier investigators to obtain the nitri- 
fying bacteria from the soil in pure cultures was due to the fact that 
these bacteria do not grow in the usual culture media. By the use 
of certain saline solutions the authors named have succeeded in iso- 
lating nitrifying bacteria in pure cultures, or nearly so. It is still ° 
uncertain whether these investigators have obtained the same bac- 
teria, but the microdrganisms described by them, and obtained from 
