( 159 ) 
by interesting agricultural experiments of P. Wacyer ') in 1895, which 
seemed to point out a danger produced by these bacteria for agriculture. 
His experiments gave direct cause to the research of Burri and 
STUTZER®), who, in the same year, elaborately described two denitri- 
fying bacteria. 
From that time this group has been laboriously studied and at 
present a number of twenty denitrifying species have been described *). 
To these I for my own part might add some ten species more, 
but of these I will only discuss those, for which I can point out an 
accumulation experiment, which gives a constant result. 
2. General considerations. 
The hitherto isolated denitrifying bacteria are all aerobic. In 
liquids containing nitrate or nitrite, they can, however, grow vigor- 
ously with a very slight or without access of air, so that in this case 
they behave like anaerobie bacteria. They then transfer the oxygen 
of the nitrate or the nitrite to the organic compounds present in the 
culture liquid. Thence nitrogen is freed and the metals of the salts 
pass into carbonates or bi-carbonates, which process may be represented 
by the formulae: 
5C..+4KNO, + 2 H,0 = 4 KHCO, + 2 N, + CO, 
3C..+4KNO, + H,O = 2 KHCO, + K‚CO, + 2N, 
The correctness of this representation has been proved by the 
observations of Gayon and Dvprtir, Gintay and ABERSON, PFEIFFER 
and LEMMERMANN, AMmPora and ULprant, and also by my own researches. 
We see from this, that in a liquid, simultaneously with the nitrate, 
the rate of organic substances decreases, and accordingly also the 
permaganate number. From a practical point of view this must 
necessarily be of signification for the explanation of the processes on 
which is based the biological purification of sewage and water *). 
My experiments have furthermore convinced me that denitrification 
is inseparably connected with the growth, for which traces of free 
oxygen are always necessary. 
1) Die geringe Ausnützung des Stallmiststickstoffs und thre Ursachen, Landw. 
Presse, -1895:"$:..92: 
2) Ueber demitrifizierende Bakterien. Centrbl. f. Bakt. Abt. II, Bd. 1, 1895, S. 257. 
5) OQ. LEMMERMANN. Kritische Studiën über Denitrifikationsvorgiinge. Jena. 1900. 
C. Horuicu. Vergleichende Untersuchungen über die Denitrifikationsbakt. ete. 
Centrbl. f. Bakt., Abt. Il, Bd. 7, 1902, S. 245. 
4) Dr. Jenny Weyerman. Biologische stelsels tot reiniging van rioolvocht, enz, 
Vragen des Tijds. Febr. 1901, Sep., blz. 38. 
