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VITAL MAXIFESTATIOyS OF BACTERIA. 33

microbes which reduced nitrates, sometimes to nitrites, some-
times to ammonia, nitrogen, and nitrous oxide (denitrification.)
Drainage-water, to which saltpetre was added (0.02 to 1 litre),
infected with spoiled urine, showed a gradual decrease of the
nitrate under development of numerous microbes.
When the culture was sterilized, or chloroform, or any other
antiseptic added to it, no process of reduction took place. The
presence of organic matter and a limited access of air were the
conditions of fermentation. The optimum of temperature was
found to lie between 35° and 40" C.
Salicylic acid, in antiseptic doses, did not impede the process of
reduction, but soon disappeared itself. Xitrates of sodium, am-
monium, and calcium were reduced in the same way as nitrates
of potassium. Deherain and Maquenne ^' obtained similar results.
Their first experiment yielded the following figures for one hun-
dred parts of the gas formed :

CO., 80.5
K,6 8.2
X 11.3
The second experi ment yielded-
CO., 67.3
H 31.5
X 1.2

These processes of oxidation and reduction have also recently
been observed in pure cultures.
It appears from Heraeus's^' investigations that they may be
occasioned by a large number of dififerent species of bacteria.
He made experiments with several known kinds, and also with
twelve unknown kinds, which he cultivated from water, soil,
etc. Of the latter, two reduced nitric acid to nitrous acid and
ammonia, and converted urea into carbonate of ammonia. Two
consumed nitric acid without reduction ; one of them transform-
ing urea into salts of ammonia, the other not. Xone of these
twelve varieties manifested any oxidizing influence. Heraeus
succeeded, however, in isolating, from garden soil, two species
which led to the formation of nitrous acid in saccharine solu-
tions, in urine, etc.
3