The NUrifijing Ferments of the Soil. 
713 
second culture, four out of five of the foreign organisms had 
disappeared ; the fifth could not be got rid of, even by a series 
of cultures, and it remained the only organism in the liquid 
that would grow on gelatine, and yet not the one producing 
I nitrification. 
i By making use of the inability of the latter to grow on 
gelatine, Winogradsky at last got rid of its one remaining 
! troublesome companion. Shaking a little of the deposit with 
I T)ure water, and dropping separate drops of this on a gelatine 
I slab, after incubation for ten days it was seen that some only of 
the drops grew the colonies of the foreign microbe — and by 
I seeding an ammonia solution with a particle of deposit carefully 
I removed from one of the sterile drops, nitrification was produced 
( and a liquid obtained in which the nitrifying ferment was 
allowed to grow until a sufficient quantity was obtained in a 
pure state for examination. 
Just before the announcement of this successful result. Dr. 
and Mrs. P. Frankland also succeeded at last in isolating a nitri- 
fying organism. A series of no less than twenty-four successive 
cultures in ammoniacal solutions free from organic matter had 
all given colonies of microbes on gelatine which had no nitrify- 
ing power ; by combining the method of attenuation with that 
of successive culture, however, i.e. by adding a few drops of the 
medium to a body of water, and then seeding a fresh solution 
with a drop of this liquid, they succeeded. By seeding succes- 
sively with dilutions of tctoj tooWo> ioooooo > ^^-^ ^ Point 
is always eventually reached where the inoculation fails to pro- 
duce nitrification, and selecting always the weakest inoculation 
which succeeds in nitrifying, it is possible to find that no foreign 
organisms have been introduced. At length they found them- 
selves with a nitrifying solution refusing to give a growth on 
gelatine, but containing a nitrifying organism which seems 
to agree with the description given by Winogradsky. 
Only a month or two after this Warington succeeded, also by 
the method of attenuation, in producing from cultivations made 
in the absence of organic matter a similar result, the absence of 
any growths on prepared gelatine being taken as a proof of the 
absence of foreign organisms. 
All these results made it clear that the exclusion of organic 
matter from the cultivation media is vital to success in isolating 
these organisms, and Winogradsky, in successive endeavours to 
obtain a solid medium on whicli they could be grown in pure 
colonies, was not successful with any gelatinising medium of an 
organic nature. But by employing a purely inorganic jelly, i.e., 
soluble silica, purified by long-continued dialysis, ^nd capsed to 
