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soil, because nitrogen does not combine directly with, oxygen unless the 

 latter is transformed into ozone ; therefore, even the soils which are 

 rich in nitrogen usually contain but infinitesimal quantities of nitrates. 

 All is thus against the plants. But here the microbes come into their 

 aid. Already in 1877, Schlosing and Miintz had demonstrated that a 

 living ferment is necessary for the production of nitric acid and nitrates 

 in the soil ; but it took full thirteen or fourteen years of laborious re- 

 searches before it became proved by Professor Percy and Mrs Grace 

 Frankland, Mr Warington and especially by Winogradsky, that the 

 process of converting ammonia into nitric acid, is really performed by 

 special microbes, and that two different bacteria are required to accom- 

 plish the full process. One of them decomposes ammonia, and trans- 

 forms it into water and nitrous acid ; whereupon the other intervenes 

 for further oxidising this acid and transforming it into nitric acid. 

 The two bacteria have finally been isolated by Warington and Wino- 

 gradsky, and they proved to be quite different, although each of the two 

 seems to be represented by several species, characteristic of different lo- 

 calities. Like all other bacteria, they multiply very rapidly, and it is 

 sufficient to introduce in a mould the slightest amount of a soil which 

 has already contained the nitrifying bacteria to provoke in it a trans- 

 formation of its nitrogen compounds into nitric acid It is also most 

 remarkable that the second bacterium was only discovered by Winogradsky 

 when he investigated a sample of soil from Quito — that is from a region 

 not very distant from the great saltpetre layers at Chili and Peru, and 

 that altogether the soils taken from South America, and South Africa, 

 act as powerful ferments, while European soils seem to contain but 

 smaller quantities of the bacteria of nitrification. 



The scientific and practical importance of this discovery cannot be 

 overrated. Without the two microbes, which continually prepare fresh 

 nitric acid in the soil, while the previous stocks of it are washed 

 downwards into the subsoil by rain-water, agriculture would remain in 

 a precarious state. Moreover, when we import nitrate of sodium from 

 Chili and spread it over our fields, we not only increase their stock of 

 assimilable nitrogen, we also import the nitrifying microbe, which will 

 help to mantain the fertility for some time to come. Of course, we also 

 may manure with costly nitrates prepared in the manufacture. Artifi- 

 cially prepared nitrates also exercise a splendid effect upon vegetation, 

 while phosphates admirably aid the plant in the development of its 

 younger parts. But if chemical manure is vivified by the living fer- 

 ment, it only becomes the better for it, the more so as it has been proved 

 that, contrary to all provisions, the nitrifying organisms flourish in li- 

 quids which contain no traces whatever of organic matter. Like green 

 plants, they can build up their protoplasm out of carbonic acid, oxygen, 

 water and ammonia. 



One of the two questions mentioned at the beginning of this chapter 

 has thus received a definite solution. As to the second question, re- 

 lative to the assimilation of nitrogen by plants, it offers some additional 

 difficulties, Already, in the earlier Rothamsted experiments, previous 

 to 1861, it had been remarked that while higher plants, as a rule, ab- 

 sorb no nitrogen from the air, the leguminosae manage somehow to get 

 some of it from this source as well. It was also known to practical 



