THE CIECULATION OF NITROGEN IN NATURE. 67 
limestone, and sewage was slowly filtered througli it. Now 
sewage contains about five grains of ammonia and one grain 
of organic nitrogen in each gallon, but no trace of nitre. 
At first the sewage passed through the filter unaltered, but 
gradually the ammonia began to diminish, whilst calcium 
nitrate made its appearance in the filtrate, until at length 
the latter contained no ammonia, but a quantity of nitrate 
almost corresponding to the nitrogen in the original sewage. 
Now the conditions of the experiment altogether were very 
suggestive of the establishment of a fermentation ; but the 
truth of this supposition was confirmed by the crucial test 
of applying a reagent that would inhibit all biological action 
without disturbing the chemical conditions. The vapour of 
chloroform was allowed to pass through the filter. All 
nitrification immediately stopped, and could only be renewed 
by the. original slow process of filtration. Much work has 
since been done in connection with this phenomenon, but 
progress has been very slow, owing to the very peculiar 
habits of the organisms. In particular, they abhor organic 
matter, and it is impossible, therefore, to cultivate them upon 
any of the ordinary media. It has been found, however, 
that they can be grown fairly easily upon a medium the 
basis of which is gelatinous silica, or a jelly prepared from 
certain seaweeds from which all soluble organic matter has 
been removed by putrefaction. 
In this way it has been shown that two distinct organisms 
are concerned, one converting ammonia into nitrous acid, the 
other completing the oxidation into nitric acid. 
These organisms are both short bacilli, frequently occur- 
ring in pairs attached to the particles of soil over which the 
ammoniacal water is passing. They flourish between the 
temperatures of 10° and 30° 0., are sensitive to bright 
light, and require a constant supply of oxygen and a base, 
