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vegetable organism has the property of with olding nil ric acid from solution, 

 either by some property of its tissues or more probably by some preli- 

 minary combination which the nitric acid undergoes in the plant itself. 

 This is easily shown by a simple experiment. If fresh and still living 

 plants be subjected to the solvent action of water, very little nitric acid 

 will be found to pass into solution. If, however, the plants are killed be- 

 fore the experiment is made, by being exposed for some time in an at- 

 mosphere of chloroform, the nitric acid which they contain is easily ex- 

 tracted by water. 



The losses, therefore, which an arable soil sustains in respect of its 

 content of nitrogenous matter must be supplied either by the addition of 

 nitrogenous fertilisers or by some action of the soil whereby the nitro- 

 gen which pervades it may be oxidised and fixed in a form suited to the 

 nourishment of plants. The discussion in regard to the possibility of 

 fixing nitrogen in the soil has been carried on with great vigour during 

 the last two decades. The proof, however, is now overwhelming that 

 such fixation does take place. It would not be proper htre to enter into 

 a discussion of the processes by which this fixation is determined, and, 

 in fact, they are not definitely known. One thing, however, is certain, 

 viz ; that it is accomplished by means of nitro-organisms or ferments, 

 similar, perhaps, in their nature, to those already mentioned, but ca- 

 pable of absorbing, assimilating, and oxidizing free nitrogen. 



METHODS OF OXIDIZING FREE NITROGEN. 



At the present time it is sufficiently well known that this operation 

 takes place in two ways. In tne first place, there are found to exist on 

 the rootlets of certain plants, chiefly of the leguminous family, colonies 

 of bacteria, whose function is known by the effects which they produce. 

 In such plants in a state of maturity, as was mentioned above, are found 

 larger quantities of organic nitrogen than could possibly have been de- 

 rived from the soil in which they were grown or from the fertilisers 

 with which they are supplied. Cultural experiments in steiilised soils, 

 with careful exclusion of all sources of organic nitrogen, have proved 

 beyond question that this gain in nitrogen is found only in such plants 

 as are infected by the organism mentioned. The logical conclusion is, 

 therefore, inevitable that these organisms, in their symbiotic develop- 

 ment with the plant rootlets, assimilate tnd oxidize the free nitrogen 

 of the air and present it to the plant in a form suited to absorption. 

 Attempts have beeu made to inoculate the rootlets of other families of 

 plants with these organisms, but so far without any pronounced success. 

 There are, however, certain orders of low vegetable life — such as cryp- 

 togams, for instance — which seem to share to a certain degree the fa- 

 culty of the leguminous plants in acting as a host for the nitrifying or- 

 ganisms mentioned. The observation above recorded becomes a sufficient 

 explanation of the fact that the fertility of fields is increased by the 

 cultivation of leguminous plants, which would not be possible except 

 they could develop some such property as that which has already been 

 described. 



Another order of organisms has also been discovered which is capa- 

 ble of oxidizing free nitrogen when cultivated in an environment from 

 which organic nitrogen is rigidly excluded. It seems p: obable. therefore, 

 even in soils which bear crops not capable of developing nitrifying or- 



