428 Fixation of Free Nitrogen hy the Lower Gfden Pldnts. 
In the prosecution of experiments made to determine to what 
extent plants are capable of availing themselves of the free nitrogen 
of the air, two methods of procedure may be employed. These are 
known respectively as the direct method and the indirect. 
In order to ascertain beyond all doubt whether free nitrogen 
gas is taken up by plants, it appeared to the investigators that the 
preferable method was to accurately ascertain the quantity of 
gaseous nitrogen which came in contact with the plants in the 
course of their development, and to record the volume of this 
nitrogen before and after growth. If a certain quantity of the 
nitrogen disappeared in the course of growth, it might be reasonably 
affirmed that some of the gas had been fixed. Then would arise 
the question as to whether such nitrogen had been fixed by the 
plants or by the soil which carried them. To settle this point, the 
same soils are maintained under exactly similar conditions, save 
that they are kept free from plants. If it wei’e found that, whilst 
in the case of the cropped soils there was gain of nitrogen, in that 
of the bare soils there was no such gain, it would be fair to attri- 
bute the fixation to the action of the plants and not of the soil. 
This is the direct method, and it is so called because it supplies a 
direct answer to the question. Is free nitrogen gas taken up by 
plants % 
The other method, the one which has been usually followed, 
involves an exact ascertainment of the quantity of nitrogen (1) 
before growth, in both soils and seeds, (2) after growth, in both 
soils and plants. If there has been fixation of nitrogen the second 
result will be higher than the first. This is the indirect method. 
If both methods are adopted, and are rigidly carried out, they 
ought to agree in their results. If there has been fixation, the 
disappearance of nitrogen gas recorded by the direct method should 
agree with the gain of nitrogen indicated by the indirect process. 
Where no fixation has occurred the direct method should furnish 
equal quantities of nitrogen gas in the air before and after growth, 
whilst the indirect method should show no gain of nitrogen by 
plant or soil. The one method controls the other, and all experi- 
ments in which the two methods do not furnish accordant results 
should be rejected 
As a matter of fact, MM. Schloesing fils and Laurent have em- 
ployed the two metliods simultaneously, as they felt that the results 
so obtained would be more likely to inspire confidence than would 
have been the case had they restricted themselves to one method 
only. It is not necessary, in this brief abstract of their memoir, to 
enter upon a description of the elaborate apparatus which they used, 
or to discuss the rigid precautions which were adopted in order to 
eliminate all sources of error. These belong to the technical details 
of manipulation in experimental chemistry and physiology. 
In their experiments of 1890 the investigators sought to verify 
the accepted fact of the fixation of free nitrogen by certain legu- 
minous plants. For this purpose peas were planted in a soil, prac- 
tically free from nitrogen and sterilised by heat, but afterwards 
