The Atmosphere as a source of Nitrogen to Plants. 
253 
interestinjj; subjects of vegetable nutrition laboured, however, 
under many disadvantages which are unknown to their succes- 
sors, and wanted many facilities for investigation which these 
latter enjoy, and we must turn to a later period for any very 
important inquiry upon the question in hand. 
in the year 1825 Professor Liebig demonstrated the presence 
of nitric acid in rain water, but only in that which had fallen 
during storms. Of 77 samples, 17 were the produce of storms, 
and all of these contained nitric acid in greater or less quantity, 
combined with lime or ammonia. In the remaining 60 he did not 
succeed in detecting any nitric acid.* Professor Liebig seems to 
have attached very little importance to this source of nitrogen for 
vegetation, believing, as he says, that the quantity present in rain 
water was very small, and would only occur at all during tiiunder 
storms, of which there are perhaps not more than ten or twelve 
in the year. We shall presently see how far he was correct in this 
opinion. 
In the year 1836 Boussingault, whose name is familiar to every 
student of agricultural chemistry, attempted to settle the question 
whence and under what form plants obtain their nitrogen. Aware 
of the difficulties and sources of error to which the experiments 
of Saussure were exposed, Boussingault employed another and 
very simple method to ascertain whether plants could obtain their 
nitrogen from the air, namely, to compare the composition of the 
seeds with that of the crops produced from them, solely at the 
expense of the air and water. The great exactness with which 
the determination of nitrogen could be made, would of course 
enable him to ascertain with certainty whether the crops contained 
a larger quantity of this element than the seeds from which they 
were produced. 
Seeds of which the weight was previously ascertained, were 
sown in burnt clay or in silicious sand, from which all traces of 
organic matter and ammoniacal salts had been removed by perfect 
calcination. Porcelain pots were used to contain the soils, and 
the growing plants were placed in a green-house at the end of a 
long garden, and watered with distilled water. If under these 
circumstances the quantity of nitrogen in the crop was larger than 
that furnished by the seeds, the inference was incontestable that 
the plants had imbibed nitrogen from the air in some form. 
The plants upon which these experiments were made were 
clover, wheat, oats, and peas. The conclusions to which he 
arrived from these experiments were that wheat and oats, although 
increasing in weight from an assimilation of hydrogen, carbon, 
and oxygen, did not sensibly increase in the proporticm of nitro- 
* It is to be borne in mind that nitric acid is a compound, from which, if it 
existed in the air in sufficient quantity, plants might readily obtain their nitrogen. 
