254 Tlie Atmosphere as a source of Nitrogen to Plants. 
gen ; tliat, on the other hand, the leguminous plants, clover and 
peas, had sensibly acquired nitrogen during the experiment. 
Boussingault, however, came to no definite conclusion as to 
whether the nitrogen so absorbed by the plant was derived from 
the gaseous nitrogen of the atmosphere, or from the small quantity 
of ammonia, which as he says, and as Saussure had pointed out, 
is always present in the air. In his ' Chemistry of Agriculture,' 
published in 1840, Professor Liebig related experiments which 
had been made at Giessen to demonstrate the presence of ammo- 
nia* in rain-water and in snow. Believing it to be constantly 
present in the air, he considered it to be the natural source of the 
supply of nitrogen to plants. He says, " there is not the slightest 
reason for believing that the nitrogen of the atmosphere" (mean- 
ing, of course, the free nitrogen forming the great bulk of the air) 
" takes part in the processes of assimilation of plants and ani- 
mals ;" and in another place, " no conclusion can then have a 
better foundation than this, that it is the ammonia of the atmo- 
sphere which furnishes nitrogen to plants." 
The importance thus attached by so great an authority in such 
matters to the presence of ammonia in the air, as the source of 
supply of nitrogen to plants, naturally led chemists to endeavour 
to ascertain the proportion of this substance which at different 
times and under different circumstances the atmosphere might 
contain. 
The first attempt of the kind was made by a German pharma- 
ceutical chemist, named Grager. His experiments, however, 
were not conducted with the precautions which are indispensable 
in so delicate an inquiry, and need not detain us. He found that 
one million parts of air contained 0 333 parts of ammonia, or one 
part of ammonia in three million parts of air. 
The experiments of Mr. Kemp, an Irish chemist, are open to 
different but equally strong objections. Kemp made one million 
parts of air to contain 3 880 parts of ammonia, or one part of 
ammonia to 258,000 parts of air, or about twelve times as much 
as found by Grager. 
Fresenius, a former pupil of Professor Liebi<j's, made a series 
of determinations of ammonia in the air, which are far more 
valuable and trustworthy than those just mentioned. He, 
however, appears to have operated upon too small quantities, 
and his results are in consequence open to some doubt. Fresenius 
found that one million parts of air contain on the average, — 
By day 0-098\ . . 
T> • t... n i/^r. } pa' ts ot ammonia, 
By night 0- 169 j ' ' 
* Ammonia is a compound of nitrogen and hydrogen. 
