258 The Atmosphere as a source of Nitrogen to Plants. 
of green-houses to increase the luxuriance of the plants growing 
therein. 
In a ])aper read before the Academy of Sciences in Paris, in 
1851, M. Barral has given the result of his experiments to deter- 
mine the quantity of ammonia and nitric acid in rain water. 
Rightly considering that every shower of rain would bring down 
in solution any portion of these substances suspended in the air, 
he determined upon the examination of rain as a means of ascer- 
taining the quantity of ammonia and nitric acid which the air 
contained. It will be remembered that Liebig had failed to 
discover nitric acid except in the rain of thunder storms. Barral, 
however, by the most careful experiments, has succeeded in de- 
tecting it in the water of rain at all times, at least of rain falling 
in Paris. The water operated upon by M. Barral was collected 
in the large rain gauges of the Observatory at Paris. The various 
quantities falling were collected and analysed for each month, the 
nitric acid and ammonia being separately determined. M. Barral 
found very considerable differences in the proportion of each of 
these present in rain in different months ; their quantity was also 
not at all proportionate to the quantity of rain falling, a circum- 
stance easily comprehended when we call to mind their solubility 
in water, which would cause them to be brought down in chief 
part by the first shower that fell. These experiments extended 
only over four or six months, and cannot, therefore, very properly 
be said to apply to the whole year. M. Barral has calculated 
that if the ammonia and nitric acid of the remainder of the year 
were in the same proportion, an acre of soil in the latitude 
of Paris would receive annually the following quantities of nitro- 
gen :— 
In the form of ammonia 8 lbs. 
In the form of nitric acid IG'9 lbs. 
M. Barral, therefore, has shown that the nitrogen in the nitric 
acid of the air exceeds that present in the form of ammonia. He 
has the credit of being the first experimenter who has succeeded 
in determining the quantity of nitric acid in rain-water. The 
objection to these experiments, as the basis of conclusions to be ap- 
plied to agriculture, is obvious, and would occur to every one, 
as it did to M. Barral himself, namely, that the air of a city of a 
million inhabitants, covering an area considerably smaller in 
proportion than that of London, might well be impregnated with 
a very much larger proportion of ammonia and nitric acid, th& 
products of animal decomposition, than the pure atmosphere of 
the country. Carefully and ably as M. Barral's experiments 
have been made, they are, from this cause, inadequate to furnish 
any reliable data for the purpose of the agricultural student. 
We turn now to the later researches of Boussingault. 
