ATMOSPHERIC AIR AS THE FOOD OF PLANTS. 61 
soil, or upon the foliage of a cultivated field, they bring 
down to the reach of vegetation in a given time a quantity 
of ammonia, far greater than what is diffused throughout 
the limited volume of air which contributes to the nour- 
ishment of plants. The solubility of carbonate of ammo- 
nia in water has already been mentioned. Ina rain-fall 
we have the atmosphere actually washed to a great de- 
gree of its ammonia, so that nearly the entire quantity of 
this substance which exists between the clouds and the 
earth, or in that mass of atmosphere through which the 
rain passes, ig gathered by the latter and accumulated 
within a small space. 
Proportion of Ammonia in Rain-water, ete.—The pro- 
portion of ammonia * which the atmospheric waters thus 
collect and bring down upon the surface of the soil, or 
upon the foliage of plants, has been the subject of inves- 
tigations by Boussingault, Bineau, Way, Knop, Bobiere, 
and Bretschneider. The general result of their accordant 
investigations is as follows: In rain-water the quantity of 
ammonia in the entire fall is very variable, ranging in the 
country from 1 to 33 parts in 10 million. In cities the 
amount is larger, tenfold the above quantities having been 
observed. 
The first portions of rain that fall usually contain much 
more ammonia than the latter portions, for the reason that 
a certain amount of water suffices to wash the air, and 
what rain subsequently falls only dilutes the solution at 
first formed. In a long-continued rain, the water that 
finally falls is almost devoid of ammonia. In rains of 
short duration, as well as in dews and fogs, which occasion- 
ally are so heavy as to admit of collecting to a sufficient . 
extent for analysis, the proportion of ammonia is greatest, 
and is the greater the longer the time that has elapsed 
since a previous precipitation of water. 
* In all quantitative statements regarding ammonia, NH3 is to be understood, 
and not NH40, 
