Ammonia.—The presence of ammonia in the atmosphere appears to have been first observed by Saussure, who found that when the sulphate of alumina is exposed to the air, it is gradually converted into the double sulphate of alumina and ammonia. Liebig more recently showed that ammonia can always be detected in rain and snow water, and it could not be doubted that it had been absorbed from the atmosphere. Experiments have since been made by different observers with the view of determining the quantity of atmospheric ammonia, and their results are contained in the subjoined table, which gives the quantity found in a million parts of air.

Kemp3·6800
Pierre{12 feet above the surface3·5000
{25 feet do. do.0·5000
Graeger0·3230
Fresenius{By day0·0980
{By night0·1690
Ville{{Maximum0·0317
{ In Paris{Minimum0·0177
{{Mean0·0237
{{Maximum0·0276
{ Environs{Minimum0·0165
{ of Paris{Mean0·0210

Of these results, the earlier ones of Kemp, Pierre, and Graeger are undoubtedly erroneous, as they were made without those precautions which subsequent experience has shown to be necessary. Even those of the other observers must be taken as giving only a very general idea of the quantity of ammonia in the air, for a proportion so minute as one fifty-millionth cannot be accurately determined even by the most delicate experiments. For this reason, more recent experimenters have endeavoured to arrive at conclusions bearing more immediately upon agricultural questions, by determining the quantity of ammonia brought down by the rain. The first observations on this subject were made by Barral in 1851, and they have been repeated during the years 1855 and 1856 by Mr. Way. In 1853, Boussingault also made numerous experiments on the quantity of ammonia in the rain falling at different places, as well as in dew and the moisture of fogs. He found in the imperial gallon—

Grs.
Rain{ Paris0·2100
{ Liebfrauenberg0·0350
Dew,Liebfrauenberg{ Maximum0·4340
{ Minimum0·0714
{ Liebfrauenberg0·1790
Fog{ Paris9·6000

It thus appears that in Paris the quantity of ammonia in rain-water is just six times as great as it is in the country, a result, no doubt, due to the ammonia evolved during the combustion of fuel, and to animal exhalations, and to the same cause, the large quantity contained in the moisture of fogs in Paris may also be attributed. Barral and Way have made determinations of the quantity of ammonia carried down by the rain in each month of the year, the former using for this purpose the water collected in the rain-gauges of the Paris Observatory, and representing, therefore, a town atmosphere; the latter, that from a large rain-gauge at Rothamsted, at a distance from any town. According to Barral the ammonia annually deposited on an acre of land amounts to 12·28 lbs., a quantity considerably exceeding that obtained by Way, whose experiments being made at a distance from towns, must be considered as representing more accurately the normal condition of the air. His results for the years 1855 and 1856 are given below, along with the quantities of nitric acid found at the same time.

Nitric Acid.—The presence of nitric acid in the air appears to have been first observed by Priestley at the end of the last century, but Liebig, in 1825, showed that it was always to be found after thunder-storms, although he failed to detect it at other times. In 1851 Barral proved that it is invariably present in rain-water, and stated the quantity annually carried down to an acre of land at no less than 41·29 lbs. But at the time his experiments were made, the methods of determining very minute quantities of nitric acid were exceedingly defective, and Way, by the adoption of an improved process, has shown that the quantity is very much smaller than Barral supposed, and really falls short of three pounds. His results for ammonia, as well as nitric acid, are given in the subjoined table.

Nitric Acid in Grains. Ammonia in Grains. Total Nitrogen in Grains.
1855. 1856. 1855. 1856. 1855. 1856.
January 230 1564 1244 5,005 1084 4,526
February 944 544 2337 4,175 2169 3,579
March 1102 866 4513 2,108 3995 1,945
April 325 1063 1141 8,614 1024 7,369
May 1840 3024 4206 18,313 3939 15,863
June 3303 2046 5574 4,870 5447 4,540
July 2680 1191 9620 2,869 8615 2,670
August 3577 2125 4769 4,214 4870 4,021
September 732 1756 3313 5,972 2917 5,373
October 4480 2075 7592 3,921 7414 3,767
November 1007 1371 3021 2,591 2749 2,489
December 664 2035 2438 4,070 2180 3,352
Total in pounds for the whole year 2·98 ·280 7·11 9·53 6·63 8·31

No attempts have been made to determine the proportion of nitric acid in air, but its quantity is undoubtedly excessively minute, and materially smaller than that of ammonia. At least this conclusion seems to be a fair inference from Way's researches, as well as the recent experiments of Boussingault on the proportion of nitric acid contained in rain, dew, and fog, made in a manner exactly similar to those on the ammonia, already quoted. According to his experiments an imperial gallon contains—

Grs.
Rain.{Paris0·0708
{Liebfrauenberg0·0140
Dew.{Maximum0·0785
{Minimum0·0030
Fog.{Paris0·7092
{Liebfrauenberg0·0718

Although it thus appears that Barral's results have been only partially confirmed, enough has been ascertained to show that the quantity of ammonia and nitric acid in the air is sufficient to produce a material influence in the growth of plants. The large amount of these substances contained in the dew is also particularly worthy of notice, and may serve to some extent to explain its remarkably invigorating effect on vegetation.