y found during the night was 5.74,
during the day 5.4. Heavy and continued rain diminishes the quantity of
carbonic acid, by dissolving and carrying it down into the soil.
Saussure found that in the month of July 1827, during the time when nine
millimetres of rain fell, the average quantity of carbonic acid amounted
to 5.18 volumes in 10,000; while in September 1829, when 254 millimetres
fell, it was only 3.57. A moist state of the soil, which is favourable
to the absorption of carbonic acid, also diminishes the quantity
contained in the air, while, on the other hand, continued frosts, by
retaining the atmosphere and soil in a dry state, have an opposite
effect. High winds increase the carbonic acid to a small extent. It was
also found to be greater over the cultivated lands than over the lake of
Geneva; at the tops of mountains than at the level of the sea; in towns
than in the country. The differences observed in all these cases, though
small, are quite distinct, and have been confirmed by subsequent
experimenters.

_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.

Kemp 3.6800

{ 12 feet above the surface 3.5000
Pierre { 25 feet do. do. 0.5000

Graeger 0.3230

{ By day 0.0980
Fresenius { By night 0.1690

{ { Maximum 0.0317
{ In Paris { Minimum 0.0177
{ { Mean 0.0237
Ville {
{ { Maximum 0.0276
{ Environs { Minimum 0.0165
{ of Paris { Mean 0.0210

Of these results, the earlier ones of Kemp, Pierre, and Graeger are
undoubtedly erroneous, as they were made without those precautions wh