ich
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 { Paris 0.2100
{ Liebfrauenberg 0.0350

Dew, Liebfrauenberg { Maximum 0.4340
{ Minimum 0.0714

{ Liebfrauenberg 0.1790
Fog { Paris 9.6000

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