We are apt to regard the rain solely as a product of distillation, and,
as such, very pure. A little reflection and a very slight amount of
experimental examination will quickly disabuse those who have this
mistaken and popular impression of their error. A great number of bodies
which arise from industrial processes, domestic combustion of coal,
natural changes in vegetable and animal matter, terrestrial disturbances
as tornadoes and volcanic eruptions, vital exhalations, etc., are
discharged into the atmosphere, and, whether by solution or mechanical
contact, descend to the surface of the earth in the rain, leaving upon
its evaporation in many instances the most incontestable evidences of
their presence. The acid precipitation around alkali and sulphuric acid
works is well known; the acid character of rains collected near and in
cities, and the remarkable ammoniacal strength of some local rainfalls,
have been fully discussed. The exhaustive experiments of Dr. Angus Smith
in Scotland, and the interesting reports of French examiners, have made
the scientific world familiar, not only qualitatively but
quantitatively, with the chemical nature of some rains, as well as with
their solid sedimentary contents.
Some years ago my attention was unpleasantly drawn to the fact that the
rain water in our use reacted for chlorine; and on finding this due
solely to the washing out from the atmosphere of suspended particles of
chloride of sodium or other chlorides or free chlorine, it appeared
interesting to determine the average amount of these salts in the rain
water of the sea coast. The results given in this paper refer to a
district on Staten Island, New York harbor, at a point four miles from
the ocean, slightly sheltered from the ocean's immediate influence by
the intervention of low ranges of hills. They were communicated to the
Natural Science Association of Staten Island, but the details of the
observations may prove of interest to the readers of the _Quarterly_,
and may there serve as a record more widely accessible.
It has long been recognized that the source of chlorine in rainfalls
near the sea was the sea itself, the amount of chlorides, putting aside
local exceptions arising from cities or manufactories, increasing with
the proximity of the point of observation to the ocean, and also showing
a marked relation to the exposure of the position chosen to violent
storms. Thus the west coast rainfalls of Ireland contain larger
quantities of chlorides than those of the east, and the table given by
Dr. Smith shows the variations in neighboring localities on the same
seafront. The chlorides of the English rains diminish as the observer
leaves the sea coast. In the following observations the waters of
thirty-two rains were collected, the chlorine determined by nitrate of
silver in amounts of the water varying from one liter to one-half a
liter, and in some instances less. While it is likely that some of the
chlorine was due to the presence of chlorides other than common salt, as
the position of the point of observation is not removed more than a mile
from oil distilleries and smelting and sulphuric acid works in New
Jersey, yet this could not even generally have been so, as the rain
storms came, for the greater number of instances, from the east, in an
opposite direction to the position of the factories alluded to. It has
also been noticed by Mr. A. Hollick, to whom these observations were of
interest, that in heavy storms a salt film often forms upon fruit
exposed to the easterly gales upon the shores of the island.
The yearly average for chlorine is 0.228 grain per gallon; for sodic
chloride, 0.376 grain. The total rainfall in our region for 1884, as
reported by Dr. Draper at Central Park, was 52.25 inches, somewhat
higher than usual, as the average for a series of years before gives 46
inches; but taking these former figures, we find that for that year
(1884) each acre of ground received, accepting the results obtained by
my examination, 76.24 avoirdupois pounds of common salt, if we regard
the entire chlorine contents of the rains as due to that body, or 46.23
pounds of chlorine alone.
In comparison with this result, we find that at Caen, in France, an
examination of the saline ingredients of the rain gave for one year
about 85 pounds of mineral matter per acre, of which 40 pounds were
regarded as common salt.
Although chlorine is almost constantly present in plant tissues, it is
not indispensable for most plants, and for those assimilating it in
small amounts, our rainfall would seem to offer an ample supply. These
facts open our eyes to the possible fertilizing influence of rains, and
they also suggest to what extent rains may exert a corrosive action when
they descend charged with acid vapors.--_L.P. Gratacap, in School of
Mines Quarterly_.
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