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.