Miscellaneous Substances in the Air

That oxidisable organic compounds are present in the air has been known for many years, the Swedish chemist Berzelius being one of the first to refer to the fact. This explains the observation of Levy and Henriet that air freed from carbon dioxide by treatment with caustic potash generates it again on standing, owing to the oxidation of the organic matter. The exact nature of the organic compounds present in the air is uncertain. Gautier finds 1.21 parts of marsh gas or methane in 10,000 of Parisian air, and 0.17 of benzene and its analogues. Formaldehyde and other organic derivatives have been detected in minute quantities. The unhealthy air of marshes is usually attributed to the volatile organic substances produced by the decomposition of the vegetation. Analyses of the gases evolved from marsh land show that methane constitutes in general more than half the total volume of gases, the remainder consisting of carbon dioxide, nitrogen, and in some cases of oxygen and hydrogen.

Oxides of nitrogen and free nitric acid occur in traces in the air, the former owing their existence to the combination of oxygen and nitrogen under the influence of lightning flashes, and possibly also to a much smaller degree to the oxidation of ammonia. These oxides unite with water, yielding nitric acid.

During thundery weather accompanied by very little rain 1 part of oxides of nitrogen in 4 or 5 million of air has been found.

Ammonia, mainly as carbonate but also as nitrate, is present in the air, and originates in the decomposition of organic nitrogenous substances. H. T. Brown found the air at Burton-on-Trent, during the years 1869 to 1870, to contain from 0.04 to 0.09 parts of ammonium carbonate in 10,000 of air. The analysis of rain-water shows the presence of ammonium nitrate in appreciable quantities.

Sulphur compounds are detectable in the neighbourhood of active volcanoes and in towns and cities where much coal is burned. Thus at Lille, Ladureau found 1.8 c.c. of sulphur dioxide in 1 cubic metre of air, which corresponds to 0.018 parts per 10,000 of air. Sulphuretted hydrogen is present in traces in coal-gas, and in the neighbourhood of decaying organic matter containing sulphur. It would appear that as much as 2.0 parts of this gas per 10,000 of air have no deleterious action upon the system, even if breathed for protracted periods. All of these sulphur compounds are rapidly oxidised to sulphuric acid, which may or may not be neutralised by the ammonia of the air yielding ammonium sulphate. Warington found the equivalent of 17.26 lb. of sulphur trioxide fell annually on each acre of land at Rothamsted, and this may be taken as a fair average, for Miller gives 20.89 lb. for Sicily, and Gray 15.2 lb. as the mean for four and a half years of observation in New Zealand.