Chemistry and physics in relation to hygiene.
- International Congress of Hygiene and Demography
- Date:
- 1892
Licence: Public Domain Mark
Credit: Chemistry and physics in relation to hygiene. Source: Wellcome Collection.
Provider: This material has been provided by London School of Hygiene & Tropical Medicine Library & Archives Service. The original may be consulted at London School of Hygiene & Tropical Medicine Library & Archives Service.
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![Piirticles are free surfiiees, and, l)ecomiiig weighted by the luoisiure they condense, tend to sink ; even tlie gaseous impurities in the air will be dissolved and carried down by the moisture present. Experiment confirms this, for it has been proved how correctly the im])uritles of an air can be ascertained by determining tlie composition of dew, e-\'en if this be artificially and locally formed. It is of imjjortance to note that even the purely gaseous emanations from our towns cannot pass away when a fog exists, as is shown by the accumulation of carbonic acid which takes place during a fog. 'raking four volumes in 10,000 volumes as the normal amount of carbonic acid in London air, some years ago 1 found that it increased in the case of a dense fog to as much as 14'1 volumes, and often to double the normal amount, wliich must represent a very serious accumulation of the general imi)urities in the air. A fog in this way becomes a useful indicator of the relative purity of the atmosphere in which it forms. If pure aqueous vapour be condensed it gives a white mist—a country fog, a sea fog—and a white light seen thi’ough it is not converted into a red light; but in town fogs the whiteness of pure mist disappears and ))econies dark, in some cases almost black, in colour, the change l)eing produced by the foreign matters floating in the air. By far tlie most abundant colouring matters of our town fogs are the products generated by the imperfect combustion of coal, but in addition to these bodies, many others must obviously find their way into the air over a town ; especially will there be dust from the universal grinding and pounding going on in street-tratfic and manv mechanical operations, from the general disintegration of sul)stauces and the decomposition of perishable materials, all of which will add something to the air, and will become an integral part of the fog. However, although it is often said that a town fog is so dense, that it may be cut with a knife, still it is difficult to condense so much of it that it can b(‘ subjected to a searching chemical analysis. In 1885 by wa.shiug foggv air I was able to determine the amount of sulphates and chlorides present, and—as indicators of organic matter—the quantity of carlion and nitrogen in the fog. 'Lhe results showed strikingly how largelv the amounts of organic matter and of ammonia salts in tlie air varied with the weather. No case of dense fog occurred when the experiments were being made, but the mean of several experiments clearly showed that in foggy weather the amount of oi’ganic matter Avas double as much as existed in the air in merely dull Aveather, and that the amount of sulphates and chlorides increased under like conditions, but not to the same extent. Eog may, however, be made to gNe its own account of its constituents; for avc haA’e only to collect and analyse the deposit Avhich it leaves to learn Avhat its more steble constituents are. We have to thank the air analysis committee of the Manchester Field Naturalists’ Society for the most complete analysis of such a deposit Avhich has yet been made. 'Fhe deposit analysed occurred during the last fortnight in February of this year, 1891, and Avas obtained from the previously AAuxshed glass roof of the plant houses at Koav, and from Messrs. Veitch’s orchid houses at Chelsea. At Kcav 20 square yards of roof yielded 30 grammes of deposit. At Chelsea the same area *gave 40 grammes,](https://iiif.wellcomecollection.org/image/b28045403_0015.jp2/full/800%2C/0/default.jpg)
