Chemistry and physics in relation to hygiene.

  • International Congress of Hygiene and Demography.
Date:
1892
Catalogue details

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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    sncli as magnesia and many others, are power£\;l fog-prodncers; no donbt their activity may in part l)e attiihnted to their l)eing good radiators of heat, and thus by becoming cooled, inducing condensation. Mr. Aitken also showed that the products of combustion, even when the combustion is perfect, are powerful fog producers,—a fact which has an important bearing on the production of town fogs. One other point in these experiments I cannot omit mentioning; it is the exceedingly minute amount of matter capable of inducing fog. In his first series of expei'iments, Mr. Aitken showed that of a grain of iron wire, however often it was heated, evolved on each heating sufficient dust to cause a, visible fog ; and afterwards, with still more delicate apparatus, that woVu of a grain of either iron or copper when treated in the same way gave a similar result; and from these last expei'iments he infers that even TooVoo g™'i of either wire, if only slightly heated, would yield sufficient nuclei to cause a visible amount of fog. It is of much importance and interest, seeing how small a (piantity of dust is required for the pro- duction of fog, to know that even this small amount may be filtered out of the air liy passing it through cotton wool, and that an air may thus be obtained in which a fog cannot be produced. Air. Aitken’s de.'5ci'iption of such an atmosphere is at first most alluring ; for he says that if there was no dust in the air there would be no fogs, no mists, and probably no rain ; but he goes on to state that when the atmosiihere became burdened with supersaturated vapour, it would convert everything on the surface of the earth into a condenser, every blade of grass and every branch of a tree would drip with moisture deposited by the passing air, our dresses would become wet and dripping, and umbrellas useless; but our miseries would not end here, for the inside of our houses would become wet, the v/alls a!id every object in the room would run down with moisture. I think if we picture to ourselves this state of things we may be thankful that there is dust—and fog. Dust in its finer forms is invisible to us, but as its delicate particles become loaded with moisture, it becomes a fine mist, which is dense if tbe number of particles are many. But if the dust particles are fewer, the amount of aqueous vapour remaining the same, each particle will luu'e a larger amount of condensed moisture to cai'rv, and will give rise to a more coarse-grained fog; the moisture, or some of it, will be more feebly attached to its nuclei, producing then what is known as a wet fog; whereas at least one most inqjortant fact in the proiluction of a dry fog is the strong affinity between the nuclei and the condensed vapour. As most of you are no doubt awai'e, Mr. Aitken has invented a most ingenious method for counting the number of dust particles in air, mid has obtained most interesting and valualile results. I ean only mention here that some of these results deal with the clearness of air in relation to the number of dust particles present; and other results show how little effect rain has in diminishing the amount of the finer dust in air. Towns obviously supply dust of all kinds, and therefore offer every inducement for fogs to form, and some at least of these dust particles will be capable of causing the con- densation of moisture even from an atmosphere which is not saturated with aqueous vapour. This condensation of moisture is a very complete process for removing all kinds of impurities from the air. Floating