The health of the city / by Hollis Godfrey.

  • Godfrey, Hollis.
Date:
1910
Catalogue details

Licence: In copyright

Credit: The health of the city / by Hollis Godfrey. 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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    vice of sand, which acts in a most remarkable way as a support for a true bacterial filter. As the affluent passes through the upper layers, the sand stops the coarser materials left in the liquid and held in suspension there. Soon there forms above the original surface a filter composed of the smaller sediments, a layer so fine that even the infinitely small micro-organ- isms cannot pass. Many of them are probably held by the sheer adhesion of the sand. Add this adhesion to the filtering powers of the sediment layer, and you have erected a strong barrier. Here is a fortress placed across the pathway of the invading germ, a barrier so effectual that water from sources polluted with disease-germs has been safely furnished to thousands after such filtration. The sediment-filter is, of course, constantly increasing in thickness, and as it increases, more and more pressure is necessary to drive water through the interstices. When the point is reached where the pressure required to force the water through is too great to be practica- ble, the surface of the filter is scraped. Since during this scraping the filter has to be out of commission, filter-plants are generally built up from a series of small filters, in order that