[Water supply of towns].

Date:
[1856]
Catalogue details

Licence: Public Domain Mark

Credit: [Water supply of towns]. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

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    The earthy matters of soils and rocks are not tlie only solid impurities that may occui- m water. Tn some cii-cumstances iron and lead may I.e found in suspension. Vegetable and animal matter, not to speak of live insects, may likewise be present in solid specks. The drainage of a town IS a compound of salts dissolved, and of solid matters suspended, m tlie ciu-rent, both classes of substances bemg useful as manure. The methods of clearing water from mud and matters in suspension are subsidence and filtration. The method of subsidence proceeds on the supposition that the soHd earthy particles are heavier than water, and will gradually smk to the bottom when the mass is perfectly stiU. For tlie grosser kmds of mud this is quite true, but the fine, impalpable drab- coloured clay is not displaced in this way. The other method, filtration we shall describe m a subsequent section. ' To separate clay powder from water, the practice has long been resorted to m India and China of puttmg m a piece of alum, which seems to act by the property it has of curdling some organic substances, and of causing others to adhere as dyes to such solid matters as may happen to be in the water. This is the oldest known device for purifying water by anythmg approachmg to a chemical process. Dissolved Impurities. The impregnations of salts and mineral mgredients gathered up from the earth's sm-face are very numerous. The following statement by Professor Clark of Aberdeen, given in evidence before the Health of Towns Commis- sion, contains an enumeration of the prmcipal impiu-ities of this class : Bemg asked, ' You have stated that the water usually contams saUne impurities; of what do those consist ?' He stated in reply—' The most material are earthy salts; that is, salts of lime and salts of magnesia. I call the earthy salts the most material, because it is the presence of earthy salts that gives rise to hardness. There is also usually present common salt, and sometunes bicarbonates of soda and potash; but none of these affects the hardness of water. The most important portion of the earthy salts may be reckoned the bicarbonate of lime. The whole salts present, whether earthy or not, may be distinguished into two parts— according as they are neutral to test paper, or alkaline to test paper—the neutral portion, and the unneutralised portion. The unneutralised portion consists entirely of bicarbonates, those of lime and magnesia, which arc the earthy bicarbonates, and in some waters those of potash and soda, wliich are the alkaline bicarbonates. The neutral portion consists of the neutral salts of earths and alkalies—such as gy^^sum and common salt. Salts of iron occur also occasionally in waters that are in use. Such salts impart an inky taste to the water, and they give a yellowish tint to linen that is washed by the water containing them. They, too, produce hardness.' Of the salts thus enumerated the foremost are salts of lime. The bicai'bonate of lime is the chief example of what Professor Clark terms the unneutralised division ; this bicarbonate is derived from chalk or limestone, and may be considered as chalk (carbonate of lime) with a double dose of carbonic acid, which second dose changes the chalk from an insoluble salt into a soluble one. The waters having bicarbonate of lime for their chief impurity are familiarly spoken of as the ' chalk waters.' G