The theory and practice of hygiene (Notter and Firth) revised and largely re-written / by R.H. Firth.
- James Lane-Notter
- Date:
- 1908
Licence: In copyright
Credit: The theory and practice of hygiene (Notter and Firth) revised and largely re-written / by R.H. Firth. Source: Wellcome Collection.
Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.
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![it is therefore good for washing or cooking purposes, although it is- less palatable than otlier kinds for drinking. Ice and Snow Water.—In freezing, water becomes purer, losing a large portion of its sahne contents. Even calcium carbonate and sulphate are partially got rid of. Tlie air is at the same time expelled. Ice-water may thus be tolerably pure, but heavy and non-aerated. Snow-water contains the salts of rain-water, Avith the exception of rather less ammonia. The amounts of carbonic acid and air are very small. An analysis of the ice supplies made by the State Board of Health of Massachusetts showed that, taking an average of all the samples examined, the organic impurities of snow ice, as measured by the ammonias, amounted to 69 per cent, of those of the water; that the organic impurities of all the ice, except the snow ice, amounted to 12 per cent., and that clear ice gave only 6 per cent, of the impurities of the waters. The salt the waters con- tained was nearly all removed by the act of freezing. There were 81 per cent, as many bacteria in the snow ice as in the waters; 10 per cent, as many in all other ice, and 2 per cent, as many in the clear ice as in the waters. It is therefore much safer to use for drmk- ing water, and for placing in contact with food, that portion of the ice which is clear. Upland Surface Water.—Tliis, of the various kinds of water, most nearly approaches rain-water. The dissolved sohd matters are larger, their amount and nature depending on the kind of soil over which the Avater rests, and consequently it is usual to subdivide this class according to the geological character of the ground from which the upland surface water is obtained. These waters do not contain any considerable amount of dissolved matters, except they are derived from calcareous strata; the organic substances present are chiefly of vegetable and not of animal origin. There is also an absence of ammonia, nitrates and nitrites beyond that in which they occur in ram-water. The cliloriae is also low and the water soft. These upland surface waters are not only valued because of their safety for drinking, but also on account of their fitness for trade purposes. Spring and Well Water.—The rain falling on the ground partly evapor- ates, partly runs ofi, and partly sinks in. The relative amounts vary with the configuration and density of the ground, and with the circumstances imped- ing or favouring evaporation, such as temperature, movement of air, &c. In the magnesian limestone districts, about 20 per cent, penetrates; in the New Eed Sandstone (Triassic), 25 per cent.; in the chalk, 42 ; in the loose Tertiary sand, 90 to 96. Evans, from twenty-nine years' observations in the chalk at Hemel-Hempstead, gives the winter average at 608 per cent., the summer at 15*5 per cent., and the Avhole year 37*5 per cent. Penetrating into the ground, the water absorbs a large proportion of _ car- bonic acid from the air in tlie interstices of the soil, which is much richer (250 times) in CO2 than the air above. It then passes more or less deeply into the earth, and dissolves everything it meets Avith which can be taken up in the time, at the temperature, and by the aid of carbonic acid. In some sandy soils there is a deficiency of CO.,, and then the water is also wanting in this gas, and is not fresh and sparkhng. The chemicarchangcs and decompositions Avhich occur in tlie soil by the action of CO2, and Avhich are ])robably infliienced by difiusion, and perhaps by pressure, as Avell as by temperature, are extremely curious, but cannot be entered upon here. The most common and simple are the solution of calcium carbonate, and the decomposition of calcium and sodium silicate by carbonic acid, or alkaUne carbonates. Salts of ammonia, also, wlien they](https://iiif.wellcomecollection.org/image/b21508379_0030.jp2/full/800%2C/0/default.jpg)
