Hygiene and public health / by Louis C. Parkes and Henry R. Kenwood.

  • Parkes, Louis Coltman, 1857-1942
Date:
1917
Catalogue details

Licence: Public Domain Mark

Credit: Hygiene and public health / by Louis C. Parkes and Henry R. Kenwood. Source: Wellcome Collection.

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    account of its softness—that is to say, its freedom from the salts of lime or magnesia in solution. When these salts are dissolved in a water they render it hard. Hardness is usually reckoned as equivalent to so many grains of chalk (or carbonate of calcium) per gallon of water. A water containing more than io grains of chalk or its equivalent in other salts (sulphate of lime or magnesia, carbonate of magnesia, etc.) to the gallon, is said to be hard. Hardness due to the presence of carbonate of calcium, held in solution by carbonic acid, is said to be temporary; for when the water boils, the carbonic acid is driven off, and the chalk, no longer able to remain in solution, is precipitated. It is this deposit of chalk which causes the fur on the bottom and sides of boilers and kettles. When meat or vegetables are cooked by boiling in hard water, a certain amount of the hard material is deposited on their surfaces, which either hinders the proper penetration of the heat into the interior, or prevents solution of the soluble materials when this is desired. The fur lining is also a non-conducting material, and impedes the passage of heat from the fire to the contents of the boiler or kettle, thus causing a waste of fuel. This fur lining is one of the causes of the boiler explosions from which loss of life not infrequently results. To reduce the possibility of such explosions the follow¬ ing precautions are desirable:—The boilers should be of wrought iron, properly tested; they should be periodically inspected and cleaned; pipes connected with them should not be carried up external walls where they may be affected by frost, and the cisterns should also be in well-protected positions; the safety valve should be accessible, easily adjusted, and sensitive to variations of pressure. Great waste of soap, too, is caused by the use of hard water in washing. When the water is hard, the lime or magnesia combines with the fatty acid of the soap, forming a curdy pre¬ cipitate; and all the lime or magnesia of the water must be so combined before a lather can be formed. Consequently a certain amount of soap is wasted. One grain of chalk wastes about eight grains of soap. The hardness of rain-water is generally less than half a degree; that is to say, it is equivalent to less than half a grain of chalk to the gallon of water; hence its value for domestic purposes. Rain-water should never be allowed to run to waste
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