The Romance of Modern Chemistry

NATURAL WATERS at once give a lather with soap. Instead of giving an immediate lather, a curd is formed—evidence that the sulphate of lime still remaining in the water is being removed or precipitated by the soap. Only after this removal is complete will the soap form a lather. The lime in a hard water may be removed also by the addition of sodium carbonate—washing-soda, as it is commonly called—or of lime water. If the water is to be obtained free from the deposit of chalk which both these substances produce, it must be allowed to stand in tanks, and then run off after the precipitate has settled to the bottom. Occasionally the impurities in a water are of an organic nature, and these may be such as to render the water unsafe for drinking purposes. This organic matter may come from decomposing vegetable substances, or it may be of animal origin, and come from sewage or surface drainage. Sometimes our senses of taste and smell will warn us of this, but in the last resort we must depend on a chemical and bacteriological examination of the water. Such an examination will reveal, in the case of a polluted water, an unduly high amount of nitrogenous compounds, and possibly also large numbers of disease germs. To make the water safe, it must either be filtered through sand or unglazed porcelain, or it must be sterilised by boiling. All germs seem to find exposure to boiling water a somewhat trying experience, and few survive the ordeal. This will be clear from a special example which has been put on record. • A particular water of bad quality was found to contain 460,000 germs per cubic centimetre. Exposure to a temperature of 194° Fahrenheit for ten minutes reduced the number to twenty-six, and even these hardy individuals 103