Water for domestic purposes is obtained from the following sources - rain, springs, wells, streams and rivers. In mountainous countries it is also obtained by melting snow.
Waters may be divided into hard and soft. If you have ever tried to wash with soap in sea-water you will have noticed that the soap will not lather, but as soon as it is dissolved it floats to the top as a greasy scum. It is a typically hard water. When washing in rainwater the soap lathers beautifully, therefore rain-water is a soft water. The degree of hardness of water is estimated by the amount of soap which is required to make a fine lather.
The hardness of water is of two kinds, temporary and permanent. Temporary hardness is removed by boiling and is due to the presence of bicarbonate of lime and magnesia. When water containing these ingredients is boiled, the soluble bicarbonates are changed into the insoluble carbonates, which deposit as the "fur" in kettles and boilers.
Permanent hardness of water is not removed by boiling. It is due to the presence of either sulphates and chlorides of lime (which are precipitated by adding washing soda to the water) or of chloride of soda (common salt), in which case nothing short of distillation will render the water soft. Such water is only found near the sea.
For drinking purposes a hard is preferable to a soft water, because it is more sparkling; soft water is very flat and unpalatable. If the water has to flow through lead pipes it will take up less lead if it be hard than if it is soft.
As I have said, boiling destroys the hardness of water, and in consequence boiled water has a very flat taste. In times of epidemics it is better to boil all water used for drinking, however much it may spoil its taste; but in cities it is usually unnecessary to boil water used for drinking when no diseases are epidemic.
If you live in the country and take your water supply from wells or springs, always boil it, as you cannot be certain of its purity. This is really important - boil your water in the country, have nothing to do with filters.
For washing purposes a soft water is most desirable, as it very materially saves the soap. A large factory in the north used formerly to use a hard water, but for some years it has been supplied with soft water. I forget what was the exact number or pounds saved per annum in soap since the change, but I know that it ran into thousands.
Rainwater is a very soft water, and though pre-eminently suitable for washing it is totally unsuitable for drinking.
Do not think that because a water is sparkling, clear and has a pleasant taste that it is necessarily free from disease germs; a minute admixture of sewage with drinking water is said by some to improve the flavour! Whether this is true or not I do not know, as I hope that I have never partaken of such water; but sewage certainly does not give any unpleasant odour or taste if mixed in minute quantities with water.
As regards the cistern I will only say that it should be made of earthenware if possible. If this is impracticable, owing to its enormous weight or to any other reason, galvanised iron is the best substitute, not lead or zinc, and above all not wood.
The cistern must have a cover. London cisterns usually promptly lose their lids and become the watery graves of defunct cats, not to mention mice, birds and other such trifles.