This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
All natural waters are more or less charged with solid mineral matter, and, indeed, a certain proportion of it seems to be necessary to health. On the other hand, if the amount of solid matter dissolved be excessive, the water is unpalatable and unwholesome.
A water that contains an excess of calcium salts is said .to be hard, while one not so rich is said to be soft. If the hardness be due to the presence of the carbonates of the alkaline earths held in solution by an excess of carbonic acid, and hence existing in solution as bicarbonates, it is temporary, for upon the application of heat the carbon dioxide gas is driven off, and the carbonates being no longer soluble, are precipitated.
A permanently hard water owes its hardness to the presence of the sulphates of the alkaline earths, and these remain in solution (simple aqueous solution). An easy method of determining the hardness of a water is with a soap solution. Dissolve a little good, white and dry castile soap in some alcohol and add a few drops of the solution to the water to be tested. If it assumes a milky appearance the water is hard, if it 1s not changed or changes but slightly, it is soft.