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.
Waters are frequently spoken of as "soft" and "hard"; those which readily give a lather with soap being classed under the former category, and those which do not, under the latter. This difference is accounted for by the fact that hard water contains lime and magnesia salts, which destroy the detergent action of the soap, thereby themselves undergoing decomposition and preventing any lathering effect until a sufficient quantity has been added to completely decompose them. Should a hard water contain only calcium and magnesium carbonates, it can be softened either by boiling or adding lime or alum, and is consequently known as being "temporarily" hard; if sulphates of those elements are present it cannot be softened by either of these means, and is then styled "permanently" hard. These compounds are often a great annoyance to users of steam, as they form that familiar and objectionable deposit known as "boiler incrustration" on the interior of boilers.