The salts of lime and magnesia communicate to water the quality termed _hardness_, a property which every one understands, but which it would be very difficult to describe. By far the most common giver of hardness is chalk, or, as chemists term it, carbonate of lime; a substance not soluble in pure water, but readily so in water containing carbonic acid. Rain water always contains this acid, and is, therefore, a solvent for the chalk disseminated in the different geological formations through which it percolates. Gypsum, familiarly known as plaster of Paris, and termed sulphate of lime by chemists, is also extensively diffused in rocks, and being itself soluble in water, becomes a very common hardening ingredient, though not of such frequent occurrence as chalk. Any earthy salt, such as chalk or gypsum, decomposes soap, and prevents its action as a detergent. Soap consists of an oily acid combined generally with soda. Now, when this is added to water containing lime, that earth unites with the oily acid, forming an insoluble soap, of no use as a detergent; this insoluble lime-soap is the curd which appears in hard water during washing with soap. Hard water is of no use as a cleanser, until all the lime has been removed by uniting with the oily acid of the soap. Every hundred gallons of Thames water destroy in this way thirty ounces of soap before becoming a detergent. But as this is an enormous waste, the dwellers in towns, supplied with hard water, resort to other methods of washing, so as to economize soap. If our readers in London observe their habits in washing, they will perceive that the principal quantity of the water is used by them not as a cleanser, but merely for the purposes of rinsing off the very sparing amount employed for detergent purposes. In London, we do not wash ourselves _in_ but _out_ of the basin. A small quantity of water is taken on the hands and saturated with soap so as to form a lather; the ablution is now made with this quantity, and the water in the basin is only used to rinse it off. The process of washing with soft water is entirely different, the whole quantity being applied as a detergent. To illustrate this difference an experiment may be made, by washing the hands alternately in rain and then in hard water, such as that supplied to London; and the value of the soft water for the purposes of washing will be at once recognized. Even without soap, the soft water moistens the hand, while hard water flows off, just as if the skin had been smeared with oil. Now, although the soap may be economized in personal ablution by the uncomfortable method here described, it is impossible to obtain this economy in the washing of linen. In this case, the whole of the water must be saturated with soap before it is available. Soda is, to a certain extent, substituted with a view to economy, as much as £30,000 worth of soda being annually used in the metropolis to compensate for