The subject of the Water supply to the Metropolis and other large towns is one of the highest importance to the well-being of the community at large, in whatever point of view it may be regarded—whether as affecting the comfort, the health, or the pocket of the consumer, its influence can scarcely be overrated. To enter, however, into this matter, affecting, as it does, so many varied and conflicting interests, would be to pass beyond the limits set to this series of papers; what remains for us to do is to avail ourselves of the vast amount of scientific knowledge which has been recently brought to bear upon the question, and to cull from it such portions as bear directly upon _Domestic Manipulation_. The quantity of water for domestic purposes depends mainly upon its degree of hardness or softness; and this in its turn depends almost entirely upon the quantity of lime dissolved in some form or other in the water. In speaking of the quality of water, the term “degree of hardness” is much used; thus we say that the water of the Thames is of fourteen degrees of hardness, that of the Hampstead springs about ten degrees, &c. &c. In these and most other cases the hardness is owing to a certain amount of chalk (carbonate of lime) dissolved, and the degrees of hardness correspond with the number of grains contained in a gallon of water. Thus the Thames water, of fourteen degrees of hardness, has in each gallon fourteen grains of chalk, and the Hampstead ten grains. It is found, upon experiment, that one gallon (weighing 70,000 grains) of _pure_ water will not dissolve more than two grains of chalk, and so acquire two degrees of hardness; and that whenever more is contained in water, the excess is always owing to the presence of carbonic acid gas, which enables it to dissolve a much larger quantity. The practical part of our subject depends on this fact; for if by any means we can get rid of the carbonic acid, the dissolved chalk is necessarily precipitated, and the hard water, unfit for culinary and domestic purposes, becomes soft, and well adapted to both these uses. Carbonic acid is in part expelled from water by heating it to the boiling point: a still larger quantity is got rid of after boiling for some few minutes, and nearly every trace disappears at the end of half an hour; and just in proportion as the carbonic acid gas is expelled, so does the chalk fall, rendering the water in the first instance turbid, and becoming deposited on the interior surface of kettles, where it forms the well-known rock or _fur_.