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sometimes an amount of free carbonic acid. A hard water, commonly considered, is one that destroys soap in washing, while a soft water is one that does not destroy soap. A soft water may derive its properties from an absence of earthy salts, or it may have become soft by reason of the presence of certain alkaline salts in the water, notably the salts of soda and potash.

Hard waters may be divided into two classes, those which are permanently hard and those which are temporarily hard. It is often found that a single sample of naturally hard water partakes of both these properties. A water which is said to be temporarily hard becomes soft by boiling, as the hardness is due to salts of magnesia or lime dissolved in the water by the agency of carbonic acid, or due to the presence of this gas in a free state in the water. Under either circumstances, the effect of boiling the water a sufficiently long time is to drive off the carbonic acid gas. and a natural softening of the water takes place from the absence of this gas, and the earthy salts that have been held in solution by it in the water.

Water that is permanently hard derives this property from the presence of the same salts as render water temporarily hard, but instead of being combined with carbonic acid, they are combined with sulphuric acid, and to soften water that is permanently hard requires very different conditions than is the case with waters that are only temporarily hard.

The qualities of a good drinking water have been described as :

ist. Freedom from vegetable and animal matter.
2nd. Pure aëration.
3rd. Softness.

4th. Freedom from earthy mineral or other foreign matter.

5th. Coolness and delivery at the minimum temperature. 6th. Lucidity or clearness. 7th. Absence of taste and smell. Although many authorities insist that for the sake of health a soft water is beneficial, on the other hand, there are those who contend that there is no evidence whatever to show that even a hard water had any influence upon health. It is clear, so far as the health statistics of this country are concerned, that if anything the results come in favour of persons inhabiting districts having hard waters. On the other hand, it has been thought that particular diseases which affect particular localities, such as cretinism and goitre, are due to certain salts of magnesia, which have been found in the waters of the district. Some waters which are of remarkable softness, in which the softness is due to the presence of certain alkaline salts, especially those of soda, may be quite unfit for drinking purposes. A type of this water is found in the well supplying the Trafalgar Square fountains, as it is stated by some authorities that the large amount of soda contained in it, if taken habitually, acts medicinally upon the kidneys. It is also unfit for washing, as the water is liable to destroy certain colours, and stains glass. It is said to be unfit for bathing, as the soda combines with the oily matter of the skin, producing a roughness and liability of chapping. Water, however, which is naturally soft, or which has been softened by means of a process like Dr. Clark's, which does not add any new element to the water, has great advantages for many purposes; it prevents incrustation of steam boilers and household utensils ; it results in a saving of fuel, less wear and tear in washing linen, and in the labour of cleansing ; it saves soap in all washing and cleansing operations, the water cleans better, and gives a better colour to linen; and it is also stated to lead to greater economy in tea-making and brewing, but whether this is correct or not is very doubtful, as water used for such purposes is always boiled, and when used in that state should be as soft as softened water.

The processes which have been used both in ancient and modern times for softening water may be comprised under the heads of Boiling, Chemical Processes, Distilling, Exposure, Freezing

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Softening by Boiling.Pliny, in his 'Natural History,' states that water that hath been once sodden, that is boiled, is far better than that which is raw. There is little doubt that the boiling of water, both as a means of reducing its hardness, and also to effect its purification, was very extensively practised in ancient times, and the practice of boiling water is still carried on amongst some of the older nations, such as China and parts of India, with the greatest possible advantage, both from an economical and from a sanitary point of view. The Emperor Nero (A.D. 50 to 68), both boiled water for drinking purposes, and subsequently cooled it in glass vessels to which snow was externally applied. The effect of boiling water is to liberate the carbonic acid which holds certain alkaline salts in solution, and on the liberation of the acid these salts are precipitated and forms the coating which furs our kettles, accumulates in our boilers, blocks the circulating pipes of our waterheating apparatus, and is often a source of danger, and always of expense. The effect of boiling water, in order to to soften it, can only be secured when this operation is sufficiently prolonged. The Commissioners appointed to inquire into the chemical properties of the water of the metropolis in 1851, made some experiments on the effects of boiling an artificially prepared hard water containing 13.5 grains of carbonate of lime per gallon, and it was found to decrease in hardness from 13.5 to 11'2 degrees by being heated to the boiling point; after boiling for five minutes, it was reduced to 6-3 degrees, for fifteen minutes to 44 degrees, for thirty minutes to 2:6 degrees, and for one hour to 2:4 degrees, so that the softening effect does not take place at once, but a prolonged boiling is required in order to produce the greatest degree of softening. In order to get rid of the temporary hardness of water, sharp boiling for not less than twenty minutes is requisite; but boiling water does not remove the hardness occasioned by the salts which are neutral; in fact the permanent hardness of the water is increased by boiling, as all the water evaporated leaves a concentration of the neutral salts in the remaining water.

It has also been shown that the alkalinity of water is more after boiling than when softening has been produced by an alkaline salt such as lime, but both have the effect of reducing the hardness to about the same degree. This increase of alkalinity after boiling is attributed to the concentration of the neutral salts consequent on the loss by evaporation. The temporary salts held in solution by water are precipitated by boiling, and it is these precipitated salts which cause the furring of kettles, hot-water boilers, steam boilers, and hot-water pipes, and have led to the adoption in certain cases of means either for retaining the salts in solution in the water or of preventing their deposit in steam boilers, but as a rule with only partial success.

Chemical Processes of Softening Water.-In a paper read before the Literary and Philosophical Society of Manchester in 1781, by Thomas Henry, F.R.S., a description is given of a mode of preserving sea-water by means of quicklime, in which the author pointed out that the earthy base of magnesia was precipitated in sea-water by lime, and its place taken by a calcareous salt. He also referred in this paper to the well-known action of quicklime on common water as a preservative. The effect of the lime, doubtless, upon the sea and fresh water was to induce abundant precipitation, which dragged down with it certain organic impurities, and as a consequence the water remained free from putrefactive influence afterwards, as was clearly shown in the course of his experiments.

The first patent for purifying water by chemical agency in this country was taken out in 1838, for precipitating by muriate of zinc and salts of soda, the latter salts precipitating the zinc from the water, leaving the water in a purified state.

In 1841, Dr. Thomas Clark, of Aberdeen, took out a patent for his well-known and beautiful process for softening water, and which has, more or less, been the basis of all other patented processes of this description which have been adopted since that period. Dr. Clark thus describes his process, in a paper published in the Fournal of the Society of Arts of the 16th May, 1856:

“In order to explain how the invention operates, it will be necessary to glance at the chemical composition and some of the chemical properties of chalk, for while chalk makes up the great bulk of the matter to be separated, chalk also contains the ingredient that brings about the separation. The invention is a chemical one for expelling chalk by chalk. Chalk, then, consists, for every one pound of sixteen ounces, of lime nine ounces, carbonic acid seven ounces.

“ The nine ounces of lime may be obtained apart by burning the chalk, as in a lime-kiln. The nine ounces of burnt lime may be dissolved in any quantity of water not less than 40 gallons. The solution would be called limewater. During the burning of the chalk to convert it into lime, the seven ounces of carbonic acid are driven off. This acid, when uncombined, is naturally volatile and mild; it is the same substance that forms what has been called soda-water when dissolved in water under pressure.

“Now, so very sparingly soluble in water is chalk by itself, that probably upwards of 5000 gallons would be necessary to dissolve one pound of sixteen ounces ; but by combining one pound of chalk in water with seven ounces additional of carbonic acid—that is to say, as much more carbonic acid as the chalk itself contains—the chalk becomes readily soluble in water, and when so dissolved is called bicarbonate of lime. If the quantity of water containing the one pound of chalk with seven ounces additional of carbonic acid were 400 gallons, the solution would be a water of the same hardness as well-water from the chalk strata, and not sensibly different in other respects.

"Thus it appears that one pound of chalk, scarcely soluble at all in water, may be rendered soluble in it by either of two distinct chemical changes; soluble by being deprived entirely of its carbonic acid when it was capable of changing water into lime-water, and soluble by comVOL. VIII.-H. C.

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