Dissolved Mineral Matter.—Spring-water, which is generally clear and sparkling, holding no solid matter in suspension, is ?seldom without a large amount of dissolved mineral matter, sometimes as much as 2 parts in 1000, commonly from 1 in 1000 to 1 in 0,000. River and surface water also contains more or less dissolved minerals (see MINERAL WATERS). The great bulk of the solid matter held in solution in ordinary waters consists of the salts of soda, potash, lime, and magnesia. The most material are the salts of lime and magnesia, as they are the causes of what is called "hardness" in water, which we shall speak of more particularly afterward. The most important salt of lime is the soluble hydric carbonate formerly known as bicarbonate, which is derived from chalk or limestone. Chalk or limestone is a carbonate of lime (otherwise cattle car is, a compound of lime with one equivalent of carbonic acid (carbonic dioxide)—and is almost insoluble in water; but when water containing an excess of car bonic acid—as is the case with spring-water especially—passes over limestone, it gives the carbonate a double dose of carbonic acid, and converts it into bicarbonate, which is soluble. The waters having bicarbonate of lime for their chief impurity are familiarly spoken of as the chalk-waters. The other salt of lime often present in water is the phate or gypsum. The important distinction between the bicarbonate and the sulphate lies in the fact, that the first, the bicarbonate, may be in great part precipitated, or thrown down iu a solid form, by boiling, which drives off the solvent carbonic acid; whereas the second, the sulphate, cannot be so precipitated. The chief effect of the boiling takes place in the first five minutes.
Apart from its hardness, it has been made a question whether water containing salts of lime is injurious or not to the human constitution. Dr. Lankester holds that there is evidence to prove that carbonate of lime in large quantity is positively injurious; and most physiologists are agreed that pure water is the best for securing the health of ani mals and man.
With regard to magnesia, it salts are well known to act as powerful medicines when taken in large doses, and it may be presumed are not alto,gether without effect in the small quantities existing in ordinary magnesian waters. A foreign physician has lately made the observation, that magnesia is the characteristic ingredients of water in the dis tricts where the diseases called cretinism and gaiter abound.—Of salts of soda and potash, the principal is common salt, or the chloride of sodium. Sodic sulphate (Glauber's salt) occurs along with the chloride in the salt-springs of watering-places as well as in the sea waters. None of all these salts have any effect on the hardness. In the case of sea water, which is very hard, the effect is not due to common salt, but to the lime and magnesian salts dissolved in it; were it not for these, sea-water would be perfectly suit able for washing, although not for drinking.—Salts of iron in considerable quantity !Mike what are called technically named beater waters, which belong to the medici nal class. When the iron exists in the spring as carbonate, which is, the most usual
case, on exposure to the air it is changed into the peroxide, and falls down in the form of an ochery precipitate. Salts of iron give an inky taste to the water, and a yellowish tint to linen washed in it.
Hardness in quality of hardness in water is commonly recognized by the difficulty experienced in washing, and by the amount of soap necessary to form a lather. This quality is injurious also in the preparation of food; but its action is most universally felt in washing operations. It occasions the chapping of the skin, an enormous waste of soap, an extra labor, and a corresponding tear and wear of clothes. Every grain of chalk contained in water decomposes 10 grains of soap; anclthus the hardening matter contained in 100 gallons of water, such as is supplied to London, will destroy 35 oz. of, soap—that is, the first 35 oz. of soap added to this quantity of the water will disappear without forming any lather, or having any cleansing effect. Soap is a compound, formed of an alkali (soda or potash) joined to an oily acid. When a salt of lime, then, is present in the water, the lime decomposes the soap, and combines with the oily acid to form a lime-soap, which is insoluble, and has no detergent properties.
The most usual hardening ingredients are the salts of lime. Salts of magnesia and of iron are also hardening salts. Salts of soda and potash have no hardening effect. Dr. Clark, formerly professor of chemistry in Marischal college, Aberdeen, has devised a scale of hardness which is now universally employed in the chemical description of waters. The hardening effect that would be produced by one grain of chalk dissolved in in a gallon of water is one degree of hardness; in like manner, four grains per gallon would produce four degrees ten grains, ten degrees; and so on. The degrees are expressed in numbers—thus, 1°, 4°, 10°, 15°, are one, four, ten, fifteen degrees respectively. The degree of hardness of any particular water can be readily and exactly determined by Dr. Clark's soap test (q.v.).
Next to washing, the deleterious consequences of hardness are felt m various culi nary operations, especially in the furring of boilers and cooking utensils, and in the infusion of tea. It is a fact of universal experience that hard water requires more tea than soft water to make an infusion of the same strength, and also renders the infusion muddy. Subcarbonate of soda in crystals, by decomposing the earthy salts, improves the water; but if more is added than what will exactly decompose the earthy salts pres ent, it injures the fine flavor of the tea. It may be stated generally, that for the pur poses of washing and cooking, a water of less titan 6° is soft, but. above this point the hardness becomes objectionable. At 8°, the water is moderately hard; at 12°, it is very hard at 113°, the hardness is excessive; and much above this, it is intolerable.