and also the carbonates of iron and magnesium. If we examine a spring water, we shall find that its dissolved impurities can be divided into two classes: for instance, taking the Kent water supplied at Greenwich, and obtained from deep wells in the chalk, we find its saline constituents in grains per gallon are:
| Calcic carbonate | 6·30 |
| Calcic sulphate | 5·37 |
| Magnesic sulphate | 0·93 |
| Magnesia nitrate | 1·20 |
| Sodic chloride | 2·64 |
| Sodic nitrate | 1·21 |
| Silica, alumina, etc. | 0·97 |
And of these the calcic sulphate, magnesium, and sodium salts are dissolved by the solvent action of the water in the same way that sugar would be, while the chief impurity, calcic carbonate, is scarcely at all soluble in the water itself, 16,000 parts of pure water only dissolving one part of the carbonate, but is readily soluble in the carbonic acid, in the water which converts it into soluble calcic bicarbonate.
In the household, waters are roughly classified as hard or soft waters, and the property of hardness manifests itself, as a rule, to the householder by its action upon soap, and also by the amount of "fur" which it causes in the kettle, these actions being due to calcic bicarbonate, calcic sulphate, and the magnesium salts present in it, all of which act upon soap and cause it to curd instead of forming a lather by converting the soluble sodic oleate and stearate into insoluble lime salts, while the bicarbonate by decomposing and depositing "chalk" causes the fur.
A more careful examination, however, reveals the fact that this property of hardness owes its origin to two different causes; for if we boil water until all the bicarbonate is broken up and the calcic carbonate deposited, the clear water left behind it is yet hard, though to a far less extent, and will still decompose a certain proportion of soap. The hardness which can be got rid of by boiling is due to bicarbonate of lime, and sometimes also bicarbonate of magnesia, and is called "temporary hardness," while the hardness left after boiling the water is due to calcic sulphate and the soluble magnesium sulphate, chloride and nitrate, and is called "permanent hardness."
The relative hardness of waters is estimated by the amount of soap they will destroy—i. e., convert from the form of soluble sodic oleate and stearate into the condition of insoluble oleates and stearates of lime; and one grain of calcic carbonate, or its equivalent in sulphate or salts of magnesia, dissolved in a gallon of water, is said to equal 1° of hardness.
