This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
Another method of "softening" or purifying water consists in removing its carbonic acid gas, whereby the carbonates of lime, iron and magnesia are precipitated, together with silica and organic matters. This is effected by the addition of a proper proportion of lime-water or slacked lime, giving time for subsidence and drawing off the clear water and filtering it.
Prepare limewater as follows: Slack one pound of lime by the gradual addition of some water, until it decomposes into a powder-slacked lime. Then add one gallon of distilled or boiled water, put the whole in a stoppered vessel and shake well. When the excess of lime shall have subsided, syphon off the clear solution, which is then ready for use. Add of this limewater to the water to be treated in tank or cistern enough to give it a slight alkaline test, and then sufficient water to cause this alkaline test to disappear. After 12 hours syphon off or filter.
The explanation of the process is as follows: Chalk is practically insoluble in pure water; but it is soluble in all ordinary water, because the water contains carbonic acid. On adding lime it unites chemically with the carbonic acid and forms a little more chalk. The chalk formed and the chalk originally present having now no carbonic acid to hold it in solution, is thrown out of solution and is slowly deposited. The other practicable method of recovering mineral substances from water is by distillation. The addition of limewater also changes the bicarbonate of lime in solution to the carbonate of lime, which is precipitated and filtered out.