LIME HELD IN SOLUTION BY FREE CARBONIC ACID.
Sometimes a large quantity of bicarbonate of lime is held in solution by free carbonic acid, which escapes as soon as the water is boiled, leaving the lime to subside in the form of mud. Water of this kind may appear very hard, yet not be very objectionable for boiler use; since the deposit can be blown out if the blow-off cock is used freely. To test this, boil a pint of water for two or three minutes in a perfectly clean dish, then pass it through a filter paper that has been previously dried and weighed. When the operation is finished, dry and weigh the filter paper again; and the added weight will indicate the amount of solid matter held in solution by free carbonic acid gas.