After the required quantity of lime and carbonate of soda which is
necessary for a total precipitation has been figured out from the
analysis of the water, respectively verified by practical experiments
in the laboratory, the heated water in the reservoir is mixed with the
lime, in form of thin milk of lime, and stirred up; we have to add so
much lime, that slightly reddened litmus paper gives, after 1/4 minute's
contact with this mixture, an alkaline reaction, i.e., turns blue; now
the solution of carbonate of soda is added and again stirred well.

After twenty or thirty minutes (the hotter the water, the quicker the
precipitation) the precipitate has settled in large flocks at the
bottom, and the clear water is drawn off into the pure water reservoir.
The precipitating and settling of the impurities can also take place in
cold water; it will require, however, a pretty long time.

In order to avoid the weighing and slaking of the lime, which is
necessary for each precipitation, we use an open barrel, in which a
known quantity of slaked lime is mixed with three and a half or four
times its weight of water, and then diluted to a thin paste, so that one
kilogramme slaked lime is diluted to twenty-five liters milk of lime.

Example.--If we use for ten cubic meters water, one kilogramme lime,
or in one day (in twenty-four hours), 240 cubic meters 24 kg. lime, a
vessel four or five feet high and about 700 liters capacity, in which
daily 24 kg. lime with about 100 liters water are slaked and then
diluted to the mark 600, constantly stirring, 25 liters of this mixture
contain exactly 1 kg. slaked lime.

Before using, this milk of lime has to be stirred up and allowed to