settle for a few seconds; and then we draw off the required quantity of
milk of lime (in our case 25 liters) through a faucet about 8 inches
above the bottom, or we can dip it off with a pail. For the first
precipitate we always need the exact amount of milk of lime, which we
have figured out, or rather some more, but for the next precipitates we
do not want the whole quantity, but always less, as that part of the
lime, which does not settle with the precipitate, will be good for use
in further precipitations. It is therefore important to control the
addition of milk of lime by the use of litmus paper. If we do not add
enough lime, it prevents the formation of the flocky precipitate, and,
besides, more carbonate of soda is used. By adding too much lime, we
also use more carbonate of soda in order to precipitate the excess of
lime. We can therefore add so much lime, that there is only a very small
excess of hydrous lime in the water, and that after well stirring, a red
litmus paper being placed in the water for twenty seconds, appears only
slightly blue. After a short time of practice, an attentive person can
always get the exact amount of lime which ought to be added. On adding
the milk of lime, we have to dissolve the required amount of pure
carbonate of soda in an iron kettle, in about six or eight parts hot
water with the assistance of steam; add this to the other liquid in the
precipitating reservoirs and stir up well. The water will get clear
after twenty-five or thirty minutes, and is then drawn off into the pure
water reservoir.


EXAMINATION OF WATER WHICH HAS BEEN PURIFIED BY MEANS OF MILK OF LIME
AND CARBONATE OF SODA.

In order to be convinced that the purification of the water has been
properly conducted, we try the water in the following manner. Take a