and Tf'ate?' used for General Purposes. 
159 
On the top of the bottom is a perforated disc, on which the 
spongy iron is placed. An opening in the curved bottom is 
connected with an earthenware pipe, which passes up to the outer 
side of the vessel to slightly above the level of the spongy iron. 
Here the pipe communicates with another pipe, passing from the 
top of the outside of the spongy iron vessel down to the centre of 
the closed bottom. The latter pipe is open at the top and bottom. 
An alternate exposure to air and water causes the spongy iron 
to become oxidised, when it loses more or less its purifying^ 
power. A screw-tap at the lower end of the latter pipe serves 
to regulate the flow of the water through the spongy iron. The 
spongy iron vessel is placed inside the casing of an ordinary 
stoneware filter, Avith perforated bottom, beneath which there 
is a reservoir for the filtered water. On the top of the per- 
forated bottom is placed a layer, some four inches thick, of finely- 
divided marble or limestone, upon which the water containing 
some iron in solution flows from the screw-tap. The effect of 
the limestone is to remove completely every trace of iron from 
the water. 
BischofTs filter has a decided advantage over ordinary water- 
filters, which soon lose their purifying properties unless the 
filtering agent is renewed from time to time, whereas Bischoff's 
filter remains in good working order for years, without requiring- 
the renewal of the spongy iron. 
3. Purification of Water hy Precipitating Processes. — An elegant 
and useful process for softening hard water is that patented by 
the late Dr. Clark, of Aberdeen. Carbonate of lime is scarcely 
soluble in pure distilled water, a gallon being capable of holding- 
only about 2 grains in solution. In liver or spring-water, how- 
ever, carbonate of lime is held in solution by carbonic acid, or, 
in other words, exists as bicarbonate of lime. On boiling, the 
second equivalent of carbonic acid in the soluble bicarbonate 
is expelled, and neutral carbonate of lime precipitated. Profes- 
sor Clark proposed to soften hard water by taking advantage of 
the property of caustic lime to remove carbonic acid from water. 
Caustic lime, when added to hard water in sufficient quantity, 
neutralizes the carbonic acid, removes the solvent, and, be- 
coming at the same time carbonate of lime, is precipitated with 
that originally in solution. In falling down, the precipitated 
carbonate of lime carries with it a portion of the organic and 
colouring matter present in most waters, and thus Clark's pro- 
cess not only softens, but in a measure also deprives hard water 
of organic impurities. 
Clark's process is peculiarly well adapted to the softening of 
chalk springs, which owe their hardness almost entirely to car- 
bonate and not to sulphate of lime, a constituent which cannot 
