ON THE PURIFICATION OF DRINKING WATER. 255 
the Nile. We have no doubt that the efficacy of all these 
seeds in clearing water depends on the albumen and casein 
which they contain. 
6. The addition of Chemical Agents. — This head includes 
several proposed methods of purifying water, which consist 
in the addition of certain chemical agents to this liquid, by 
which its composition is altered. 
a. Addition of Alum. — In England as well as in France 
a popular method of clearing muddy water is to add a few 
grains of powdered alum to it (two or three grains are 
usually sufficient for a quart of water.) This process our 
neighbors call the alunage de Veau ; and Arago states that 
when practised on the Seine water it causes the mud to 
agglomerate in long thick striae which are very quickly 
deposited. The theory of the process appears to be this : 
the alum decomposes the bi-carbonate of lime, and gives 
rise to the formation of sulphate of lime, which, with sul- 
phate of potash, remains in solution, while carbonic acid is 
evolved, and hydrate of alumina being precipitated in a 
floccu'ent form, carries with it various mechanical impuri- 
ties. 
KO,S03-h A1203,3S03-{-3CCaO,2C02) = 6 CO2 K0.S03 -{- 3(Ca0,S03}-{- AI2O3 
V y 1 v y ' C^v^» V v , v , / J V^-y^ 
Alum (drj) Bicarbonate Carbonic Sulphate Sulphate Alumina. 
of lime. acid. ofpotasli. of lime. 
This process then is a kind of mechanico-chemical one. 
It clears the water, but at the same time alters its chemical 
composition, and by converting bi-carbonate into sulphate 
of lime augments the hardness of the water. 
b. Addition of Caustic or Carbonated Alkalies. — Caustic 
alkalies added to water holding in solution bicarbonate of 
lime, saturates the excess of carbonic acid, throws down 
carbonate of lime (chalk,) and leaves an alkaline carbonate 
in solution. If soda be the alkali used the result will be as 
follows : 
CaO, 2 C0 2 + Na = Na 0, C0 2 -f CaO, CO2 
v v v v 
Bi carbonate of lime. Soda. Carbonate soda. Chalk. 
