256 ON THE PURIFICATION OF DRINKING WATER. 
If an alkaline carbonate be employed, all the earthy salts 
(calcareous and magnesian sulphates, chlorides, and bi-car- 
bonates,) carbonates of the earths, are precipitated, while 
alkaline sulphates, chlorides, and bicarbonates which do 
not communicate hardness to water, are left in solution. 
If carbonate of soda be employed, its reaction on sulphate 
of lime and chloride of magnesium will be as follows : 
CaO,S0 3 +MgCl-}-2(NaO,C0 2 )=NaO,S03-4-NaC14-CaO,C0 2 .f-MgO,C02 
Sulphate Chloride of Carbonate Sulphate of Chloride Carbonate Carbonate 
of lime, magnesium, of soda. soda. of sodium, of lime, of magnesia 
c. Addition of Lime. — A few years ago Professor Clark 
of Aberdeen took out a patent for the purification of water. 
His process consists in the addition of caustic lime to water; 
by which the bi-carbonate of lime held in solution is de- 
composed: the caustic lime saturates the excess of carbonic 
acid, and forms carbonate of lime, which is precipitated. 
^Ca 0, 2C0 2 + Ca = Ca ? C0 2 
Bi-carbonate of Caustic Carbonate 
of lime. lime. of lime. 
We believe Clark's process to be virtually impracticable 
on the large scale, while its efficacy is but slight.. 
It must be remembered that this process affects the bi- 
carbonate of lime, not the more troublesome earthy salts, 
such as the sulphates and chlorides, on which the hardness 
of spring waters mainly depends. The difficulty of mixing 
lime and water in definite proportions, on the large scale, 
must be obvious to every one. If too much be employed 
the companies would furnish their customers with lime- 
water! If too little, the bi-carbonate of lime would not be 
completely destroyed, and the process would be a failure. 
Altogether the difficulties of carrying out the process are 
such that we believe it to be impracticable for water com- 
panies. 
But the advantages of the process have been greatly ex- 
aggerated. The calculation of the patentee that, by the 
adoption of his process, the metropolis would annually save 
