ON THE PURIFICATION OF DRINKING-WATER. 247 
the isolated acid to direct analysis, and have obtained the 
same numbers as those furnished by the combustion of 
aspartic acid. — Chemical Gazette, Jipril 15, 1S5G, from 
Comples Rendas, March 18, IS 50. 
ART. LIX.— ON THE PURIFICATION OF DRINKING 
WATER. 
Of the various important topics which have been brought 
under discussion in connection with the metropolitan water 
supply, not the least important is that which relates to the 
depuration of the water furnished by the several companies 
to the inhabitants of this great metropolis. We propose, 
therefore, on the present occasion, to take a general survey 
of the various possible methods of effecting the purification 
of water, and then to consider which of them are practicable 
and necessary to be adopted by the metropolitan water com- 
panies. 
Water may be purified by subsidation and decantation, by 
filtration, by ebullition, by distillation, by clarification 
or fining, and by the addition of certain chemical agents, 
which effect a chemical change in the composition of the 
fluid. Of course some of these methods are practicable on 
the large scale, but it appears to us to be desirable to examine 
the whole subject, and to notice also those which, though 
not applicable to public companies, have been, or still are, 
practised by individuals on the small scale. 
1. Subsidation and Decantation. The depuration of water 
is greatly aided by repose, by which various suspended or 
mechanical impurities are allowed to subside gradually, and 
from these the supernatant water is drawn off. Tanks, cis- 
terns, and reservoirs become, therefore, important depurating 
agents. All the metropolitan water companies are provided 
with large deposite reservoirs in which this form of purifica- 
