and Water used for General Purposes. 
157 
Purification of Water. 
Spring, river, well, or lake-waters, as it has been shown, are 
rendered impure to a greater or less extent — 
1. By suspended animal and organic substances, such 
as finely-divided clay, marl, flaky organic matters, 
decaying vegetable matter, and similar mechanical 
impurities : 
2. By soluble organic impurities, which generally colour 
the water yellow or brownish : and 
3. By certain saline matters and soluble earthy compounds, 
which, in the shape of a more or less considerable 
and generally slightly coloured residue, are left be- 
hind when a measured quantity of any kind of 
natural water is evaporated to dryness. 
In other words, suspended — or mechanical, organic, and 
mineral, — and soluble — vegetable, and animal, — matters, are the 
ordinary impurities of natural waters, to which have to be added, 
in exceptional cases, sulphuretted hydrogen, traces of copper, 
arsenic, or more frequently lead. 
The means available for the purification of water are : — 
1. Distillation. 
2. Filtration. 
3. Precipitative processes, which remove certain soluble 
earthy compounds. 
1. Distillation. — When river, or spring, or sea-water, is kept 
boiling in a glass retort or metal still, it is converted into steam, 
which carries with it all the gaseous or volatile impurities that 
may have been present in the natural water, and leaves the 
whole of the solid saline and earthy matter behind. By suitable 
cooling apparatus the steam is readily condensed ; and if the 
first part of the distillate, containing most of the volatile im- 
purities, is rejected, nearly pure distilled water is obtained. 
Except at sea, or for chemical use, this method of purification 
is seldom resorted to for effecting the purification of water. 
2. Filtration. — On a large scale turbid river-water is effec- 
tually clarified by passing it through gravel and sand filter-beds. 
By this means the mechanical impurities, such as fine clay or 
marl, dead leaves, and similar accidental impurities, are arrested 
in the filter-beds, and the water is rendered bright. Filtration 
through sand also removes to some extent soluble organic matters, 
which sometimes give a yellowish tint to river-waters, for by 
passing through gravel or sand, a portion of such organic matters 
is oxidised, and the filtered water is in a measure deprived of the 
original yellow tint. The saline and earthy matters dissolved 
