164 WATER, AND ITS IMPURITIES. 
The quantity of grit or sand in suspension in water varies 
with the condition of the water and the state of the weather. 
If the water be kept in continual agitation, as by the passage 
of boats, barges, and steam- vessels, it will always contain a 
very large quantity of earthy matter diffused through it, suffi- 
cient, indeed, to impart an opaque appearance and an earthy 
colour. The water of the Thames near to London is usually in 
the state described, and a sample of it is generally opaque and 
discoloured, by reason of the immense quantity of earth sus- 
pended in it. 
Again, water kept in a state of motion by storms, wind, and 
rain, will be affected in a similar way, and will be laden with 
earthy matter. 
The water just poured into a dirty reservoir or cistern re- 
mains in the state described for some hours after, in consequence 
of its stirring up the accumulated sediment contained in it. 
Inorganic Impurities in Solution . 
The salts contained in water may be divided into the earthy , 
alkaline , and metallic ; and it is to their presence that many of 
the sensible properties of water are due, as its softness, hard- 
ness, alkalinity, and acidity. 
Softness is a property common to all the purer waters, as 
distilled, rain-water, &c., and is possessed in greatest proportion 
by those waters which contain the least amount of earthy salts. 
The alkalies or alkaline salts, however, possess the property 
of imparting softness, and hence their employment in soap and 
washing powders. 
The degree of hardness of water depends upon the amount 
of earthy salts dissolved in it, that is, of sulphates and bicar- 
bonates of lime and magnesia, but particularly of the bicar- 
bonates. 
The alkalinity of water is dependent upon the amount of 
alkalies present in solution, the principal of these being the 
carbonates of soda and potash. 
The acidity arises, of course, either from the presence of acid 
salts, or some free acid, as the carbonic. 
When we come to consider “ the different kinds of water,” 
we shall have again to refer to the sensible qualities we have 
thus briefly alluded to. 
The two great metallic impurities or contaminations of water 
are those by lead and iron, each of which, but especially the 
first, we shall consider in detail. 
