A good criterion of the purity of water fit for domestic purposes, is

its softness. This quality is at once obvious by the touch, if we only

wash our hands in it with soap. Good water should be beautifully

transparent; a slight opacity indicates extraneous matter. To judge of

the perfect transparency of water, a quantity of it should be put into a

deep glass vessel, the larger the better, so that we can look down

perpendicularly into a considerable mass of the fluid; we may then

readily discover the slightest degree of muddiness much better than if

the water be viewed through the glass placed between the eye and the

light. It should be perfectly colourless, devoid of odour, and its

taste soft and agreeable. It should send out air-bubbles when poured

from one vessel into another; it should boil pulse soft, and form with

soap an uniform opaline fluid, which does not separate after standing

for several hours.

It is to the presence of common air and carbonic acid gas that common

water owes its taste, and many of the good effects which it produces on

animals and vegetables. Spring water, which contains more air, has a

more lively taste than river water.

Hence the insipid or vapid taste of newly boiled water, from which these

gases are expelled: fish cannot live in water deprived of those elastic

fluids.

100 cubic inches of the New River water, with which part of this

metropolis is supplied, contains 2,25 of carbonic acid, and 1,25 of

common air. The water of the river Thames contains rather a larger

quantity of common air, and a smaller portion of carbonic acid.

If water not fully saturated with common air be agitated with this

elastic fluid, a portion of the air is absorbed; but the two chief

constituent gases of the atmosphere, the oxygen and nitrogen, are not

equally affected, the former being absorbed in preference to the latter.

In agitating water with atmospheric air, consisting of 79 of nitrogen, and 21 of

oxygen, the water absorbs 1/64 of 79/100 nitrogen gas = 1,234, and 1/27 of

21/100 oxygen gas = 778, amounting in all to 2,012.

Water is freed from foreign matter by distillation: and for any chemical

process in which accuracy is requisite, distilled water must be used.

Hard waters may, in general, be cured in part, by dropping into them a

solution of sub-carbonate of potash; or, if the hardness be owing only

to the presence of super-carbonate of lime, mere boiling will greatly

remedy the defect; part of the carbonic acid flies off, and a neutral

carbonate of lime falls down to the bottom; it may then be used for

washing, scarcely curdling soap. But if the hardness be owing in part to

sulphate of lime, boiling does not soften it at all.

When spring water is used for washing, it is advantageous to leave it

for some time exposed to the open air in a reservoir with a large

surface. Part of the carbonic acid becomes thus dissipated, and part of

the carbonate of lime falls to the bottom.

The more any spring is drawn from, the softer the water becomes.