74 
THE MIDDLE LIAS OF NORTHAMPTONSHIRE. 
waste and decay, and zymotic germs, together with the fumes 
of manufacturing processes. These various substances hang 
suspended in the air, and may be carried long distances by the 
wind until condensation of moisture takes place, and then 
they are entangled in the minute globules of water forming 
clouds, and are so brought down with the rain. 
Half-a-pint of rain water often condenses out of 3,873 
cubic feet of air, the quantity of air a man would breathe in 
eight days, so that in drinking that quantity he swallows an 
amount of impurity that would only reach his lungs from the 
air in eight days.* 
However unsuited ordinary rain water may be for drinking, 
it is very valuable for washing, owing to its softness, and 
therefore the small amount of soap required for detergent 
purposes compared with that required by hard waters. 
If then rain water is so impure, and at the first contact 
with the ground it usually becomes much more so, it is 
evident that some highly-efficient purifying processes are at 
work to enable us to have a.ny water to which the term pure 
may be applied. By carefully considering the way in which 
alone this purification is accomplished naturally, viz., by 
oxidation and filtration during the passage of the water 
through soil and rock, and the amount of it which around 
Northampton gives a reputedly pure spring water, the 
efficiency of the various arrangements proposed for filling up 
the Marlstone will be very evident. 
A Pure Water is never met with naturally, but a water is 
usually regarded as pure that is free from all those substances 
that can be injurious to persons drinking it. It may contain 
impurities that are objectionable for other household, or 
for manufacturing purposes, some of which may and others 
may not be removed by artificial processes, save that of 
distillation. 
Hard and Soft Waters.— Of good waters, some are hard 
and some soft, i.e., some destroy much soap in the process of 
washing, others little; and the hardness may be temporary or 
permanent. 
Temporary hardness is due to the presence of the carbonates 
of lime or magnesia in the water. Perfectly pure water will 
not dissolve much of these substances, only about two grains 
to the gallon (70,000 grains) of the most common one of the 
two—carbonate of lime ; but water containing carbonic acid, 
as all rain and most other water does, will dissolve more of 
these carbonates in proportion to the amount of acid (carbonic 
* 
Sixth Report of Rivers Pollution Commission. 
