cXVv1x 
the first-named metal ; and it should not contain more than two parts 
of iron or zinc, or one part of lead or copper, in a million parts of 
water. It should be free from the slightest suspicion or possibility of 
contamination with sewage or drainage or foul gases of any kind, from 
houses, cesspools, church yards, slaughter yards, tanneries, farm 
yards, manured fields, etc. No sediment should form on standing, and 
only a moderate amount on boiling. It should be moderately cool and 
well aerated, containing seven or eight cubic inches of dissolved gases 
per gallon, two cubic inches at least being oxygen. Such water will 
be entirely free from taste, smell, and colour ; soft, clear, bright, and 
transparent, and entirely wholesome and palatable. 
‘ Classes of Water. 
Rain-water in falling takes from the air traces of nitric acid, ammonia, 
mineral and organic matter, including the germs of animals and plants ; 
and if collected from the roof of a house, willsweep into the tank much 
additional impurity, as the droppings of birds, dust, decayed leaves, zinc 
from gutters, etc. Some of these germs are the producers of fermenta- 
tion, putrefaction, and sometimes, doubtless, of disease. 
Springs may contain excess of mineral or vegetable metter, or 
poisonous metals. Lakes and ponds or water-holes will contain various 
impurities, according to position and the source from which they are 
fed, but usually yield; water inferior to good river water. Rivers may 
receive drainage from manured land, pastures, houses, farm yards, etc., 
and thus contain the germs of various diseases of men and animals, and 
of intestinal worms and other parasites ; or sheep may have been washed 
in them. Wells frequently receive leakage from cesspools, farm yards, 
etc., and may be much polluted, although the water remains perfectly 
clear, bright, and tasteless. Marsh water usually contains much vege- 
table matter, and has a ‘‘ peaty ” taste. 
Purification. 
Tanks and barrels used for storing, and filled from any of these 
sources, are liable, if not frequently emptied and cleaned out, to be- 
come foul from the accumulation of sediment, and the possible presence 
of drowned rats, mice, and insects, and the absorption of foul gases 
from neighbouring cesspools, pigstyes, stables, etc.; water in this condi- 
tion also dissolving greater quantities of harmful metals, as lead, copper, 
and zinc (from galvanised iron) with which it may come in contact. 
In addition to frequent cleansing of store tanks and barrels, it is always 
advisable to use a filter, which, if efficient, will retain suspended matter 
and the larger organisms, including the ova of fluke, tape, and other 
intestinal worms, and some of the dissolved albuminous matter will be 
oxidised ; but many of the minuter forms of life, including the dan- 
gerous ones, will pass through an ordinary filter. I mention this not to 
discredit filters, but to prevent too implicit reliance being placed upon 
them. 
Boiling for some time, either without, or, better, with the addition 
of a very small quantitysof permanganate of potash, is necessary if the 
quality be doubtful ; but it is much safer to obtain, if possible, a supply 
quite free from suspicion, as it is not certain that all dangerous germs 
will be destroyed even by boiling, the spores of some forms bearing a 
still higher temperature. 
Filtering, especially through a dripstone, will re-aerate the water after 
boiling, and so remove its insipid flavour. The dripstone should of course 
be out of reach of noxious gases from ill-kept yards, etc. Filters should 
be cleansed every two months by pouring through a quart of pure water 
containing 30 grains of permanganate of potash, and 10 drops of strong 
sulphuric acid (oil of vitroil), then two to four gallons of pure water con- 
taining a quarter to half an ounce of pure hydrochloric acid (spirits of 
salt), and followed by a like quantity of pure water only; the_filter is 
