Water in Relation to Health and Disease. 727 
foreign matters, inasmuch as it brings down with it in its fall 
various substances, solid and gaseous, living and dead, which the 
atmosphere holds in suspension. Of these some are desirable 
and useful additions, others are harmless, but some are distinctly 
hurtful. In regard to the solids thus acquired, they are neces- 
sarily of a miscellaneous character, and comprise dust in its 
many and varied forms, the ubiquitous germs of putrefaction, 
and too frequently, also, others out of which arise specific con- 
tagious disorders. 
The quantity of solid matter contained in rain-water is esti- 
mated to be about two grains per gallon in rural districts, but 
in large towns, where traffic is heavy and manufacturing indus- 
tries are carried on, it is much greater. The gaseous constitu- 
ents of rain-water comprise oxygen, carbonic acid, nitrogen, and 
ammonia, and in the vicinity of factories where much coal is con- 
sumed, and the ore of metals is smelted, sulphurous acid and 
other injurious gases may also be present in greater or less 
amount. Of these it is computed that each gallon contains four 
cubic inches of nitrogen, two cubic inches of oxygen, and one 
cubic inch of carbonic acid, with a small fractional part of 
ammonia. Although regarded as impurities in a strictly chemi- 
cal sense, they are not only harmless, but impart to water a 
coolness and freshness which it would not otherwise possess. 
Airless water is flat and insipid, and in other respects objection- 
able. In a biological as well as in a sanitary sense oxygen is 
indispensable to water. Without it aquatic animals could not 
live. Not only does it minister to respiration in the living, 
but it serves to oxidise dead organic matter and to confer upon 
our rivers a self-purifying power, by converting substances 
which would probably prove hurtful into harmless products of 
oxidation. In the case of rivers contaminated with the sewage 
of towns this action of dissolved oxygen is of great importance. 
Sulphurous acid as ordinarily found in rain-water is in itself 
of little moment, but the action it exercises upon lead renders 
such water hurtful, and even poisonous, in passing through lead 
pipes, or over lead gutters, or when stored in lead tanks. 
Besides impurities gathered from the air, others are also 
added to rain-water as it passes over the roofs of houses, stables, 
sheds, and other buildings. In places where pigeons are 
kept in large numbers such water becomes not only foul but 
actually poisonous, its effects being specially marked on young 
animals when supplied to them continuously for long periods. 
In two instances in the experience of the writer it has caused 
considerable mortality and sickness, and there is reason to 
believe that some of those mysterious outbreaks of putrid fever — 
