DISEASE GERMS 
83 
criminal carelessness, because in the eye of the law 
ignorance is not accepted as an excuse. Somewhere 
there has been neglect of the cleanness or care which 
ought to have made infection impossible. 
When rain starts from the clouds it is pure, but in 
falling through the air it washes out from the air large 
quantities of dust, so that the first fall of any shower 
is very dirty. Where rain water is collected for drink¬ 
ing or cooking purposes this first fall should be allowed 
to waste or the whole be thoroughly filtered before 
its use in cooking. The cistern also must be kept clean 
and free from dust pollution. It should be sheltered, 
but not air-tight. Such a water supply is seldom pol¬ 
luted by sewage or any human wastes. It is water 
running on the surface of the ground or draining 
through it which may encounter sewage pollution and 
thus be most liable to take up disease germs. 
Snow filters the air even more than rain, each con¬ 
gealed flake usually containing many bacteria. The 
first snow, although white and pure to look at, is not 
clean and should not be used as a source of drinking 
water except in emergencies. However, after the snow 
has been falling for some time the water from it is 
practically clean. 
Light always retards and in many cases prevents 
the development of harmful micro-organisms. But this 
disinfectant action does not extend to all depths, prob¬ 
ably not much beyond nine feet, so that its purifying 
agency in open water supplies is only partial. A water 
Criminal 
Carlessness 
Pollution 
of Water 
Effect 
of Light 
