PURIFICATION OF WATER. 253 
to expose to the air for longer or shorter periods nutrient 
agar spread upon the surface of the Petri dish. After 
exposure the plates are put either in the incubator at 
37°C. or kept at room-temperature. The more careful 
examination is made by drawing a given quantity of 
air through tubes containing sterile sand, which is kept 
in by pieces of metal gauze. When the operation is 
completed the sand is poured into a tube containing 
melted nutrient gelatin or nutrient agar, and after 
thoroughly shaking the mixture is poured into a Petri 
dish and the bacteria allowed to develop, either at 
37° or 20° C., according as a growth of the parasitic 
or saprophytic varieties is desired. 
THE CONTAMINATION AND PURIFICATION OF 
DRINKING WATERS. 
Brook and river water is contaminated in two ways: 
through chemicals, the waste products of manufactur- 
ing establishments, and through harmful bacteria by 
the contents of drains, sewers, etc., the latter method 
being by far the more dangerous. 
When water, which has been soiled by waste products 
of manufactories only, becomes so diluted or purified 
that the contamination is not noticeable to the senses 
and shows no dangerous products on chemical analysis 
it is probably safe to drink. When sewage is the con- 
tamination this rule no longer holds, and there may be 
no chemical impurities and no pathogenic bacteria 
found and yet disease be produced. That river water 
which has been fouled by sewage will, in the course of a 
few miles, through the dilution of additional supplies, 
through sedimentation, and through oxidation, become 
