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 



