Page:First course in biology (IA firstcourseinbio00bailrich).pdf/631

 trary to the usual belief). They are most abundant in rivers that flow through populous regions. They are comparatively scarce in dry, sandy soils, and very numerous in moist, loamy soils. The water of cities should never be taken from a stream or lake into which sewerage flows unless it is thoroughly filtered. Filters are constructed thus: first a layer of small stones, next a layer of coarse sand, lastly a layer of very fine sand on top, the total thickness being four or five feet. Beneficial microbes live upon the grains of sand and destroy all, or nearly all, of the dangerous microbes as the water slowly soaks through. The construction of such waterworks is left to sanitary engineers, of course, and the average citizen does not need to know the details.

The department of street cleaning should receive the willing coöperation of all citizens. Banana peelings, paper, etc., should not be thrown upon the street or school grounds. Garbage, ashes, and rubbish should be placed in separate cans, as the rules provide. Garbage cans, if not thoroughly cleaned, acquire unpleasant odors and breed flies and bacteria. They should be thoroughly washed with very hot water and sal soda and scalded with boiling water and scrubbed with an old broom.

The chief duties of the Health Department are: quarantine isolation and disinfection, with the purpose of preventing or controlling contagious and infectious diseases;