Moreover, so foul are many of our rivers and streams becoming in thickly settled regions that fish can no longer live in them, and it is hardly safe to bathe in them.[17] Fortunately, however, a great deal of the worst contamination can be prevented by using modern methods of disposing of sewage, such as filter-beds and sewage farms.

All of these methods use the bacteria of the soil, or crops growing in it, to eat up the waste and thus purify the sewage.
[Illustration: SCRAPING THE SEDIMENT FROM THE BOTTOM OF A RESERVOIR] HOME METHODS OF PURIFYING WATER Boiling.

Where the water that you are obliged to drink is not known to be pure, then it can be made quite safe for drinking purposes by the simple process of boiling it for about ten or fifteen minutes.

But this, except in travelling or in emergencies, is a lazy, slipshod substitute for pure water, and extremely unsatisfactory as well; for the boiling drives off all its air and other gases, and throws down most of the salts, so that boiled water has a flat, insipid taste.

These salts, although sometimes regarded as impurities, are not such in any true sense; for the lime and soda especially are of considerable value in the body, so that boiled or sterilized water is neither a pleasant nor a wholesome permanent drink.