108 THE NATURE STUDY COURSE. 



that look clear and transparent may contain disease germs. 

 Many people have an erroneous notion that a powerful 

 microscope will reveal living forms in any and every drop of 

 water. In most samples of ordinary drinking water, when 

 taken from the well or river or lake, the best equipped 

 examiner might search a long time without finding a single 

 living organism. Some open springs and forest rivulets 

 abound in microscopic life, and yet people derive no harm by 

 drinking water from them. Generally, however, waters that 

 contain life that can be seen with the microscope are not safe 

 to drink. 



Reservoirs of drinking water are lakes, streams, springs, 

 dug wells, artesian wells, rain cisterns, surface-water cisterns, 

 cisterns for hauled-water. (To many people, clear sparkling, 

 hard water is more palatable than clean rain water, but it is 

 not more wholesome.) 



What causes may make water taken from each of these 

 sources of supply unwholesome or of suspected purity ? 



Tests of Purity. — Water may contain mineral impurities, 

 vegetable impurities, animal impurities. 



Dissolved minerals remain and may be seen when water is 

 evaporated in a clean glass vessel. 



Organic impurities — vegetable or animal — are probably 

 present if any odor develops in a sample of water kept in a 

 warm place for a day or two in a clean, tightly-corked bottle. 

 Such impurities are present if a two or three hours' exposure 

 to light bleaches out the purplish-pink color imparted to the 

 sample by a few drops of solution of potassium permanganate. 

 In a teaspoonful of the water dissolve a few crystals of the 

 salt just named. Have a sample of the water to be tested in 

 a clean bottle or clean cup. Add the solution, drop by drop, 

 until the sample becomes pink. Set it in the light, protected 



