SCIENCE OF COMMON THINGS. 55 



Purity of waters. Air in water. Do fishes breathe air? 



ionic j that is, it strengthens and invigorates the sys- 

 tem. 



331 What quantity of mineral matter is generally contained in com- 

 paratively pure natural waters ? 



Any water which contains less than fifteen grains of 

 solid mineral matter in a gallon, is considered as com- 

 paratively pure. Some natural waters are known so 

 pure that they contain only gV th of a grain of mineral 

 matter to the gallon, but such instances are very rare. 



Waters obtained from different sources may be classed as regards com. 

 parative purity as follows : 



Rain water must be considered as the purest natural water, especially 

 that which falls in districts remote from towns or habitations; then 

 comes river water; next, the water of lakes and ponds; next, spring 

 waters ; and then the waters of mineral springs. Succeeding these, arc 

 the waters of great arms of the ocean into which immense rivers dis- 

 charge their volumes, as the water of the Black Sea, which is only 

 brackish ; then the waters of the ocean itself; then those of the Mediter- 

 ranean and other inland seas ; and last of all, the waters of those lakes 

 which have no outlet, as the Dead Sea, Caspian, Great Salt Lake of 

 Utah, etc. etc. 



333 How much solid matter is ordinarily contained in a gallon of sea 

 water f 



From twenty-two hundred to twenty-eight hundred 

 grains. 



333 H<JW much solid matter is contained in a gallon of water from the 

 Dead Sea? 



From eleven thousand to twenty-one thousand grains, 

 or nearly one-fourth part of its weight. 



334 Does air exist in all natural waters f 



It does : fishes and other marine animals are depend- 

 ent on the air which water contains for their existence. 



335 Would absolutely pure water act as a poison to a fish f 



The fish would die of suffocation in such water. 



336 Where is the purest water to be found as a natural product f 



The purest natural water that can be procured is- 

 obtained by melting freshly-fallen snow, or by receiv- 

 ing rain in clean vessels at a distance from houses. 



337 Why is flowing water not liable to become stagnant? 



Because its currents carry away all contaminating 

 substances to the sea. 



