232 DESCRIPTIONS OF MINERALS. 



minute crystals, and stars, like the figures on page 4, may 

 often be detected with a glass. Various other allied forms 

 are also assumed. The "rays meet at an angle of 60°, and 

 the branchlets pass off at the same angle with perfect regu- 

 larity. The density of water is greatest at 39° 2' F.; below 

 this it expands as it approaches 32°, owing to incipient 

 crystallization, and in the state of ice it is only 0*920. It 

 boils at 212° F. A cubic inch of pure water at 62° F. and 30 

 inches of the barometer, weighs 252*458 grains, which equals 

 16*386 grams; and a cubic foot of water weighs 62*355 

 pounds avoirdupois. A pint, United States standard mea- 

 sure, holds just 7,342 troy grains of water, which is little 

 above a pound avoirdupois (7,000 grains troy). 



Water, as it occurs on the earth, contains some atmo- 

 spheric air, without which the best would be unpalatable. 

 This air, with some free oxygen also present, is necessary 

 to the life of aquatic animals. In most spring water there 

 is a minute proportion of salts of calcium (sulphate, chloride 

 or carbonate), often with a trace of common salt, carbonate 

 of magnesium and some alumina, iron, silica, phosphoric acid, 

 carbonic acid, and certain vegetable acids. These impuri- 

 ties constitute usually from ^ to 10 parts, in 10,000 parts 

 by weight. The water of Long Pond, near Boston, con- 

 tains about ^ a part in 10,000 ; the Schuylkill of Philadel- 

 phia, about 1 part in 10,000 ; the Croton, used in New York 

 city, 1 to 1^ parts in 10,000. Nitric acid is usually found 

 in rain water combined with ammonia ; river w r aters are 

 ordinarily the purest of natural waters, unless they have 

 flowed through a densely populated region. 



Sea water contains from 32 to 37 parts of solid substances 

 in solution in 1,000 parts of water. The largest amount in 

 the Atlantic, 36*6 parts, is found under the equator, away 

 from the land or the vicinity of fresh-water streams ; and 

 the smallest in narrow straits, as Dover Straits, where there 

 are only 32*5 parts. In the Baltic and Black Seas, the pro- 

 portion is only one-third that in the open ocean. Of the 

 whole, one-half to two-thirds is common salt (sodium chlo- 

 ride). The other ingredients are magnesium salts (chloride 

 and sulphate), amounting to four-fifths of the remainder, 

 with sulphate and carbonate of calcium, and traces of bro- 

 mides, iodides, phosphates, borates and fluorides. The water 

 of the British Channel affords water 964*7 parts in 1,000, 

 sodium chloride 27*1, potassium chloride 0*8, magnesium 



