939 DESCRIPTIONS OF MINERALS. 
minute crystals, and stars, like the figures on page 4, may 
often be detected with a olass. 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 oreatest at 39S Ae below 
this i¢ expands as it approaches 32°, owing to incipient 
crystallization, and in the state of ice it is only 6920. It 
boils at 212° F. A cubie 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 ayoirdupois. <A pint, United States standard mea- 
sure, holds just 7,342 troy grains of water, which is little 
above a poand 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 
bs 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 7; to 10 parts, in 10,000 parts 
by weight. The water of Long Pond. near Boston. con- 
tains about 4 a part in 10,000 ; “the Schuylkill of Philadel- 
phia, about i part in 10, 000 ; the Croton. used in New York 
city, 1 to 13 parts in 10, 000. Nitric acid is usually found 
in rain water combined with ammonia; river waters are 
ordinarily the purest of natural w aters, 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 sait (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 
