A large portion of this planet is covered by the waters of the ocean, of lakes and rivers. Water forms the best means of communication between remote parts of the earth. It is in every respect of the utmost importance to the animal and vegetable kingdom; and, indeed, it is indispensable in all the great phenomena of the inorganic world. The peculiarities of saltness or freshness in water are dependent upon its solvent powers. The waters of the ocean are saline from holding dissolved various saline compounds, which are received in part from, and imparted also to, the marine plants. Perfectly pure water is without taste: even the pleasant character of freshly-drawn spring-water is due to the admixture of atmospheric air and carbonic acid. The manner in which water absorbs air is evidently due to a peculiar physical attractive force, the value of which we do not at present clearly perceive or correctly estimate. It is chemically composed of two volumes of hydrogen gas--the lightest body known, and at the same time a highly inflammable one--united with one volume of oxygen, which excites combustion, and continues that action,--producing heat and light,--with great energy. By weight, one part of hydrogen is united with eight of oxygen, or in 100 parts of water we find 88·9 oxygen, and 11·1 of hydrogen gas. That two such bodies should unite to furnish the most refreshing beverage, and indeed the only natural drink for man and animals, is one of the extraordinary facts of science. Hydrogen will not support life--we cannot breathe it and live; and oxygen would over-stimulate the organic system, and, producing a kind of combustion, give rise to fever in the animal frame; but, united, they form that drink, for a drop of which the fevered monarch would yield his diadem, and the deprivation of which is one of the most horrid calamities that can be inflicted upon any living thing. Water appears as the antagonist principle to fire, and the ravages of the latter are quenched by the assuaging powers of the former; yet a mixture of oxygen and hydrogen gases, in the exact proportion in which they form water, explodes with the utmost violence on the contact of flame, and, when judiciously arranged, produces the most intense degree of heat known;--such is the remarkable difference between a merely mechanical mixture and a chemical combination. Beyond this, we have already noticed the remarkable fact that water deprived of air is explosive at a comparatively low temperature, less than 300°; gunpowder requiring a temperature of nearly 1000° F.