ON the 1st of August, 1874, it will be exactly a hundred years since oxygen gas was first made known to the world. This discovery is one of the most important ever made in science, and we commemorate its centennial by doing something to make more widely known the character of the illustrious man whose name will be associated with it as long as science is cultivated or civilization continues.
A hundred years of advancing knowledge has steadily exalted the importance of Priestley's discovery. It formed a great epoch in the progress of modern chemistry, and gave a profound clew to the internal constitution of Nature. The element first revealed, examined, and described by Priestley, is the most extensive in its distribution, and the most potent in its influence, of all the material constituents of the world. We now know with some definiteness the proportions in which oxygen exists in the various parts of Nature, but the aggregates are so stupendous as utterly to baffle the imagination. It exists in the smallest proportion in the atmosphere, forming but one-fifth of its weight. As there are fifteen pounds weight of air on every square inch of the earth's surface, it follows that there are three pounds of oxygen to the same area. By a simple calculation, it therefore turns out that the amount of oxygen in the earth's atmosphere is one quintillion, one hundred and seventy-eight quadrillions, one hundred and fifty-eight trillions of tons—a quantity absolutely inconceivable by the human mind.
In the world of waters, the scale of proportions is enormously increased, as eight-ninths of the weight of this liquid consists of oxygen. The ocean is assumed to cover two-thirds of the earth's surface, and to have an average depth of two miles, which would be sufficient to cover its entire surface to the depth of one mile and one-third. This would give us twenty-seven hundred pounds of oxygen for every square inch of the earth, or an amount in the oceans equal to nine hundred atmospheres.
Chemical analysis has also shown us the proportions of oxygen in the various classes of rocks. It forms one-half the weight of silica, one-third that of alumina, and two-thirds that of lime; and, as the great bulk of the geological formations are made up of these minerals, it follows that the entire crust of the globe, so far as it has been explored, with its twenty miles thickness of stratified rocks and its underlying granites, consists of oxygen to the extent of one-half of its weight.
If we turn now to the world of life, although the absolute magnitudes are much less, the relative proportions of oxygen are very high, and the grandeur of its operations is simply amazing. Three-fourths the weight of the entire animal world, and four-fifths the weight of the whole vegetable kingdom, consist of this element alone. Moreover, the operations of life in both branches are intimately dependent upon its activity and the rapid changes of which it is the main agent; while the vegetable kingdom is a grand laboratory, worked by the power of the solar rays to liberate oxygen from its combinations, and pour it back into the atmosphere in a free and active state. The animal kingdom, on the other hand, through all its grades, depends for its existence upon the incessant withdrawal of oxygen from the air. Each adult