More than a thousand years before the birth of Christ a great Phœnician philosopher believed that all matter—solids, liquids, and gases—was built up from infinitely small aggregations of atoms. The learned men of Greece enlarged upon his views but this philosophy passed into oblivion with the destruction of Rome and the coming of the Dark Ages, and it was not revived until about one hundred and fifty years ago. The ancients could not prove their theory, while we to-day can count the atoms and determine their size and motions; and, exceedingly small though they be, we no longer believe them to be indivisible in structure. On the contrary, we know that each atom consists of particles of positive and negative electricity. The negative electrons arrange themselves about a positive electron for a nucleus and, rotating about it as if it were a central sun with planets, constitute an atom. All matter reduced to the ultimate electron is precisely alike. The difference in matter is determined by the number of negative electrons that are attracted and held in place by the positive nucleus that is at the center of each atom of which a particular kind of matter is composed. Each of the ninety-two elements which we believe constitute the ninety-two different forms of simple matter has an atom with its own peculiar type of nucleus, which nucleus differs from those of the others only in the amount of positive electricity it contains. Thus hydrogen, the lightest of all gases, whose weight is taken as unity in measuring the magnitude of other gases, has a nucleus whose positive charge of electricity is only sufficient to attract one negative electron. The next element, helium, has a nucleus with a double positive charge and consequently holds two electrons or planets to pay it homage. In like manner the carbon atom contains six electrons; oxygen, eight; aluminum, thirteen; nitrogen, fourteen; sulphur, sixteen; iron, twenty-six; copper, twenty-nine; silver, forty-seven; gold, seventy-nine; mercury, eighty; lead, eighty-two; bismuth, eighty-three; radium, eighty-eight; thorium, ninety; and uranium, ninety-two. The chemical union of these elementary forms of matter creates other forms. For instance, the union of two atoms of hydrogen and one of oxygen constitutes a molecule of water. But the gases of the atmosphere are not in chemical union; they exist in the form of a mechanical mixture, each acting as though the others were not present.