Take a molecule of water--the _physical_ unit, you remember. When it is chemically separated or analyzed, it is found to contain two atoms of hydrogen, and one atom of oxygen--both chemical units, remember--which when united and combined, form water, but which when separated are simple atoms of certain chemical gases. The proportion in water is always the same, two of hydrogen and one of oxygen--this is the only partnership that will form water. The molecule of table salt contains one atom of sodium and one of chlorine. The molecule of air contains five chemical gases, of which nitrogen and oxygen are the principal ones, the proportion being about three parts of nitrogen to one of oxygen. Some molecules are far more complex, for instance the molecule of sugar is composed of _forty-five_ chemical atoms, and sulphuric acid of seven. An atom is estimated at one-250,000,000th of an inch in diameter. But this is not all. The old theory of the finality, and ultimateness of the Atom has been shattered by the recent discoveries of Science. The atom of Hydrogen was formerly considered to be the refinement of Substance--the Ultimate Atom--the smallest and finest Atom possible or known--the last thing that could be imagined about Substance. Some even went so far as to declare that the Atom of Hydrogen was the Ultimate Element, that is the Element out of which all other atoms were made--the mother of Atoms--the Origin of Substance. It was supposed that all other Atoms of Matter were composed of a varying number of hydrogen Atoms, which themselves were "vortex-rings in the Ether"--and that analysis could go no further. Science rested on its oars, and pronounced the work of a century completed. But alas! no sooner was this position reached, than the discovery of Radiant Matter and the formulation of the "Corpuscle Theory" brought down the whole theoretical structure, and Science was compelled to take up the hunt again, and to probe further into the inner recesses of Things for the Ultimate Thing. But, nevertheless, Atoms still exist, although their finality is no longer urged. The facts remain, although the theory has fallen.