Already within that realm in which the theory of atoms is a supplemental hypothesis directly indispensable at present--_i.e._, within their application in physical sciences--we meet suppositions which raise great doubts and difficulties. Such a scientific difficulty occurs when the atomism of the natural philosophers supposes a double complexity of atoms, material atoms and atoms of ether: complexities which both penetrate one another, and are supposed to follow partly totally different, partly the same, elementary laws of force. Material atoms are subordinate to the law of gravitation, while atoms of ether are not; and yet both act legitimately upon one another,--as, for instance, when heat passes into motion and motion into heat, which certainly presupposes a law of power acting in common for both. Another difficulty lies in the atomism of the chemists; and still another {143} in the divergency of the aims at which the physical theory of atoms on the one hand and the chemical theory of atoms on the other seem to point. Chemistry is inclined to explain the difference of its numerous elements from the original difference of the atoms; and yet it is not at all certain that the elements of chemistry themselves are not composed of still more simple and less numerous primary elements. Many indications seem to point to such primary elements which are more simple in number and quality, and investigators even mention an element--hydrogen--in the direction of which we have to look for the way that will lead us to those primitive elements of matter. The divergency of aims, finally, consists in the fact that physical atomism prevailingly points to a conformity of the atoms of bodies; chemical atomism, on the contrary,--at least, according to its present state,--points to a dissimilarity among these.