For pith-balls read atoms, and we have an explanation of the univalent atoms like those of chlorine and sodium which act as single poles; and this is confirmed by the fact that such atoms are never found isolated, but are always associated in a molecule with at least one other atom which forms the opposite pole of the molecular system. Bivalent or magnetic atoms, on the other hand, which have two poles, like those of mercury and zinc, may constitute a complete polar system and be found isolated, and form the class of molecules which consist of single atoms. This conception of the polarity of atoms enables us to understand the way in which the almost infinite variety of substances existing in the world is built up from a comparatively few simple elements. Atoms and radicals, which are multipolar, can attract and form molecules with as many other atoms or radicals as they have poles. This is called their degree of atomicity, which is the same as their quantivalence; and each of these atoms or radicals may be replaced by some other atom or radical, which presents to any pole a more powerful polarity. Thus, compounds may be built up of great and varied complexity, for the quality of any compound may be greatly altered by any one of the substitutions at any one of the poles. And the molecules, or small specimens of matter, may be thus built up into very complex aggregations of atoms, some single molecules containing more than a hundred atoms. Thus, carbon has four poles, or is quadrivalent, and its atoms possess the power of combining among themselves to an almost indefinite extent and forming groups of great stability. Thus, carbon radicals may be formed in very great number, each affording a nucleus upon which compound radicals may be built up, so that carbon has been aptly called the skeleton of almost all the varied compounds of the more complex forms of inorganic matter as well as the principal foundation of organic life. Nor is this all, for the qualities of substances depend not only on the qualities of their constituent elements, but also on the manner in which these elements are grouped. Two substances may have exactly the same chemical composition and yet be very different. We may suppose that the same elements affect us differently according as they are grouped. Thus, the same bricks may be built up either into a cube or pyramid, which forms are extremely stable and can only be taken in pieces brick by brick; or into a Gothic arch, which all tumbles to pieces if a single brick forming the keystone is displaced. As an instance of this, butyric acid, which gives the offensive odour to rancid butter, has exactly the same composition as acetic ether, which gives the flavour to a ripe apple. They consist of the same number of atoms of the same elements—carbon, hydrogen, and oxygen—united in the same proportions. This applies to a number of substances, and is called Isomerism, or formation of different wholes from the same parts.