atoms and chemical elements, incapable of being decomposed by purely chemical means, but that such are not absolute atoms or absolute elements, and consequently not impervious to all forms of action. But of this more will be said later. [91] The term “valency” is not altogether an easy one to define; we will, however, here do our best to make plain its significance. In a definite chemical compound we must assume that the atoms constituting each molecule are in some way bound together (though not, of course, rigidly), and we may speak of “bonds” or “links of affinity,” taking care, however, not to interpret such terms too literally. Now, the number of “affinity links” which one atom can exert is not unlimited; indeed, according to the valency theory as first formulated, it is fixed and constant. It is this number which is called the “valency” of the element; but it is now known that the “valency” in most cases can vary between certain limits. Hydrogen, however, appears to be invariably univalent, and is therefore taken as the unit of valency. Thus, Carbon is quadrivalent in the methane-molecule, which consists of one atom of carbon combined with four atoms of hydrogen; and Oxygen is divalent in the water-molecule, which consists of one atom of oxygen combined with two atoms of hydrogen. Hence, we should expect to find one atom of carbon combining with two of oxygen, which is the case in the carbon-dioxide--(carbonic anhydride)--molecule. For a development of the thesis, so far as the compounds of carbon are concerned, that each specific “affinity link” corresponds in general to a definite and constant amount of energy, which is evolved as heat on disruption of the bond, the reader is referred to the present writer’s monograph _On the Calculation of Thermo-Chemical Constants_ (Arnold, 1909). The phenomena of valency find their explanation in modern views concerning the constitution of atoms (see § 81). The Determination of the Atomic Weights of the Elements.