ing by themselves they would signify comparatively little; but considered with other analogous evidence they help to found an almost overwhelming argument. The concurrent testimony supplied by the specific or atomic volumes of the elements is particularly strong.
The specific volume of any substance is the quotient obtained upon dividing its atomic weight by its specific gravity. This value may be supposed to represent the volume of an atom of the substance plus the sphere of unoccupied space immediately surrounding and belonging to it. Leaving theoretical definitions out of account, however, we shall find, upon comparing the specific volumes of solid and liquid substances, many extraordinary relations. Often, all the members of an elementary group have equal values. This is the case with the closely-related metals platinum, iridium, osmium, palladium, rhodium, and ruthenium. They have different atomic weights and different specific gravities; yet the quotient obtained upon dividing the former by the latter is the same in every instance. The same thing holds good of the group formed by iron, cobalt, nickel, chromium, manganese, copper, and perhaps also uranium. Here the regularity extends even beyond the elements themselves, for their corresponding compounds have, with few exceptions, equal specific volumes also. An altogether different, but on the whole more remarkable, relation is furnished by the alkaline metals lithium, sodium, potassium, and rubidium; whose specific volumes are respectively 11.9, 23.7, 45.1, and 56.2. These values are almost exactly multiples of the first, standing to it in the ratio of 1 : 2 : 4 : 5. The slight variations from accuracy in this case are very far within the limits of experimental error. Almost as remarkable multiple relations are found in several other series, and apply not only to the specific volumes of the solid elements, but to their values in liquid compounds also. Closely connected with this subject is that of crystalline form. As a general, though not invariable rule, elements having equal specific volumes are isomorphous; that is, crystallize alike; a fact which may be extended to a very large number of compound series as well.
It would be easy to go on to almost an indefinite extent multiplying examples of relationship between the elements. There is hardly any set of physical properties which may not be made to emphasize the idea that these substances are internally related. Take, for example, their specific heats, which, multiplied by their atomic weights, give a constant quantity in the neighborhood of 6.3. That is, according to the law of Dulong and Petit, all elementary atoms have equal capacities for heat. But space is limited, so that we must omit the consideration of many important facts, and pass to the theoretical discussion of those already cited. All this evidence suggests quite emphatically that the elements are not totally distinct and independent bodies. Are they, then, compounds formed from a few simple substances, or are they modifications of but one primal matter? Strong
