Page:Popular Science Monthly Volume 82.djvu/587

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WHEN we consider the marked resemblances and striking interrelations of the elements as expressed by the Periodic System, the conviction grows more and more strongly upon us that this system is the external expression of a fundamental process in nature, to which are due the general properties, as well as the individual characteristics, of the elements. On the present occasion I shall endeavor to point out that between the Periodic classification and the ordinary zoological classification, such analogies exist as tend to indicate an identity in fundamental principle. We shall then consider some of the phenomena which are at the foundation of the law of organic evolution, and here, too, we shall find among the elements conditions exactly corresponding.


A Family of the Elements Compared with a Homologous Series

Before proceeding farther, however, it is of interest to note the similarities which exist between a family of the elements and a homologous series of organic compounds. For the purpose of this comparison it is most useful to select the homologous series of fatty acids, . If we should arrange the normal acids of this series in order of molecular weight, we should find that between such a series and a family of the elements there exist certain close analogies, which are tabulated below in parallel columns.

Fatty Acids () Family of the Elements
1. There is a constant difference in molecular weight between consecutive members of 14, due to the constant group difference . 1. There is a fairly constant difference in atomic weight between consecutive elements of the same family of about 45, except between the first and second (and in some cases between the second and third), where it is about 16.
2. The first member of the series, formic acid, differs somewhat in properties from the other members of this homologous series. Thus, it manifests the characteristics of an aldehyde, reducing ammoniacal solutions of silver nitrate, etc. It has no corresponding chloride or anhydride, is readily decomposed into and , etc. 2. The first member in each family of the elements differs somewhat from the other members. Thus, lithium differs from the other elements of its family in forming an almost insoluble carbonate and phosphate. Oxygen, again, differs from sulphur, selenium and tellurium in that its hydride is a colorless and odorless liquid, while those of the others are gases of disagreeable odor; in that it is seldom, if ever, more than divalent; in being gaseous under ordinary conditions of temperature and pressure, etc.