# Page:Popular Science Monthly Volume 82.djvu/588

 3. Several compounds of this series, which theoretically may exist, are unknown. Thus, between arachidic and behenic acids there is no acid corresponding to C_{20}. Between behenic and lignoceric there is none corresponding to ${\displaystyle {\ce {C_{22}}}}$. Similarly, several acids are missing between cerotic and melissic. 3. Many elements which theory predicts should exist are unknown in the Periodic Table. Thus, elements are missing between silver and gold, between cadmium and mercury, etc. 4. The vacant places are all found in the lower part of the series, i. e., among the heaviest molecules. 4. The vacant places all occur in the lower part of the Periodic Table, i. e., among the heaviest atoms. The first four periods are complete (excepting the manganese family). In the last three periods many empty places appear. 5. Isomeric forms occur in the series, e. g., butyric and isobutyric, caproic and isobutyl acetic acids. 5. Allotropic forms occur in several of the families, e. g., the various forms of phosphorus, of sulphur, of carbon. 6. In a homologous series in general, the melting points, boiling points and specific gravities change uniformly and progressively with increase in molecular weight. In this particular series (considering, as before, only the acids with normal structure) the boiling points and specific gravities show this progressive change, and the melting points do also from caprylic acid on.[1] 6. Generally speaking, the melting points, boiling points and specific gravities change progressively and uniformly in each family of the elements with increase in atomic weight. 7. The acidity decreases with increasing molecular weight. 7. The oxides of the elements become successively less acidic (or more basic) in each family with increasing atomic weight.
The above relations show that a family of the elements possesses all the characteristics of a homologous series. There is evidently some identity of principle in the two things compared. We know that in the one case there is in the whole series a common plan of molecular structure, the differences in the structures of the successive normal acids being due to the constant and progressive addition of the same group of atoms, ${\displaystyle {\ce {CH_{2}}}}$; and hence it seems reasonable to suppose that there is likewise in each family a common plan of atomic structure,[2] to which are due the properties common to a family.