��TEE POPULAR SCIENCE MONTHLY
��3. Several compounds of this series, which theoretically may exist, are un- known. Thus, between arachidic and behenic acids there is no acid corre- sponding to Ch). Between behenic and lignoceric there is none corre- sponding to C22. Similarly, several acids are missing between cerotic and melissic.
4. The vacant places are all found in the lower part of the series, i. e., among the heaviest molecules.
��5. Isomeric forms occur in the se- ries, e. g., butyric and isobutyric, ca- proic and isobutyl acetic acids.
6. In a homologous series in gen- eral, the melting points, boiling points and specific gravities change uniformly and progressively with in- crease in molecular weight. In this particular series (considering, as be- fore, only the acids with normal struc- ture) the boiling points and specific gravities show this progressive change, and the melting points do also from caprylic acid on. 1
7. The acidity decreases with in- creasing molecular weight.
��3. Many elements which theory pre- dicts should exist are unknown in the Periodic Table. Thus, elements are missing between silver and gold, be- tween cadmium and mercury, etc.
��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 (ex- cepting the manganese family). In the last three periods many empty places appear.
5. Allotropie forms occur in several of the families, e. g., the various forms of phosphorus, of sulphur, of carbon.
6. Generally speaking, the melting points, boiling points and specific gravities change progressively and uniformly in each family of the ele- ments with increase in atomic weight.
��7. The oxides of the elements be- come 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, 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.
1 The physical constants here used (as well as the tabulation of the acids) are those given by Leathes, The Fats," pp. 10-11. Other authors include several acids (e. g., pelargonic, undecylic) not mentioned by Leathes.
3 For example, grouping of electrons, according to the well-known theory of J. J. Thomson.