Page:Popular Science Monthly Volume 82.djvu/588

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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, ; and hence it seems reasonable to suppose that there is likewise in each family a common plan of atomic structure, to which are due the properties common to a family.