Page:Outlines of Physical Chemistry - 1899.djvu/117

Rh 2. Substances which are active in solution are frequently inactive in the crystalline state, and vice versa. Rammelsberg ('Berichte d. d. ch. G.' 1869) distinguishes the following cases:

(a) Substances active only in the crystalline state: quartz, sodium chlorate, sodium bromate, and Schlippe's salt.

(b) Substances which are active in solution but inactive in the crystalline state: tartaric acid, malates, asparagine, the sugars, camphor, &c.

(c) A single substance active both in solution and in the crystalline form: strychnine sulphate (with water of crystallisation). (See note on page x.)

From these peculiarities Rammelsberg concluded that crystalline particles are not simple molecules, but are multimolecular associations. For substances of series a, a helical structure must be attributed either to the molecules in the crystalline particle or to the crystalline particles in the crystal. For substances of series b the dissolved isolated molecules must possess a rotatory power which they lose in forming the aggregates of the solid.

These conclusions have been partly confirmed. Thus, when the association factor of a solid substance is determined by I. Traube's method, it is generally found to be almost 2. There remains, however, the question how the juxtaposition of two active and identical molecules can produce an inactive aggregate. Here no pretence is made of giving a final solution of this difficult problem, but a tentative interpretation by means of figures may not be out of place. The separate tetrahedra of fig. 23 represent two active and identical molecules (because they are asymmetric and superposable). After joining two homonymous edges (2—8), we get a double molecule, and the inactivity of this can be shown by applying the principles on which we based the theory of optical isomers.

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