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combine say an anion possessed of this power with an inactive cation, wMch in combination with most ions gives inactive salts, equivalent quantities of all the salts of this anioD must, in dilute solution, have equal rotatory powers. This has been confinned in one or two instances ; the salts of quinic acid in i⁠1/7⁠-normal solution give the following molecular rotations : —

Potassium 48.8° Barium 46.6° Sodium 48.9 Strontium 48.7 Ammonium 47.9 Magnesium 47.8.

The differences are quite inconsiderable. For the sake of comparison, it may be added that the molecular rotation of a ⁠1/7⁠-normal solution of quinic acid, which is very little dissociated, is 43.4°.

The molecular rotation of tartaric acid (little dissociated) is 15° at 20°, that of the salts is from 26.30° to 27.62°. Oudemans (9) and Landolt (10) have examined these salts of optically active acids and bases.

Light Absorption of Solutions. —Another optical An image should appear at this position in the text.Fig. 39. property of dilute solutions which is of great importance is their power of absorbing light. Ostwald, who carried out experiments in this direction, applied a photographic method (Jl). He produced on the same photographic plate spectral images of different solutions, which contained the same "coloured " ion in equivalent quantities with various cations. In most cases these absorption spectra are identical; Fig. 39 shows the absorption spectra of 0002-normal solutions of permanganates, MnO2 with 10 different cations. Only in a few cases were