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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 if-normal solution give the following molecular rotations : —

Fotaaainin 48-8" 1 Barium -HS-fi"

Ammonium .... 47*9 I MBgneaium .... 4T'8.

The differences are quite inconsiderable. For the sake of comparison, it may be added that the molecular rotation of a f-normal solution of quinic acid, which is very little dis- sociated, is 43'4°.

The molecular rotation of tartaric acid (little dissociated) is IS" 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.

Li8:ht Absorption of Solutions. — Another optical property of dilute solutions which is of great importance is ^ = | |

their jjower of absorbing light. ° ^

Ostwald, who carried out ex- ^ periments in this direction, applied a photographic method (Jl). He produced on the same photographic plate spec- tral images of different solu- " tious, which contained the **' same "coloured " ion in equiva- Mg lent quantities with various ca cations. In most cases these h absorption spectra are identi- ^, cal ; Fig. 39 shows the absorp- tion spectra of 0002-normal Fw. 3h. solutions of permanganates, MnOi with 10 different cations. Only in a few cases were

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