Secrion III, 1902 [118 ] Trans. R. 8. C. 
X1L.— Oudemans’ Law and the Influence of Dilution on the Molecular 
Rotation of Mandelic Acid and its Salts. 
By J. WazLacE WaLKER, M.A., Ph.D. 
(Read May 27, 1902.) 
From an examination into the influence of dilution upon the 
property possessed by solutions of the salts of the alkaloids of rotating . 
the plane of a beam of plane-polarized light, Oudemans deduced the 
law that in dilute solution the molecular rotatory power of an alkaloid 
is independent of the nature of the acid with which it is combined. 
For example, he found that solutions containing a molecular weight 
of quinine in 20 litres of water showed approximately the same rota- 
tory power when combined with the following acids: 
Acid HCl HNO, HCIO, C,H,0, CH,0, H,SO, C,H,0, H,PO, HClO, 
WE, NET Faea ses 270°. 2819. 2792 272°. 2805) 2882 
Similar results were obtained for a number of other alkaloids by 
Oudemans, Schwebel, Carrara, and others. 
Landolt examined in a similar way the metallic salts of the 
optically active tartaric acid and arrived at a similar conclusion, viz., 
that in dilute solution the molecular rotation is independent of the 
nature of the metallic constituent of the salt. The values for the 
neutral salts were 
i 
Li Na K NH, Mg 
: +38°6° 39-:9° 43° 42° 412, 
for the Acid Salts +-28°5° 21:5° 28:3° 280: = 
van’t Hoff in his “ Die Lagerung der Atome im Raume,” p. 103, gives 
a table of fifteen such acids whose salts have been examined in this 
way, and I have since, along with Professor Purdie, investigated lactic 
acid along the same lines. In the main the results obtained sustain 
the conclusion that, although the molecular rotations of the various 
salts of an optically active substance may differ from each other in 
concentrated solutions, they all, on dilution, approach the same value. 
This law, like so many others regarding the dilute aqueous solu- 
tions of salts fell at once into line with the ionic theory of solution, 
for, according to the latter, these dilute solutions of the salts of the 
alkali metals and of the strong acids contain but few whole molecules 
of the salt. The molecules have become dissociated by dilution into 
separate ions, and, therefore, the dilute solutions of different salts 
of an optically active acid contain the same unit upon which the 
amount of this activity depends, viz., the optically active ion. Excep- 
