ON DYNAMIC. ISOMERISM. 221 
disguised by admixture with the solvent. Thus it would be almost 
impossible to detect with certainty the slight change in density or refrac- 
tive index which would result from a partial isomeric change in solution, 
and the change in magnetic rotatory power, though still appreciable, 
would be much smaller than in a fused substance. For this reason the 
method of dissolution only gives the best results when the physical 
property utilised has a zero value in the case of the solvent, and differs 
considerably in the different isomerides. These conditions are fulfilled by 
(1) conductivity, (2) optical rotation, (3) solubility, (4) colour. 
The electrical conductivity has been used with remarkable success by 
Hantzsch,! who has followed the gradual decrease of conductivity in 
solutions of the pseudo-nitroparafiins and similar compounds, freshly 
prepared by mixing a solution of the sodium, barium, or silver salt with 
a mineral acid-—e.g. :— * 
atin sit +-— —i+ 
H Cl+CH,.CH : NO, Na2.Na Cl+CH,.CH : NO, H 
ete 
2Na 'Cl+CH,.CH,.NO,. 
Unfortunately the method is somewhat limited in its application, and 
cannot readily be applied to substances that are insoluble in water, or are 
only feeble electrolytes. 
Perhaps the most generally applicable method is that which consists 
in observing the mutarotation of freshly prepared solutions of optically 
active bodies. The isomerides usually differ widely in rotatory power, 
and observations can be made in any solvent and in fairly dilute solution. 
The rapidity and accuracy with which measurements can be made render 
it possible to detect changes involving only a small percentage of the 
material or taking place so rapidly that equilibrium is reached in the 
course of a few minutes. Moreover, the conditions are such that the 
behaviour of highly purified materials can be successfully investigated. 
The method has already been applied in a large number of typical cases, 
and can be extended to nearly every type of isomeric change. 
Hitherto the solubility has only been utilised in a limited number of 
cases,” but the method, though more tedious and perhaps less accurate 
than those just described, is even more widely applicable, and has the 
unique advantage that it gives information as to the proportions of the 
isomerides in solution, even when only one of these can be isolated. 
The recent observations of Dobbie, Lauder, and Tinkler on the ultra- 
violet absorption-spectra of cotarnine * show that colowr may be made the 
basis of a quantitative method, and the valuable results that have been 
obtained justify the hope that the method will be applied in many other 
cases. 
7. Colour,—As a qualitative method the colour of dynamic isomerides 
has proved most valuable in indicating the occurrence of isomeric change, 
and it has the unique merit of rendering visible to the eye the progress of 
isomeric change both in the solid and in the liquid state. Moreover, 
since it has become possible to associate colour with definite types of 
structure, and even roughly to predict the probable colour of a compound 
having a given formula, it is often possible to determine, by means of the 
colour, the constitutions that must be assigned to a series of solid 
1 Ber. 1896, 29, 699, 2256; 1899, 32, 607, 628, 641. 
2 Lowry, Proc. 1993, 19, 156; Proc. 1904, 20, 108. 3 Trans. 1903, 88, 598. 
