198 REPORT—1904. 
raised. A high temperature can scarcely do more than increase the 
activity of the impurities present in the material but these may easily be 
introduced at high temperature owing to incipient decomposition or to 
action on the walls of the containing vessel. 
5. Equilibrium is determined by the Velocities of Isomeric Change in 
opposite directions.—The proportions of the isomerides in the ultimate 
mixture is determined by the ratio of the velocities with which they 
undergo isomeric change under the given conditions. If these velocities 
are equal there will be 50 per cent. of each isomeride, but if one undergoes 
change ninety-nine times as fast as the other, there will only be | per cent. 
of it in the mixture. ‘Complete’ isomeric changes are merely limiting 
cases in which one velocity is small compared to the other, and as it is 
difficult to detect a back action in which the ratio of the velocities is greater 
than 100 to 1, the distinction between complete and incomplete changes is of 
very small importance. It may be added that solids are usually incapable 
of undergoing isomeric change but if such a change should occur (owing, 
for instance, to incipient fusion or to the presence of a trace of solvent), 
it is usually complete, back action being possible only in the liquid or 
gaseous state. Thus Walker and Hambly ' have shown that the conversion 
of ammonium cyanate into urea, which is complete when the solution is 
evaporated to dryness, is reversible in solution. In a normal solution at 
59-6°, 14:4 per cent. of ammonium cyanate undergoes isomeric change in 
a minute, and 0:0038 per cent. of urea. Equilibrium is reached when 
there is 5 per cent. of cyanate and 95 per cent. of urea, as indicated by 
the equation 
[5 %] NH4.0.0 N 2 CO(NH,), [95 %]. 
The concentration of the ammonium cyanate is then only N/20, and 
its velocity of change is reduced to half that in normal solution. Equi- 
librium is reached when the ratio 95/5 of the concentrations is equal to 
the ratio 7:2/0-0038 of the velocities of change in opposite directions.” 
Ammonium cyanate changes slowly in aqueous solutions at ordinary 
temperatures ; at 100° equilibrium is reached almost immediately. Am- 
monium thiocyanate, on the other hand, does not change in aqueous 
solution, but in the fused state at 170° it changes at about the same rate 
as ammonium cyanate in aqueous solution at 60°. The predominance of 
the thiocyanate in the equilibrium 
[75-7 %] NH,S.C: N @ CS(NH,), [24:3 %] 
is in marked contrast to the instability of the cyanate. 
6. Isomeric Change proceeds according to a simple Logarithmic Law, and 
the Period of Change is the same for different Isomerides.—The logarithmic 
law has been verified in the case of glucose and other sugars,‘ in the case 
of z-bromonitrocamphor’® and in many other instances. If, however, 
one of the isomerides is an active catalyst, or if by reason of association or 
dissociation only part of the material is in a condition to undergo isomeric 
change, the velocity-constant may vary as the concentration changes. In 
the absence of such disturbances the period of change must be the same 
1 Trans. 1895, 67, 746. 
2 For observations on the alkyl thiocyanates see Walker and Appleyard, Trans. 
1896, 69, 193 
$ Reynolds and Werner, Frans. 1903, 88, 1. 
4 Osalca, Zit. phys. Chem. 1900 35, 661, 5 Lowry, Zrans. 1899, 75, 227. 
