220 REPORT—1904. 
limits. Statical methods of this kind are, therefore, of little use in detect- 
ing dynamic isomerism, and their chief value consists in the information 
that they give as to the constitution that must be assigned to the domi- 
nant form of the parent substance and to its isomeric derivatives. Thus 
Briihl,! by measuring the molecular refraction and dispersion, has shown 
that hydroxymethylene (formyl) camphor is mainly enolic, but its bromo- 
derivative has the aldehydic constitution ;— 
C : CH.OH Pepe. 
Cth | 
and, again, that ethyl camphocarboxylate is mainly ketonic, whilst its 
acetyl- derivative is enclic.? 
CH.CO,Et 0.CO,Et 
C,H, 7% HOSE pe Hold 
8 LN | 8 
co 
Perkin? has also made use of the magnetic rotatory power, a property 
that has the advantage of being much more sensitive to changes 
of structure; thus, whilst the calculated molecular refractions of the 
two forms of ethylic phenylformylacetate differ by less than 2 per 
cent., the magnetic rotations of the two possible forms of ethyl 
acetoacetate differ by 20 per cent. ; some of the results obtained by 
this method have already been indicated (p. 203). (2) An analogous 
method depends on the fact that the composition of the stable mixture 
may be influenced to a very considerable extent by physical condi- 
tions, and properties like the molecular refraction and the magnetic 
rotation which are normally almost independent of temperature, solvent, 
and concentration may vary widely when the material examined is 
an isodynamic mixture. Such variations may indicate the existence of 
dynamic isomerism,’ though similar effects are produced by reversible 
polymeric change and by association with the solvent. (3) A more 
sensitive method of detecting dynamic isomerism consists in following the 
physical changes which accompany isomeric change. In the case of a 
pure liquid compound the physical properties reach a constant value as 
soon as the temperature and other physical conditions are steady, but the 
existence of a time-factor is a sure indication of chemical change. Thus, 
when ethylic acetoacetate is distilled the proportions of the isomerides are 
altered, and some hours elapse before the substance recovers its normal 
density,” though the total change only amounts to 0:0013 gram per c.c. 
Since crystallisation normally results in the complete separation of one of 
the isomerides a maximum amount of isomeric change is observed when 
the crystals revert to the liquid state either by fusion or by dissolution. 
Thus it is sometimes possible by repeated fusion to follow the gradual fall 
in the melting-point as the homogeneous crystals revert to an equilibrium 
mixture, but usually isomeric change is so rapid that a single fusion is 
sufficient to bring about a condition of equilibrium. Dissolution has the 
advantage that observations can be made at atmospheric temperatures, 
and that isomeric change then proceeds much more slowly than in the 
fused state, but the properties of the solute are often very seriously 
1 Zeit. phys. Chem. 1900, 34, 31-61. 2 Ber. 1902, 35, 3510. 
3 Trans. 1892, 61, 801. 4 Perkin, Briihl, Joc. cit. 
5 Schaum, Ber. 1898, 31, 1964, 
