488 REPORT—1904. 
Section B.— CHEMISTRY. 
PRESIDENT OF THE SECTION—-Professor SypNey Youne, D.Sc., F.R.S. 
THURSDAY, AUGUST 18. 
The President delivered the following Address :— 
THe researches of Hermann Kopp on the molecular volumes and boiling-poinis of 
chemical compounds extended over half a century, beginning with his inaugural 
dissertation on the densities of oxides in 1838, and concluding in 1889 with a 
review of the whole of the work done on the subject. In his second paper Kopp 
considered the molecular volumes of solid compounds, and arrived at the conclusion 
that truly isomorphous substances have the same atomic or molecular volume, but 
that in other cases the volumes are usually different. Schréder also made the 
same observation at about the same time. 
Now, isomorphous substances have analogous chemical formule, and are 
usually of similar chemical character, and it is interesting to notice that at this 
early date the fact was recognised that close chemical relationship is associated 
with similarity in physical properties. 
For about the first six years Kopp was engaged in the consideration of the 
results obtained by other observers, and from these results he deduced the most 
important of his generalisations. 
As regards boiling-points, Kopp, in 1842, concluded that a constant difference 
in chemical composition is accompanied by a constant difference in boiling-point, 
and he adopted the value 18° as the rise due to the replacement of the methyl] by 
the ethyl group in organic compounds, although the observed differences varied 
between 11°-0 and 24°°8. Two years later he found in sixteen comparisons differ- 
ences varying from 8° to 33°; but he doubted the correctness of the extreme 
values, and took 19° as the true value; he further suggested that this is the 
constant difference for an addition of CH, in any homologous series, and he pointed 
out that the observed difference was most regular in the case of the fatty acids. 
Kopp was also of opinion that isomeric compounds with the same composition 
and the same vapour density have the same boiling-point. 
The paucity of experimental data and the wide discrepancies between the 
results obtained by different observers induced Kopp to undertake the determina- 
tion of the boiling-points of various compounds, and, later, their molecular volumes 
at a series of temperatures, and it is interesting to note the comparative crudeness 
of his first attempts and the increasing attention which he paid to the purification 
of his compounds and to the elimination of thermometric and other errors, He 
first examined three pairs of esters in order to find whether isomeric compounds 
have really the same boiling-points. But he employed only calcium chloride as a 
dehydrating agent, and this would remove neither water nor the alcohol com- 
pletely; he was much troubled by the ‘ bumping’ of the liquids, and the tempera- 
tures he actually observed—with the thermometer bulb in the liquid—fluctuated 
considerably, and he could onlv, in most cases, take the lowest temperature 
observed as the most probable boiling-point. By so doing, and by making a fairly 
liberal allowance for residual errors, Kopp arrived at the erroneous conclusion 
