TRANSACTIONS OF SECTION B. 489 
that the boiling-points of isomers were the same in the three cases examined, and 
therefore, probably, in all cases. 
The boiling-point of methyl alcohol was of great interest to Kopp, because, 
taking that of ethyl alcohol—about which there was general agreement—as 
correct, it should, according to his law, be 78°—19°=59°, while the temperatures 
actually observed varied from 60° to 66°. Kopp prepared a specimen of methy} 
alcohol, and found that it boiled at about 65°; but he had more faith in his law 
than in his experimental result, and he concluded that the methods of determining 
boiling-points were not sufficiently accurate to give results correct to within even 
1° or 2°. 
In 1854 he discussed the corrections which should be applied to thermometer 
readings, giving a table of corrections for the unheated column of mercury, and 
adopting the value 27 mm. per degree as the value of = for all substances, in 
order to reduce the observed boiling-point to that at normal pressure. He pointed 
out, also, that the height of the barometer should be reduced to 0° C. Taking 
advantage of Delff’s improved method of preparing and purifying methyl alcohol, 
Kopp made a fresh specimen from methyl oxalate and dried it with lime; but 
while Delff observed the boiling-point to be 60°, Kopp obtained the value 
65°°2—65°'8. He was still, however, inclined to think that, owing to bumping, the 
observed boiling-point was too high and that the true temperature should be 
about 60°. 
Meanwhile, in 1847 Kopp had examined sixteen liquids, including water, two 
alcohols, three fatty acids, and seven esters, and in 1854, as a result of his further 
determinations, he was able to compare the boiling-points—and also the molecular 
volumes—of a large number of substances, most of which were either alcohols, acids, 
or esters, and he at first adhered to his previous value of 19° for the rise of boiling- 
point due to the addition of CH,. Later in the same year, however, taking a 
wider survey and including hydrocarbons and their halogen derivatives, ethers, 
sulphides, and other compounds, he was obliged to admit that tbe difference is in 
some cases higher, in others lower, than 19°, but he still regarded these cases 
merely as exceptions to the law. In 1867 Kopp admitted that isomeric aromatic 
hydrocarbons have not always the same boiling-point, and that the difference for 
an addition of CH, was not always 19°; but he still believed that the difference 
for CH, was constant in any really homologous series—for example, 20°°5 for 
homologues of toluene, 18°°5 for those of xylene, and 16°5 for those of trimethyl- 
benzene. He also recognised the fact that isomeric alcohols have widely different 
boiling-points. 
Kopp published no later papers on the boiling-points of organic compounds, 
although he dealt fully with the question of molecular volumes in his final com- 
munication in 1889. 
Asa pioneer, Kopp had very great difficulties to contend against when he 
began his researches ; data were scanty and far from accurate, and the substances 
which could be most easily obtained and, it was thought, readily purified were, 
unfortunately, those which were the least likely to lead to normal generalisations. 
Water, the alcohols, and the organic acids all contain a hydroxyl group, and we 
now know that the physical properties of these substances are abnormal in nearly 
all respects, owing, probably to the fact that their molecules tend to associate 
together ; moreover, the esters, which are formed by the interaction of acids and 
alcohols, do not behave quite normally, and there is probably molecular association, 
though to a much smaller extent than with the hydroxyl compounds. 
There can be little doubt that if Kopp had been able, in the first place, to 
obtain a considerable number of pure substances of normal behaviour, such as the 
paraffins or their halogen derivatives, he would not have been led to the erroneous 
conclusions which he defended with such vigour for so many years. If we 
take the normal paraffins as the simplest class of organic compounds, we find 
that, instead of the boiling-points rising by equal intervals as the series is 
ascended, the rise, which is very large for the lowest numbers, becomes. smaller 
and smaller as the molecular weight increases. This fact is, of course, now well 
