RESEARCHES ON THE VOLATILE HYDROCARBONS. 169 
loss of the substance, by which the quantity had become so much reduced that I could 
ill afford further loss. In the case, also, of cymole from oil of cumin, and cumole from 
cuminic acid, in which the boiling-point difference varies only 1°.5 from the common 
difference of 30°, the want of perfect agreement may be fairly accounted for by the 
fact that the quantity of cumole at command was too small to admit of continuing the 
process of fractioning far enough to obtain perfect constancy of boiling-point. In con- 
sequence, also, of the quantity being so small, the determination of the boiling-point of 
cumole is less reliable, as this had to be conducted in a test-tube. It came into full 
ebullition at 148'4, the temperature rising gradually to 151°.6 (observed temperatures), 
at which latter temperature it had distilled nearly to dryness. The distillation occu- 
pied thirteen minutes in passing over the range of three degrees. The average of the 
extremes, with the usual corrections for pressure, &c., was taken for the boiling-point. 
Abel,* who probably operated on a larger quantity, found the boiling-point of cumole 
to be 148°. It does not appear that he applied the corrections for pressure and the 
upper column of mercury. I do not doubt that the true boiling-point of this body 
will be found to be 150°, which would establish the difference of 30° between it and 
cymole. 
I would here remark that this difference of 30° for the addition of C,H, was first 
observed while engaged in fractioning Pennsylvania petroleum, and the oil from 
Albert coal, — substances the most difficult to separate, on account of the presence in 
each of two parallel series of constituents, whose boiling-points lie so near together. 
As no one had preceded me in the investigation of these substances, my mind was 
as far as possible unbiased as to the boiling-points of the constituents of these mix- 
tures. I was, however, aware of the beautiful relation between elementary constitu- 
tion and boiling-point which Kopp had discovered, and familiar with the fact that the 
more recent investigations had shown the boiling-point difference among homologous 
hydrocarbons to be about 22°.5. If there was any one thing which more than another 
tended to bias me, it was the recent work of Church} on the boiling-points in the 
benzole series, in which he made the boiling-point difference invariably 22° and a frac- 
tion, a number varying but 3° from the theory of Kopp. Soon after the publication 
of Church’s results, however, Kopp{ accepted the number 22°.5 as about the boiling- 
point difference in this series, therefore regarding it as one of the exceptional series in 
which the boiling-point difference is greater than 19°. The work of Church had cer- 
* Annalen der Chemie und Pharmacie, 1847, LXIII. 308. 
t Philosophical Magazine, 1855, (4.) IX. 256. 
{ Annalen der Chemie und Pharmacie, 1855, XCVI. 29. 
VOL. IX 26 
