272 
DE. TTEEMANIV KOPP OX THE EELATIOX BETAVEEX BOILIXG-POIXT 
alcohol, CgHgOa, boils (Cahoues and Hofmaax) at 103°. (From Table V. this boiling- 
point was calculated at. 107°.) Acetone, CgHgOj, isomeric with the former, boils at 56° 
(Liebig, Dumas, Kopp). 
In cases lilce the last, the determination of the boiling-point of a compound, together 
with an examination of the substances with which the compound is serially allied in 
boiling-point, may be of great assistance in fixing its character, the t^'pe to which it 
belongs, and the series of homologous bodies of which it is a term. The examples just 
quoted of the fact that isomeric substances of different series possess different boiling- 
points, may here again serve as illustrations. The boiling-point of the so-called eugenic 
acid (Beuning, 248°; Williams, 251°) shows that this substance, althougli represented 
by the formula C 20 H 12 O^, cannot possibly be homologous to benzoic acid, Hg (boil- 
ing-point observed by Kopp, 250°; calculated. Table VI., 253°). This result might be 
inferred with the greatest probability from the boiling-point of the two substances, even 
if cuminic acid, isomeric with eugenic acid, and the homologue of benzoic acid, were not 
known. On the other hand, when the boiling-points of eugenic acid and of hydride of 
salicyl, Cj^HgO^ (boiling-point 196°, Pieia), are considered, it becomes extremely probable 
that these two bodies are homologous. (To the difference in composition, CgH 6 = 3 C 2 H 2 , 
corresponds in this case the difference in boiling-points 250 — 196 = 54 = 3x18°.) 
The boiling-points of polymeric compounds are also different, and may serve to deter- 
mine the formula representing a substance. 
Among the isomeric compounds, in the narrower sense of the word, acids and ethers 
(of the common formula !!„ O 4 , for example), or alcohols and ethers (of the common 
formula €„ FIn +2 O 2 , e. g.\ exhibit in all their bearings an essentially different character. 
Less striking is the difference of chemical character in the various volatile bases ; and it 
is only since the last ten years that distinctive features have been traced in several 
classes of these bases, which have accordingly been subdivided into primary, secondary, 
and tertiary monamines. The basic character possessed by all these substances has 
induced many to compare their boiling-points also, without duly considering to which 
of the several classes they might belong. But in this case, again, a comparison of boil- 
ing-points is admissible only for such bases as belong to one and the same of the above- 
mentioned classes. The same formula, G’n, Hjg N, represents amylamine, propyl-ethyl- 
amine, and methyl-diethylamine ; but of these three bases, only the first can be com- 
pared, as to boiling-point, with methylamine and ethylamine as a true homologue. The 
three isomeric bases above mentioned have indeed different boiling-points, just as ethyl- 
alcohol and methyl-ether, isomeric compounds (both C 4 Hg Oj), have different boiling- 
points. We may, however, expect that methyl-propylamine and diethylamine, both 
isomeric bases of the same chemical character (both secondary monaniines), will have 
the same boiling-point, just as in the case of the isomeric ethers of the same chemical 
character, propionate of methyl and acetate of etlij l. 
Homologous bases of the common formula C„ H „_5 N appear to exhibit, at least 
approximately, for a difference in the foimulte =.'r C 2 ITj a difference in boiling-points 
