314 
Wisconsin Academy of Sciences , Arts and Letters . 
salzsaures citronenol ”) with hydrogen chloride, and regard it as iso¬ 
meric with “ Dadyl ” that hydrocarbon of turpentine oil which is cap¬ 
able of yielding a crystallizable compound with hydrochloric acid. 
Soubeiran and Capetain (1840) distinguish for the first time between 
limonene and dipentene. They had determined the rotatory power of 
their original material (for the “ essence de citron rectifiee [or] = 80°.916 
dextrorotatory), but found the “ camphre de citron” (dipentene dihydro¬ 
chloride) obtained from it, as also the regenerated “citrene” (dipentene) 
to be optically inactive. The difference, they state, is the same as that 
between the “terebene” (o-Pinene) and the essence of turpentine (-Pi- 
nene). They also call attention to the fact that the “ essence de tereben- 
thine” (-Pinene) retains in the “ camphre solide” (Pinene-monhydrochlo- 
ride) its optical activity, and that “S’il est possible de supposer que 
l’essence de terebenthine soit entree sans alteration dans le camphre 
solide, on ne peut se refuser a reconnaitre que dans ancune de ses com- 
binaisons l’essence de citron n’a conserva son etat meleculaire primitif.” 
Soubeiran and Capetain had thus recognized and pointed out the differ¬ 
ence beeween the hydro-carbons we now designate as dextrogyrate limo¬ 
nene and dipentene, and had also indicated the inversion of one into 
the other. But, although they isolated and identified the dextrogyrate 
limonene from the oil of orange, they do not at all call attention to th e 
identity of the same with that from lemon oil. 
In ] 840 Schweizer 4 isolated from the oil of caraway a hydro-carbon, 
which he designated “ carven ” (+ Limonene). By passing hydrogen 
chloride into it he obtained a dihydro chloride melting at 50° (Dipentene 
dihydro chloride). The hydro carbon regenerated from this compound 
by means of lime he regarded as unchanged carvene. Based upon his 
analyses and vapor density determinations he assigned to the “ carven ” 
the formula C 10 H 16 Boiling point 173° C. 
It is quite apparent from what has been stated above that the different 
capacity for the saturation of hydrogen chloride was regarded as chief 
distinction between pinene and limonene. Based upon the direct for¬ 
mation of dipentene dihydro chloride from turpentine oil, Berthelot 5 
(1852) claimed that the distinction between turpentine oil and lemon oil, 
i. e., between pinene and limonene, was not at all essential. He seems to 
regard both as identical, though he admits a slight difference on account 
of the somewhat varying stability of the molecule of the two. As a 
further proof for the latter statement, he shows later that limonene is 
not as easily affected by heat as pinene 6 ~ 7 . 
Already in 1840 Volckel 8 had surmised the existence of a hydrocar¬ 
bon besides carvol in the oil of caraway. In 1853 9 he pointed out the 
apparent genetic relations existing between carvol and this hydrocar¬ 
bon, the so-called carvene (+ limonene). Although Volckel’s formulas 
have long been discarded the fact remains and is of historic interest. 
