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312      Wisconsin Academy of Sciences, Arts and Letters.
          THE LIMONENE GROUP OF TERPENES.'
                     By EDWARD KREMERS.
  The following historical study was undertaken with the desire to throw
soni light upon thehistory of the terpenes. This-class of compounds
,has been of interest to chemists ever since organic chemistry may be re-
garded as a science. Year after year the material accumulated until by
the beginning of the last decade it constituted a special lumber-chamber
of chemical literature. The reasons for this disorder are of a varied
character. The thought that every volatile oil contained its peculiar
terpene, when such was present, for a long time seems to have prevailed
in the minds of chemical investigators. Another grave fault is to be
found in the fact that one investigator often gave but little or even no
attention to the researches of others. This brought about serious con-
fusions in the chemical nomenclature, which in ttirn gave rise to mis-
understandings everywhere. The reason why, even in later years, so
little could be done to clear up the subject is to be sought in the fact
that for the most terpenes, no characteristic reactions we're known.
Since at present, at least some systematic knowledge has been acquired,
it may not be without interest to look back and see who has identified
himself with the problems under consideration, who has aided in their
solution and who has retarded the same. In the course of years the
amount of material has accumulated to such an extent that a survey of
the same is a, difficult matter, even for the person who has made a special
study of the subject. In fact an understanding of the Limonene group
of terpenes became possible only after Prof. Wallach, in 1888, had
demonstrated the relatioiis existing between the members of this group.
  For the better understanding of the subject the following explanatory
remarks may serve as a brief introduction. The limonene group of ter-
penes consists of three hydrocarbons: the optically active, dextrogyrate
and lawvogyrate limonene and the optically inactive dipentene. Whether
the inactive compound resulting from the mixture of equal parts of the
optically active components is identical with dipentene still remains an
open question. Suffice it is to say that all crystallizable derivations of
such an optically inactive mixture are identical with the corresponding
* These notes are translated f rom a series of articles published on this
subject by the
writer in the " Pharmaceutische Rundschau " of 1891 and
1892.