BuMa_2005_05 - Deutsche Bunsengesellschaft für Physikalische ...

DEUTSCHE BUNSEN-GESELLSCHAFT
Lehmann's polarizing microscope
Lehmann immediately launched a systematic study, first of cholesteryl
benzoate, and then of related compounds which exhibited the
double-melting phenomenon. With his microscope, he was not only
able to make observations in polarised light, but also, and this was a
key advantage, his microscope possessed a hot stage enabling in situ
high temperature observations. The intermediate cloudy phase clearly
sustained flow, but other features, particularly the signature under a
microscope, convinced Lehmann that he was dealing with a solid.
By the end of August 1889 he had his own article ready for submission
to the Zeitschrift für Physikalische Chemie [2] . The tone of this article,
whose first paragraph will be of some interest to readers, not only
isolated the key problem, but also gives some idea of the nature of
Lehmann's personality.
A S P E K T E
The flowery and somewhat pompous language was to remain a characteristic
both of Lehmann's scientific and of his more popular works.
He found that the cloudiness of the intermediate fluid occurred when
what we would now call nucleating droplets merge, and that sometimes
the individual droplets exhibited a black cross when viewed between
crossed nicols. The cloudiness itself was the macroscopic manifestation
of 'large star-like radial aggregates of needles'.
Lehmann was certain that the cloudy liquid possessed simultaneously
liquid and crystal attributes, and believed truly to have discovered
'crystals that flow'. Much of the rest of the article is concerned with
advocating the coexistence of liquidity and crystallinity in the same
material, and is not without rhetorical flashes. He must clearly have
expected to meet with significant opposition.
In a series of papers over the period 1890-1900 [3] , Lehmann made
exhaustive studies of the phenomenon. Because the essence of the
phenomenon seemed to occur in droplets, he made a virtue out of
necessity and often deliberately prepared fluid mixtures from which
the intermediate phase would then precipitate in droplet form. The
coloured photographs of droplets in Fig 3. (and reproduced on the front
page of this issue of the Bunsen Magazine) are taken from Lehmann's
review article published in 1900.
Lehmann's photographs of liquid crystal droplets. These dramatic pictures, originally
taken only in black and white, were coloured by hand so as to resemble what he
saw under the microscope.
Lehmann found materials some of which exhibited, as in cholesteryl
benzoate, two melting points, and some of which even exhibited
three melting points. He found a phase which he called Fliessende
Kristalle (flowing crystals) or Schleimig flüssige Kristalle (slimy liquid
crystals), and another which he named Kristalline Flüssigkeit (crystalline
fluid) or Tropfbar flüssige Kristalle (liquid crystals which form drops). If
both phases existed in the same material the latter was always the
higher temperature phase.
The latter was cloudy, but the former was clear, although very viscous.
All this culminated in a generously-sized 260 page tome [4], including
no less than 483 illustrations drawn from his microscopic observations,
published by Wilhelm Engelmann in Leipzig in 1904.
His observations were quick to attract the attention of colleagues. As
early as 1890, the organic chemist Ludwig Gattermann (1860-1920),
at that time a Dozent from the University of Heidelberg and later to
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