120 SCIENCE PROGRESS. 



extremely guarded manner ; and rightly so, for Lehmann is 

 an acute observer and experimentalist who has contributed 

 innumerable discoveries in the region of microscopical crys- 

 tallography, and it was during the study of certain crystal- 

 line preparations by the microscopical methods of which he 

 is a master that he encountered the remarkable phenomena 

 which are the subject of the present article. 



It was a botanist who, as in the case of so many 

 interesting physico-chemical discoveries, contributed the 

 initial observations. Dr. Reinitzer, of Prag, found that 

 Benzoate of Cholesteryl had ^wo melting-points, melting 

 first at 145° to a dull and then at 178° to a clear liquid ; also 

 (more curious still), that the former appeared to possess a 

 certain double refraction. He forwarded the substance to 

 Lehmann for microscopical study. 



Shortly afterwards, Dr. L. Gattermann prepared samples 

 of three other compounds — Azoxyphenol, Azoxyanisol and a 

 substance having the composition Ng O3 (Cg H^)^ CH3 C2 Hj 

 — which behaved in a somewhat similar manner. 



The crystals of Azoxyphenol, which we may take as 

 illustrating the behaviour of all the three latter compounds, 

 exhibit the following peculiarities. Warmed on a micro- 

 scope slide they are suddenly transformed, at a temperature 

 of 134°, into a substance which preserves the outline of the 

 crystal, is strongly doubly-refractive, becomes dark four 

 times when rotated on the microscope stage between crossed 

 Nicols, and therefore behaves in all these respects like a true 

 crystal. As is well known to all students of the subject, 

 crystals differ from other substances in being anisotropic 

 while they are homogeneous ; that is to say, all the pro- 

 perties of a crystal, while the same along parallel lines 

 within it, are in general different in different directions. In 

 the matter of their optical properties this character ex- 

 presses itself in the double refraction exhibited by all 

 crystals save those which belong to the cubic system ; and 

 as a result of this birefringence, if the crystal be placed 

 between two polarising- Nicol prisms whose principal planes 

 are at right angles, light is in general transmitted through 

 the combination and is only extinguished four times as 



