LIQUID CRYSTALS. 121 



the crystal is rotated through 360° on the microscope 

 stage. 



Azoxyphenol at 134°, therefore, behaves in these respects 

 hke a crystal, but, incredible as it may seem, is nevertheless 

 a liquid ; it does not retain a geometrical form, but is free 

 to move in all directions. 



If the preparation be still further warmed, it passes at 165° 

 into a third modification which Is also liquid, but not doubly 

 refractive. It is possible to contrive that this molten sub- 

 stance shall contain small portions of the first, birefringent, 

 liquid which float about in it as perfectly spherical drops. 



Crystals of Azoxyanisol and the remaining substance 

 resemble Azoxyphenol in their behaviour, the changes 

 taking place at 116" and 134° with the former, and at Sy" 

 and 140° with the latter. The drops are regarded by 

 Lehmann as really liquid crystals. It must be confessed 

 that from his account of their appearance their optical 

 characters do not seem to be precisely those of ordinary 

 crystals, although, as we shall see, they do somewhat 

 resemble those of warped or deformed crystals ; the direc- 

 tions of extinction of one of the drops viewed through the 

 crossed Nicol prisms constitute a series of radial and con- 

 centric lines corresponding in form to the equipotential 

 curves in a spherical conductor which is traversed by an 

 electric current entering at one end of a diameter and 

 leaving at the other end. Viewed along the diameter, the 

 drops present a black cross resembling that produced by 

 sphaero-crystals or by material having a spherulitic struc- 

 ture. 



Further, when viewed through one Nicol prism alone 

 they are seen to be dichroic and to present different colours 

 in adjacent quadrants. We have to do then at any rate 

 with doubly refractive drops which possess a symmetrical 

 disposition of their peculiar optical properties, and are un- 

 like any other drops or any other liquid previously de- 

 scribed. 



In recent years many crystals apparently belonging to 

 the cubic system have been found to be birefringent, and 

 long and fierce has been the strife between those who have 



