LIQUID CRYSTALS. 125 



Now Lehmann has found that in many crystals the limit of 

 elasticity is so low that without actual disruption they may 

 by the application of very slight force be made to flow like 

 shellac or certain other amorphous substances. He has 

 long expressed his opinion that the essential feature of a 

 crystalline structure is not, as is commonly supposed, the 

 regular arrangement of particles which are held together by 

 elastic forces to form a more or less rigid structure. If this 

 were the case, he argues, a sufficient deformation would de- 

 stroy the structure and reduce the crystal to the amorphous 

 condition, and this has never yet been effected. 



He inquires, therefore, whether the limit of elasticity 

 may not in certain crystals be not only extremely small, as 

 in the soft substances previously described by him, but 

 actually zero, so that the material may be liquid and yet 

 crystalline. If a liquid exhibits polarisation phenomena 

 similar to those of a deformed crystal, is there any reason, 

 he asks, why we should not regard it as a liquid crystal ? 

 In a bent crystal the optic axes and the directions of extinc- 

 tion are no longer straight lines but curves, and the bire- 

 fringence is possibly to be attributed not to the relative 

 arrangement of the particles but to their anisotropic nature. 

 According to this view, the fusion of a crystal is simply its 

 conversion into a fresh crystalline modification in which the 

 limit of elasticity is zero, so that the substance behaves as 

 a liquid ; if the molten crystal is devoid of birefringence it 

 may be that it represents a modification belonging to the 

 cubic system. 



It is suggested that the soft crystals prepared by Dr. 

 Reinitzer and the liquid globules of Dr. Gattermann's 

 compounds are crystalline liquids which possess a very 

 strong double refraction. Reference must be made to the 

 original memoir of Lehmann for a full account of the 

 remarkable manner in which the liquid drops behave. 

 His figures represent the normal drops viewed in the 

 polarising microscope, when each resembles a globe with its 

 meridians of longitude and its parallels of latitude marked 

 upon it ; the axis appears to resemble in some degree the 

 optic axis of a uniaxial crystal. The spheres might be sup- 



