102 proceedings: Washington academy of sciences 



influence of mechanical deformation, as demonstrated by Maxwell, 

 Mach, de Metz, and others; under dielectric stress, as shown by Kerr, 

 and also, according to Majorana, in strong magnetic fields. 



In all these cases the common point is that the birefringence disap- 

 pears always with the external forces which produced it— with most 

 liq ids instantly, with some gradually. The time required for the 

 birefringence to fall to the 1/e part of its original value has been defined 

 by Maxwell as the "time of relaxation," and is directly proportional to 

 the internal friction. The time of relaxation has never been found 

 greater than one and one-half hours, and this is much reduced with a 

 slight increase of temperature. 



It might be supposeel that liquid crystals are only liquids of very long 

 relaxation periods, but this view is not supported by accurate measure- 

 ments of viscocity nor does it agree with the observed close analogy 

 between the birefringent and the ordinary isotropic liquids. On the 

 other hand, it is a well known fact that crystals can be plastic to any 

 degree. In fact thepossiLLity of deforming crystals without destroying 

 them is of the highest practical importance in drawing and forging metals 

 and their alloys as Ewing, Rosehain, and others, have demonstrated 

 with the microscope. We might therefore ask what manner of substance 

 is a crystal that shows such a high degree of plasticity? 



Consider, for instance, the needle-like crystals that form on the carbon 

 plates of exhausted Leclanche cells, and have the composition ZnCl 2 

 + 2NH 3 . These can be wound spirally around an ordinary lead pencil 

 without breaking them or destroying their orthorhombic character. 

 Experiment shows that they possess one or more systems of so-called 

 "gliding planes" along which the molecules roll and glide, under the 

 influence of feeble forces, incomparably more easily than along any other 

 plane. The deformation of the crystal in these cases is called "singular 

 deformation along gliding planes" and does not alter the internal struc- 

 ture. Exact investigations have shown that the original symmetry 

 of the molecular aggregation remains wholly intact. A deformed crys- 

 tal of barium bromide, for instance, will continue growing in its saturated 

 solution and become a quite normal individual, which would be impos- 

 sible if the molecular arrangement of the crystal had been modified dur- 

 ing the deformation. Crystals may have a number of gliding planes; 

 those of the Leclanche cells have six, situated in two different zones. 

 Crystals can thus be deformed without alteration of their internal molecu- 

 lar structure, and this deformation can take place in several directions 

 at the same time, and the crystals therefore be highly plastic. 



Suppose we had a crystal of this kind whose molecules roll so easily 

 along a number of such gliding planes that even a weak force like gravi- 

 tation is sufficient to move them; it would be deformed under its own 

 weight without any change of internal structure — the crystal would 

 "flow." Looked at in this way the expression "fluid crystal" loses its 

 seeming absurdity. Such crystals exist and are closely related to ordi- 

 nary crystals from which they differ only in having a much smaller rate 

 of recovery after deformation, and a preponderant surface tension. 



