LIQUID CRYSTALS. 123 



grounds : one of his drops suspended in a liquid of a 

 density equal to its own assumes the form of a perfect 

 sphere, like Plateau's drops of oil in a mixture of alcohol 

 and water, and this would be impossible if the action of the 

 surface tension called into play any opposing elastic force. 

 He concludes that the particles of the drops are free to 

 move as in a true liquid. 



In the course of his numerous microscopic observations 

 Lehmann has adduced many examples of crystals which 

 have been mechanically deformed and continue to grow 

 with bent faces and curved outline. He argues that a 

 crystal does not cease to be one when it is bent ; and that 

 the extent to which it will recover its original form when 

 the tension is removed depends merely upon the elasticity of 

 the substance and not upon any essential difference between 

 crystallised and uncrystallised material. 



"According to current views," he says, "the crystalline 

 condition consists herein, that the molecules are arranged 

 in a regular order in which they are retained more or less 

 rigidly by the elastic forces. Under certain conditions the 

 system can be disturbed by heat, pressure, etc. ; it then 

 passes suddenly by virtue of local pressure into a new 

 system which is more stable under the altered conditions, 

 just as a half-opened pocket knife closes with a snap." 

 Against this view he enters a forcible protest. 



It is true that many crystals do behave in this manner ; 

 the researches of Mallard and of Lehmann himself have 

 disclosed numerous instances in which a crystal when 

 heated passes suddenly from one solid modification to 

 another without any transition through the liquid condition. 

 Nothing can be more startling than the behaviour of such 

 crystals when seen for the hrst time. 



A section of Boracite viewed in the polarising micro- 

 scope is seen to be traversed by a number of doubly re- 

 fracting lamellae ; but when the section is warmed to a 

 temperature of 265' a cloud seems to pass suddenly over 

 the crystal, which becomes quite dark; as the section cools 

 twin lamellae suddenly make their appearance again at the 

 same temperature. 



