PKOCEEDINGS: WASHINGTON ACADEMY OF SCIENCES 103 



Fluid crystals, like ordinary crystals, possess form, elasticity, sur- 

 face tension and molecular vector forces. Presumably, therefore, a 

 real distinction exists between these fluid crystals and the true "aniso- 

 tropic liquids," like p-azoxyphenetol, for here no trace of form elasticity 

 remains. The numerous trunsi.ious of these latter and their whole 

 physico-chemical behavior are quite analogous to those of fluid crystals, 

 but th( ir behavior under the microscope differs in so many respects from 

 that of the true liquid crystals that their real nature is still enigmatic. 



All substances showing the phenomena of fluid crystals, or anis tropic 

 liquid phases, behave exactly like polymorphic substances. All the 

 transition phenomena shown by these compounds during the gradual 

 rise or fall of temperature of the system can be explained in just the same 

 way as in the case of polymorphic substances undergoing a series of 

 phase-transitions. Even the same two kinds of polymorphic trans- 

 formations, the "enantiotropie" (reversible) and the "mcnotropic" 

 (irreversible) are met with again here; and just as there are substances 

 with two, three, four and more different solid polymorphic modifica- 

 tions, so there are substances which exhibit two, three, four or more 

 liquid states. The liquid which appears last, or at the highest transition 

 temperature, is always the isotropic one; the others are turbid-looking 

 aggregations of fluid crystals, and notwithstanding the fact that they 

 are stable at lower temperatures, the fluid crystals are often of much 

 lower viscosity than the isotropic liquid. Fluid crystals are true homo- 

 geneous phases as has been proved in a number of ways. 



All anisotropic liquid phases have a turbid appearance. They look 

 as if they were emulsions like oil and water. However, this appearance 

 of turbidity is not at all indicative of a close analogy to emulsions, but 

 is a necessary consequence of the particular structure of these phases, 

 for they are really aggregations of innumerable little fluid crystals each 

 of which is completely transparent. These little crystals, like ordinary 

 crystals, are birefringent, and when in great multitude and irrregularly 

 oriented must be turbid as a whole from the innumerable refractions 

 and total reflections of the transmitted light at the boundaries between 

 adjacent individuals. 



It has been asked why liquid crystals do not flow together. Real 

 liquids do so; but they possess no molecular directive force. On cooling, 

 an isotropic liquid is transformed into an aggregation of liquid crystals 

 and not into a single one, for the same reason that a molten substance 

 never solidifies to one big solid crystal but always to a turbid aggregate 

 made up of a great number of irregularly oriented crystals. 



The strange and disturbing phenomena which are displayed by fluid 

 crystals and in particular this misunderstood turbidity, have led some to 

 suppose that these substances are not pure compounds but are mixtures 

 of imperfectly miscible liquids. To be sure, liquid emulsions feebly 

 depolarize transmitted light; but they can never account for the system- 

 atic variation of the enormously strong birefringence with the direction 

 of transmission through these flowing bodies — a birefringence which is, 

 in some cases, several times that of calcite. And there are many other 



