IX] OF LIQUID OR FLUID CRYSTALS 485 



we could account for it, would make it comparatively easy to 

 explain the superficial configuration. But there are no obvious 

 mechanical forces by which we can so explain this peculiar 

 polarity. This at least is evident, that it arises in the central 

 mass of protoplasm, which is the essential living portion of the 

 organism as distinguished from that frothy peripheral mass whose 

 structure has helped us to explain so many phenomena of the 

 superficial or external skeleton. To say that the arrangement 

 depends upon a specific polarisation of the cell is merely to refer 

 the problem to other terms, and to set it aside for future solution. 

 But it is possible that we may learn something about the hues in 

 which to seek for such a solution by considering the case of 

 Lehmann's "fluid crystals," and the light which they throw upon 

 the phenomena of molecular aggregation. 



The phenomenon of "fluid crystalhsation " is found in a 

 number of chemical bodies ; it is exhibited at a specific temperature 

 for each substance; and it would seem to be limited to bodies 

 in which there is a more or less elongated, or "chain-like" arrange- 

 ment of the atoms in the molecule. Such bodies, at the appropriate 

 temperature, tend to aggregate themselves into masses, which are 

 sometimes spherical drops or globules (the so-called "spherulites"), 

 and sometimes have the definite form of needle-like or prismatic 

 crystals. In either case they remain liquid, and are also doubly 

 refractive, polarising light in brilliant colours. Together with 

 them are formed ordinary solid crystals, also with characteristic 

 polarisation, and into such solid crystals all the fluid material 

 ultimately turns. It is evident that in these liquid crystals, 

 though the molecules are freely mobile, just as are those of water, 

 they are yet subject to, or endowed with, a "directive force," 

 a force which confers upon them a definite configuration or 

 "polarity," the Gestaltungskraft of Lehmann. 



Such an hypothesis as this had been gradually extruded from 

 the theories of mathematical crystallography*; and it had come 

 to be beheved that the symmetrical conformation of a homo- 

 geneous crystalhne structure was sufficiently explained by the 

 mere mechanical fitting together of appropriate structural units 

 along the easiest and simplest fines of "close packing": just as 



* Cf. Tutton, Crystallography, p. 932, 1911. 



