IX] OF LIQUID OR FLUID CRYSTALS 731 



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

 the definite form of needle-like or prismatic crystals. In either case 

 they remain Hquid, and are also doubly refractive, polarising Hght 

 in brilhant colours. Together with them are formed ordinary sohd 

 crystals, also with characteristic polarisation, and into such soHd 

 crystals all the fluid material ultimately turns. It seems that in 

 these hquid crystals, though the molecules are freely mobile, just 

 as are those of water, they are yet subject to, or endowed w4th, 

 a ''directive force," a force which confers upon them a definite 

 configuration or "polarity," the " Gestaltungskraft " of Lehmann. 



Such an hypothesis as this has been gradually extruded from the 

 theories of mathematical crystallography*; and it has come to be 

 understood that the s^onmetrical conformation of a homogeneous 

 crystalhne structure is sufficiently explained by the mere mechanical 

 fitting together of appropriate structural units along the easiest and 

 simplest lines of "close packing": just as a pile of oranges becomes 

 definite, both in outward form and inward structural arrangement, 

 without the play of any specific directive force. But w^hile our 

 conceptions of the tactical arrangement of crystalline molecules 

 remain the same as before, and our hypotheses of "modes of 

 packing" or of "space-lattices" remain as useful and as adequate 

 as ever for the definition and explanation of the miolecular arrange- 

 ments, a new conception is introduced when we find something hke 

 such space-lattices maintained in what has hitherto been considered 

 the molecular freedom of a liquid field; and Lehmann would per- 

 suade us, accordingly, to postulate a specific molecular force, or 

 " Gestaltungskraft " (not unlike Kepler's "facultas formatrix"), to 

 account for the phenomenon^. 



Now just as some sort of specific " Gestaltungskraft " had been 

 of old the deus ex machina accounting for all crystalline phenomena 

 (gnara totius geometriae, et in ea exercita, as Kepler said), and as 



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



t Kepler, if I understand him aright, saw his way to account for the shape of the 

 bee's cell or the pomegranate-seed; and it was for want of any such mechanical 

 explanation, and as little more than a confession of ignorance, that he fell back 

 on a facultas formatrix to account for the six rays of the snow-crystal or the five 

 petals of the flower. He was equally ready, unfortunately, to explain, by the 

 same facultas formatrix in acre, the appearance of a plague of locusts or a swarm 

 of flies. 



