July io, 1913] 



NATURE 



491 



units arranged so that their representative points are 

 repeated in space with extraordinary accuracy of 

 position, with production of unit cells or "bricks" 

 of absolutely identical dimensions throughout the 

 crystal, but the shapes of the grosser units themselves 

 arc identically similar and identically similarly orien- 

 tated in space. Suppose, however, that the force 

 of crystallisation, the directive molecular force con- 

 cerned in bringing the molecules together in this 

 regular order of marshalling, is only adequate just 

 to attain this marshalling of the grosser units into 

 a space-lattice formation, without being able to fix 

 the units about their own centres of gravity, a certain 

 amount of wobbling about the latter being still per- 

 mitted. We might, in' such circumstances, expect 

 that some of the properties of a crystal, dependent on 

 the space-lattice formation on lines of definite sym- 

 metry, such as the optical properties of double refrac- 

 tion and polarisation of light, would be developed and 

 exhibited, while the production of exterior plane faces 

 would be either only partial, with rounded edges and 

 the exhibition of plasticity and viscosity, or would 



IP % 



^*> -- . -• e - > - - . -<: 



■ 





• y 1 \ ,t 



fe£ 



Fig. i.— Liquid crystals of ammonium oleate. 



not be achieved at all, the objects produced being 

 still fluid. One cause of such a condition of partial 

 sin cess at crystallisation might well be that the sub- 

 stance was composed of a large number of atoms 

 arranged in a long chain, such as the well-known 

 " long' chain compounds " of organic chemistry, which 

 would offer considerable resistance to marshalling. 

 The author believes that herein lies the explanation 

 of the remarkable " liquid crystals " wjiich Prof. 

 Lehmann has made the subject of his particular 

 study, many of which are of just such long-chain 

 character. 



By the kindness of Prof. Lehmann, who has sent 

 over specimens of some of the most characteristic 

 of his substances for the special purpose of this lec- 

 ture, and of Mr. Poser, of Messrs. Zeiss, who con- 

 struct an admirably convenient form of heating micro- 

 scope and projection arrangement for demonstrating 

 the formation of liquid crystals and their behaviour 

 in polarised light, it is possible to exhibit some of 

 the typical phenomena of these interesting objects on 

 the screen. The substances in question are chiefly 

 such as form two or more polymorphous forms, each 

 NO. 2280, VOL. 91] 



stable within a limited range of temperature, and the 

 liquid crystals are usually the second phase observed 

 on allowing the truly liquid heated substance to cool ; 

 the liquid crystal phase is produced at a definite tem- 

 perature during the cooling, and persists through- 

 out a definite interval of temperature during 

 the continued cooling.' The view here put 

 forth is apparently in agreement with that of 

 Lehmann himself, as most recently expressed both 

 in letters to the lecturer and in a memoir of July 27, 

 1912, to the Heidelberg Akademie der Wissen- 

 schaften, in which he says that in all probability: 

 " Die Rundung der Formen hange zusammen mit 

 der Plastizitat der Stoffe und habe ihren Grund in 

 unzureichender molekularer Richtkraft, welche wohl 

 geniigt, ein Raumgitter herzustellen, nicht aber regel- 

 massige Treppenstufen, wie es nach Hauys Theorie 

 zur Btldung ebener Krystallflachen notig ware." The 

 formation of regular stepped faces (of invisibly minute 

 steps, "Treppenstufen") the lecturer considers to 

 occur only when the grosser units become fixed about 

 their centres of gravity or representative points, with 

 production of a truly solid crystal. 



But now let us pass to the considera- 

 tion of the internal structure of the 

 grosser or space-lattice units themselves. 

 Their symmetry may be, in simple cases, 

 similar to that of the space-lattice, but in 

 general this will not be so. Whatever the 

 stereometric arrangement of the chemical 

 atoms in the molecule may be, and, if 

 more than one molecule goes to form the 

 space-lattice unit, whatever their mutual 

 arrangement, and therefore whatever be 

 the outer configuration of the whole unit, 

 when the crystal is a truly solid one, the 

 force of crystallisation (now no longer 

 denied) is adequate to fix each space- 

 lattice unit, not only considered as a point 

 with reference to its neighbours, but as 

 regards its shape and its whole character, 

 pa"rallelwise and sameways orientated 

 with respect to its adjacent fellows, and 

 as close as possible to them. Also, if 

 more than one molecule goes to each 

 space-lattice unit, their mutual arrange- 

 ment is achieved on a definite plan, and 

 is the same for every space-lattice unit; 

 these constituent molecules of the latter 

 are also as closelv packed as possible. 

 The final result is thus to produce an 

 assemblage of chemical atoms in which not only the 

 demarcation frontier between the space-lattice units 

 disappears, but also that between the constituent 

 molecules in the cases of polymolecular grosser units. 

 We come, ultimately, in consequence, to a structure 

 of atoms each of which we may represent by a point. 

 Now, just as the genius of Frankenheim and 

 Bravais revealed to us the fourteen kinds of space- 

 lattices, so Sohncke made us acquainted with sixty- 

 five regular systems of points, including many of the 

 thirty-two classes of symmetry, but not all, which 

 von Lang had shown crystals to be capable of possess- 

 ing. Later the number was brought up to 230^ by 

 simultaneous and wonderfully concordant geometrical 

 researches by Schonflies in Germany, von Fedorow 

 in St. Petersburg, and Barlow in England, and 

 among these 230 all the thirty-two crystal classes are 

 represented, and no others. 



Hence, we come to the conclusion that the skeletal 

 framework of crystal structure is the molecular or 

 polymolecular space-lattice, and the detailed ultimate 



l Ammonium olrate (Fig. i\ rara-a70x< an'sol, para-nzexyphenetol, and 

 cholesteryl acetate were il'ustrated on the screen. 



