286 



NA TURE 



[January 7 1909 



is quite possible tliat sucli displacements as are caused 

 hy earthquakes might be troublesome to the map- 

 maker. Topography, however, can never be final; 

 never (under some conditions) complete. Col. Burrard, 

 in his admirable preface, aptly quotes the shifting- 

 Indus as a case in point. Could the whole Indus 

 A-alley be surveyed in any one year we could then 

 say " that was the course of the Indus in the year 



. " As it is we can never hope to possess an 



accurate topographical representation of the Indus 

 from the mountains to the sea at any one time — nor 

 does it much matter if we cannot. 



The expense and the labour of geodetic triangula- 

 tion undoubtedly imposes certain limitations on its prac- 

 tical use, and probably no record in scientific historv of 

 its misapplication is more remarkable than that which 

 may be found in the Government report on the Boundary 

 Survey between British Bechuanaland and German 

 South-west Africa. Here an elaborate series was 

 e.\tended at a ridiculous cost, and involving the labour 

 of several years, in order to determine the position of 

 a meridian line (running through the Kalahari desert) 

 which had been defined by diplomats in England as 

 the only possible boundary. The possibility of the 

 existence of gold or diamond mines demanded an 

 exact and visible demarcation no doubt ; but where 

 that demarcation was carried through the undeveloped 

 and waterless wilderness was not a matter of signifi- 

 cance, provided it were somewhere near the defined 

 line. It may be that the meridian (almost the worst 

 boundary definition possible) was without an alterna- 

 tive, in which case a most important word must have 

 been inadvertently omitted from the protocol, or 

 agreement. That word was " approximate." A free 

 use of it in the original definition, and a liberal 

 interpretation of it in the field, would have 

 enabled a topographer to run a plane-table traverse 

 quite sufficiently close to the meridian on a 

 " chronometric " longitude to have fixed up the 

 boundary marks as he proceeded, and so to have com- 

 pleted the whole boundary in, say, one-fifth the 

 time and at one-tenth the expense of the geodetic 

 determination. It is not as if this geodetic determina- 

 tion resulted in rigid (and unnecessary) accuracy. 

 Col. Burrard's preface to his eighteenth volume at 

 once disposes of any such possible pretension ; nor is 

 it as if it formed the basis for useful topography, for 

 not a square mile of topographv resulted. The only 

 result is a possibly useful basis for the extension of 

 future triangulation in German territory — and for this 

 the German Government should have paid. 



T. H. H. 



THE ROLE OF LIQUID CRYSTALS IN 

 NATURE. 



THIRTY-SIX years have elapsed since Prof. Otto 

 Lehmann, while a student at Stuttgart, de- 

 signed a novel form of microscope which permitted 

 of the optical examination of substances at tempera- 

 tures differing considerably from that of the 

 surrounding air, and thus obtained access to an 

 almost virgin field for research, to the cultivation of 

 which he has strenuously devoted himself. The 

 results of a long series of observations were collected 

 and published in the form of the fine v^olume entitled 

 " Flussige Krystalle," which was noticed in Nature 

 in 1904 (vol. Ixx., p. 622). Prof. Lehmann, however, 

 by no means intended that work to constitute his last 

 word on the subject, and, as is testified by the 

 numerous papers which have since that date appeared 

 from his pen in various journals, he has in no way 

 relaxed his efforts in the prosecution of his investiga- 

 tions. Of recent years, moreover, other workers have 



XO. 2045, VOL. 79] 



in greater number been attracted to the subject, and 

 their observations are, on the whole, in harmony with 

 his, and confirm the substantial correctness of the 

 views he has put forward. In particular, mention 

 may be made of Prof. D. Vorliinder's extensive in- 

 vestigations of the azoxy-compounds. Although there 

 was in early days, not unnaturally, considerable 

 scepticism regarding the correctness of Prof. Leh- 

 mann's observations and the deductions he made from 

 them, there is at the present time little reason to 

 doubt the reality of the existence of anisotropic 

 liquids and the importance of the rdle they play. 



At first sight it may seem ridiculous and absurd to 

 suppose that any immediate relation can subsist 

 between the properties of liquids and crystallised 

 matter. The study of the characters of crystals has 

 demonstrated that the molecules composing a crystal 

 are regularly arranged at the nodes of the corre- 

 sponding space-lattice. Such a structure possesses 

 great rigidity, a character incompatible with the 

 iiiobility of a liquid. It is, indeed, very probable, as 

 Mr. William Barlow suggests, that the spheres of 

 influence of the constituent atoms are all in contact 

 with their immediate neighbours, and the molecule has 

 no separate entity in the crystal. On the other hand, 

 in a gas the molecules have clearly a distinct exist- 

 ence; they are in constant motion, and for the greater 

 part of their course are remote from one another, 

 and, if not kept within bounds in some way, would 

 altogether part company. It is not ditficult to suppose 

 that a liquid may be a compromise between such 

 different states; it may retain, though to a lesser 

 degree, both the regularity of the solid and the 

 mobility of the gas. That extreme rigidity is not an 

 essential property of a crystalline structure is evinced 

 by certain minerals — mica being a conspicuous 

 example — which are susceptible of considerable bend- 

 ing without permanent derangement of the structure. 

 .Solid substances break when the limit of elasticity is 

 reached, or, in other words, when no further slipping 

 of the spheres of influence upon one another is possible 

 without a collapse of the equilibrium. There are, 

 however, substances with small rigidity in which a 

 greater amount of shear is possible; to these viscous 

 substances, of which the melted modification of silver 

 iodide is a familiar instance. Prof. Lehmann applied 

 (he term " fliessende Krystalle." Finally there are 

 substances with almost negligible rigidity in which 

 so much relative slipping is possible without a collapse 

 that, though anisotropic, they are as mobile as water; 

 these he h.'is called " flussige Krystalle." 



No sharp distinction can, however, be drawn 

 between the three groups. Indeed, one curious sub- 

 stance, the cthvl /)n)'a-a20xycinnamate, has been dis- 

 covered which is solid in the direction of the principal 

 axis, but fluid at right angles thereto. Further, some 

 substances, such as cholesterylcaprinate, have two 

 liquid modifications. Certain of them — /lara-azoxy- 

 anisol, for instance — become turbid on melting, but 

 on increased heating suddenly clarify at a definite 

 temperature. The turbid liquid was at first supposed 

 by many physicists to be an emulsion ; but recent 

 investigations by Dr. R. .Schenk and Dr. A. C. de 

 Kock indicate that the turbid liquid is a homogeneous 

 phase. The mutually repulsive action — possibly an 

 electromagnetic phenomenon — that characterises the 

 molecules of a gas takes in a liquid the form of an 

 "expansion-force," as it is termed by Prof. Leh- 

 mann. This force varies in different directions 

 according to the svmmetry of the molecule, and con- 

 sequently the envelope of the liquid crystal, as seen 

 in the microscope, is polyhedral, the corners being 

 rounded owing to the effect of surface-tension. The 

 contour is circular when the expansion-force is nearly 



