CRYSTALLOGRAPHY.1. INTRODUCTION.THIS Report is divided into four main sections, dealing with the morcphysical aspects of crystallography (Z), metal structures (3), inorganicstructures (4), and organic structures (5). Space does not permit a veryfull treatment of all these subjects, and in some cases it has been necessaryto confine the account to certain special topics only. Attention should bedrawn, however, to a number of general articles which have recently appeareddealing with various aspects of X-ray technique, including the determin-ation of equilibrium diagrams,l the accurate determination of lattice spac-ings,2 the construction of powder cameras,3 photometry,* and relatedsubjects.In last year’s Report we referred briefly to the subject of “diffuse”reflections from crystals, which are additional to the normal Laue pattern,and can be obtained with monochromatic X-rays.A very large amountof new literature on this subject has been published during the year, andthis is reviewed in Section 2 . At first sight, and taken as a whole, theeffect of these new contributions may be to confuse rather than to clarify.Nevertheless, the experimental study of the subject is now much morecomplete, and it is clear that the extra, non-Laue reflections may havemore than one origin. Some of these reflections are “ structure sensitive ” :they depend on the history and treatment of the particular crystal speci-men, vary but little with temperature, and are no doubt associated withsome kind of internal strain. On the other hand, we have the true “ tem-perature sensitive ” diffuse spots, which are greatly enhanced at hightemperatures and disappear at low temperatures. The balance of theevidence appears to indicate that the latter are due to elastic thermalvibrations in the crystal, which give rise to new regularities in the densitydistribution, and hence to new crystal reflections at high temperatures.In the field of metal and alloy structures (Section 3) relatively few newtypes have been discovered in recent years.There have been many im-provements in technique, however, and details of some of the earlier struc-tures can now be more accurately defined. A notable example of suchwork is the precise location of the carbon atoms in cementite, Fe,C.Anotherimportant field which is now being actively pursued lies in the study ofintermediate structures during transformations in the solid state, such asthe nature of the precipitation of copper in an alloy of copper and aluminium.Another interesting study of structural imperfections of another sort hasbeen carried out in the case of cobalt, and an explanation of the existencel A. J. Bradley, (Sir) W. L. Bragg, C. Sykes, J . Iron Steel Inst., 1940, 141, 63.H. Lipson and A. J. C. Wilson, J . Sci. Iwtr., 1941, 18, 144.A. J. Bradley, H. Lipson, and N. J. Petch, ibid., p. 216.J. M. Robertson, R. H. V. M. Dawton, and A. H. Jay, ibid., pp. 126, 12892 CRYSTALLOGRAPHY.of aharp and " fuzzy " diffraction lines in the X-ray photographs has beengiven.A large amount of new data has been obtained from the general in-organic structures (Section 4) and this includes many new values for inter-atomic distances, by both X-ray and electron-diffraction methods.A fewgeneral papers have been published, and a rather drastic revision of co-valent single bond radii for F, 0, and N has been proposed. One feelsthat the new values should be treated with some reserve until the numerousstructures which involve their use have been further studied by the mostaccurate methods. Some further interesting results have been obtainedduring the year among structures containing complex ions, and these includedata bearing on the constitution of the ferrocyanides. From the niobatesfurther information has been obtained on the nature of the 7-co-ordinatedcomplex, with the analysis of two almost equally stable types.Very few complete investigations have been made for organic structuresduring the year, and this field is reviewed briefly in Section 5.The natureof the intramolecular fold in a-keratin and a-myosin has been discussed atlength, and the result promises to be fundamental in protein structure, notonly for the fibrous proteins, but for the corpuscular or globular proteinsas well. It remains to be seen whether more detailed analysis of existingdata, when that becomes possible, will bear out and refine the new ideas.J. M. R.2 . TEMPERATURE EFFECTS IN THE REFLECTION OF X-RAYS FROMCRYSTALS.The Report for 1940 1 described preliminary experimental and theoreticalwork on the " diffuse " reflections found on well-exposed Laue photographsof many crystalline substances.The year 1941 has greatly increased theliterature on this subject, notably by means of a Discussion in the Pro-ceedings of the Royal SocietyY2-l1 by a series of papers in the Proceedings ofthe Indian Academy of Sciences,12-15 and in the Phygical Review,16-24 andby articles and letters inAnn. Reports, 1940, 37, 167.* G. D. Preston, Proc. Roy. SOC., 1941, A, 179, 1 (cf. 1939, A , 172, 116).3 (Mrs.) I(. Lonsdale and H. Smith, ibid., 1941, A, 179, 8 [28 Plates].4 (Sir) W. H. Bragg, ibid., pp. 51, 94.6 C. G. Darwin, ibid., p. 65. G. I. Finch, ibid., p. 67.8 M. Born and (Miss) K. Sarginson, ibid., p . 69.9 (Sir) C. V. Raman, ibid., pp.289, 302.10 (Mrs.) I(. Lonsdale, ibid., p . 315.l2 (Sir) C. V. Raman and P. Nilakantan, Proc. Indian Acad. Sci., 1940, 11, A, 379,13 ( S i r ) C. V. Raman and N. S. Nagendra Nath, ibid., 12, A , 83, 427.14 (Sir) C. V . Raman, ibid., 1941, 13, A, 1.15 S. Bhagavmtam and J. Bhimasenachar, ibid., 1940,12, A, 337; 1941,13, A, 266.l6 G. E. M. Jauncey md 0. J. Baltzer, Physical Rev., 1940,68, 1116; 1941, 59, 699.17 G. E. M. Jauncey, ibid., p. 456.18 G. E. M. Jauncey, 0. J. Bdtzer, and D. C. Miller, ibid., p. 908.6 (Sir) W. L. Bragg, ibid., p. 61.l1 H. A. Jahn, ibid., p. 320.389, 398; 12, A, 141LONSDALE : TEMPERATURE EFFECTS. 93The earliest example of diffuse reflection (by white radiation) is to befound on Laue photographs taken by Friedrich in 1913; this was correctlyinterpreted as a temperature effect by H.F a x h in 1923.35 A more com-plete mathematical treatment of the effect of thermal vibrations in crystalsgenerally upon the interference of X-rays was given, in terms of the earlierquantum mechanics, by I. Waller in 1925.36have verified Waller's formulae by means of a more rigorous quanfum-mechanical deduction. The first deliberate experimental test of this theory,which indicates that the diffuse reflection depends upon the crystal orient-ation and thus conflicts with the better-known Debye theory, was madoby J. L a ~ a l , ~ ~ who, using ionisation spectrometer methods and mono-chromatised Cu and Mo radiation, showed that the general predictions ofthe Waller theory were essentially correct for the crystals examined (potas-sium and sodium chlorides, aluminium, calcite, diamond, and a powderedspecimen of silver, the first over a temperature range of 289" to 665" K.).Preston 2* 29 independently made similar observations by photographicmethods; but his suggestion that the results could be interpreted by theassumption of small groups of atoms scattering independently of neigh-bouring groups, although receiving considerable early support,43 6~ 16, 179 25* 28was also adversely criticised.59 193 323 33(Sir) C. V. Raman and his collaborators, who put forward an entirelydifferent theory, vix., that the X-rays excite the characteristic lattice vibra-tions and are reflected with a very small change of frequency by the newstratifications of density thus set up,12, 139 149 26n 279 *O have published someexcellent diffuse spot photographs of sodium nitrate, calcium carbonate,and sodium chloride; and have shown that in the first case a big tem-perature effect exists which varies for the different crystal planes.Asomewhat different derivation of the thermal theory has been given byZachariasen 19 and it has been shown that the various reported results forpotassium and sodium chlorides 12, 16, 2o are in quantitative agreement withOther mathematicians 379 389W. H. Zachariasen, Physical Rev., 1941, 59, 207, 766, 860, 909.Zo S. Siege], ibid., p. 371 ; R. Q. Gregg and N. S. Gingrich, ibid., p. 619.21 P. Kirkpatrick, ibid., p. 452.23 (Sir) C. V. Raman and P.Nilakantan, ibid., p. 63.24 (Mrs.) K. Lonsdale and H. Smith, ibid., p. 617.2 5 (Sir) W. H. Bragg, Nature, 1940, 146, 509; 1941, 148, 112.26 (Sir) C. V. Raman and P. Nilakantan, ibid., 1940, 146, 523, 686 ; 1941, 147, 1 1 8.* 7 (Sir) C. V. Raman, P. Nilakantan, and P. Rama Pisharoty, aid., p. 805.28 G. E. M. Jauncey, ibid., p. 146.30 (Mrs.) K. Lonsdale, ibid., 1940, 146, 806; 1941, 147, 481.s1 H. A. Jahn and (Mrs.) K. Lonsdale, ibid., p. 88.32 M. Born, ibid., p. 674. 33 H. A. Jahn, ibid., p. 511.34 (Mrs.) K. Lonsdale and H. Smith, ibid., 1941, 148, 112, 257, 628.35 2. Physik, 1923, 17, 266. 36 Diss., Uppsala, 1926.37 M. v. Lam, Ann. Physik, 1926, 81, 877; 2. Krist., 1927, 65, 493.38 H. Ott, Ann. Physik, 1935, 23, 169.39 Conipt. rend., 1938, 207, 169; 1939, 208, 1612; Bull.SOC. franc. Min., 1939, 62,40 (Sir) C. V. Raman and P. Nilakantan, Current 8&, 1940, 9, 165.0. J. Baltzer, ibid., 1941, 60, 460.29 G. D. Preston, ibid., pp. 358, 467.137 ; C. Mauguin and J. Laval, Compt. rend., 1939, 208, 144694 CRYSTALT,OCtRAPT€Y.his formula, which, like those of Faxen and Waller, take account of theelastic constants of the crystal, since these influence the frequencies andamplitudes of the elastic (thermal) vibrations. Raman 93 14 and Nila-kantan23 claim, however, that their results for diamond provide the mostrigorous proof that it is the optical (characteristic) frequencies and not theacoustical (elastic) frequencies which are chiefly instrumental in giving theextra non-Laue reflections. Lonsdale and Smith, who have contributed awide experimental survey of the geometrical and physical conditions govern-ing the appearance of diffuse reflections for crystals of organic and inorganicc~mpounds,~ have shown that diamond is very far from being either anideal or a typical crystal.l0.24* 32 The relatively sharp anomalous reflec-tions from diamond, to which Raman and Nilakantan first called attention,are in fact due to some structure-sensitive cause. They are given in varyingintensity by normal diamonds, but are completely absent from photo-graphs of the (more perfect?) diamonds classed as type 11.41 They areonly slightly, if at all, temperature-sensitive. All diamonds show thediffuse " temperature " spots predicted by the thermal theory; these aregreatly enhanced at sufficiently high temperatures and disappear a t lowtemperatures, thus conforming to the usual behaviour of such diffnsereflections.The most reasonable conclusion appears to be that the structure-sensitivereflections found for normal diamonds are associated with some kind ofinternal strain.Lonsdale and Smith have pointed out 3* 24 that cleaved orfractured crystals frequently give non-Laue reflections which are quitedifferent in quality from the true diffuse reflections; and Kirkpatrick 21has reported an increase in anomalous reflecting power of cleaved calcitesurfaces, when ground, which is undoubtedly due to the presence of verysmall disordered crystal particles.It is clear, therefore, that care must be taken to distinguish betweenanomalous, structure-sensitive reflections due to crystal strains, whetherintrinsic or externally applied, and those diffuse reflections which aretemperature-sensitive and due to crystal vibrations.Jahn 33 has pointedout that if the extension of reflecting power about the normal (Bragg)reflecting positions is due to the finiteness of the reflecting crystal particles,the distribution will be essentially similar for different planes,42 whereas thedistribution of diffuse reflecting power due to the existence of elastic vibra-tions will be different for different planes; and he has illustrated the vari-ations to be expected in the latter case by application of the Waller formulato sodium single crystals,11,33 which are soft and elastically very aniso-tropic.An experimental investigation of sodium and lithium singlecrystals 34 has shown that the observable diffuse reflections, which aredetailed, intense, and persistent, are in entire agreement with the pre-4 1 (Sir) R. Robertson, J. J. Fox, and A. E. Martin, PhiZ. Tram., 1934, A, 232,42 M. v. Lam, Ann. Physik, 1936, 26, 55; M. v. Laue and K. H. Riewe, 2. Kri.st.,463; Proc. Roy. Sac., 1936, A , 157, 579.1936, 85, 408; P. P. Ewald, Proc. Physical Sac., 1940, 52, 167LIPSON : METAL STRUCTURES. 95dictions of the elastic vibration (thermal) theory, as interpreted by Jahn.The fact that these monatomic cubic crystals, for which no optical vibrationsare ~ I ~ O W I Q ~ do show diffuse reflections at all, is a strong indication that theobserved effects are due to the elastic thermal atomic vibrations.The factthat “ layer ” and “ chain ” type structures each give typical and easily-recognisable diffuse photographs has already proved to be of assistance incrystal-structure determination.a* If, as has been suggested, these diffusepatterns can in time be used to determine the elastic properties of singlecrystals at various temperatures, then it is clear that a new and usefulfield of research is being opened up. K. L.3. METAL STRUCTURES.It is perhaps natural that the rate of discovery of new alloy structuresshould now tend to diminish. In the first place, it is probable that allthe simple types of structure are known, so that any left are those whosedeterminations require much more time and patience.In the second place,interest in complicated structures is less, since they cannot be expected toproduce any important modification of the general rules for the occurrenceof the commoner structures such as the “ electron compounds ” (W. Hume-Rothery),l the ‘( interstitial compounds ” (G. Hiigg),2 and the (‘ AB,compounds ” (G. E. R. Schulze).3 Of the structures determined in aboutthe last two years only three-C~Mg,,~ A u ~ A ~ , ~ and CaZn, 6-seem t o benew ; others, such as V,Si 7 and Li,,Pb,,* are similar to established structures.There is evident, however, a tendency to go over the ground of earlierwork in order to clear up problems left unsolved. Cementite, Fe3C, is anexample of this.The detection of the carbon atoms by X-rays had previouslybeen considered almost impossible, but with accurate photometry and theuse of three-dimensional Fourier series, H. Lipson and N. J. Petch haveshown that they can be located unequivocally. Petch 10 has also beenable to detect the carbon atoms in austenite, the face-centred cubic solidsolution of carbon in iron.Improvements in technique have also shown that there are still problemsassociated with structures previously considered well established. H. Lipsonand A. R. Stokesll have found that thallium has a body-centred cubicS. Bhagavantam, Proc. Indian Acad. Sci., 1941, 13, A , 543.O4 (Mrs.) K. Lonsdale, PTOC. Roy. Soc., 1941, A , 177, 272; (Mrs.) K. Lonsdale,J. M. Robertson, and (Miss) I.Woodward, ibid., 1941, A, 178, 43.“ The Metallic State,” 1931, p. 328, Oxford.a 2. physikal. Chem., 1931, B, 12, 33.G. Ekwall and A. Westgren, Arkiv Kemi, Min. Geol., 1940, 14, B, 7.0. E. Ullner, ibid., 1940, 14, A , 3.W. Haucke, 2. anorg. Chem., 1940, 244, 17. ’ H. J. Wallbaum, 2. MetaElk., 1939, 31, 362.M. Albert0 and E. Arreghini, 2. Krist., 1939, 101, 470.J . Iron Steel Inst., 1940, 142, 95.a 2. Ekktrochem., 1939, 45, 849.lo I&id. (in press).l1 Nature, 1941, 148, 43796 CRYSTALLOGRAPHY.structure at 262", although it is commonly accepted as face-centred cubica t this temperature.12 A. Taylor and D. Laidler l3 have pointed out thatgraphite, when it is in a well-crystallised state, gives extra diffraction lineswhich have as yet received no convincing explanation.There is thusevidence that a survey of older work by the present more accurate methodswould be well worth while.Other improvements, such as the use of crystal-reflected radiation 1*and of cameras of large resolution,15 have opened up a new and importantfield of investigation-the study of the intermediate structures that formduring transformations in the solid state. Perhaps the most importantinvestigations have been those of G. D. PrestonI6 and A. Guinier 1' onthe change with time of an alloy of 4% copper in aluminium. At 500"this alloy is a solid solution; at lower temperatures the copper is graduallyprecipitated. This precipitation is accompanied by changes in hardness,and the effect-known as " age-hardening "---has been widely discussed.l*From X-ray photographs, Preston and Guinier, independently but in strik-ing agreement, have produced evidence of the exact nature of the pre-cipitation, and this must form an important point in the ultimate explanationof the phenomenon.Photographs of a single crystal of the alloyquenched from 500" show faint streaks as well as the ordinary reflections.That these streaks are associated with structural changes in the alloy isshown by the fact that their intensity increases with time.The conclusionwas reached that they are due to the formation of plates of copper atoms,and analysis showed that these plates must be parallel to the (100) planesof the aluminium lattice. The fact that the streaks are directed awayfrom the origin confirms the suggestion that they are due to the precipitationof atoms smaller than aluminium, and from their lengths it was decidedthat the plates are only about two atoms thick.Preston19 has continued the work by maintaining the crystal at 200"in the X-ray camera.As expected, the process continues a t a faster ratethan at room temperature; it also goes further, as the streaks, after becom-ing more intense, break up into spots. These, it was found, can be accountedfor by the formation OE a definite structure with a definite orientation withrespect to the aluminium lattice. The atomic arrangement of this, however,is not the same as that of the 8 phase20 which has been established byD. Stockdale21 as the equilibrium structure. Instead, the atoms adoptthe arrangement typified by calcium fluoride,22 and this is such that, withthe orientation found, it fits neatly on the (100) planes of the aluminiumlattice.The 8 structure cannot do this, and so it appears that the presenceThe evidence is as follows.l2 S. Sekito, Z. . K T i B t . , 1930, 74, 189.14 I. Fankuchen, ibid., 1937, 139, 193.l6 Froc. Roy. SOC., 1938, A , 167, 526.l8 " Age Hardening of Metals," 1940, Amer. SOC. for Metals, Cleveland.la Nature, 1940, 146, 130.l6 H. Lipson, ibid., 1940, 146, 798.l i Compt. rend., 1938, 206, 1641.Phil. Mag., 1938, 26, 855.J. B. Friauf, J . Arner. Chem. SOC., 1927, 49, 310721 J. Inst. Metals, 1933, 52, 111. " Strukturbericht," I, 1931, 148LIPSON ; MET& STBUCTURES.97of ready-made structural elements in the parent lattice can favour theformation of a structure that is not the one of lowest free energy.C. S. Barrett and A. H. Geisler 23 have made similar investigations ofan alloy of 20% silver in aluminium. This alloy is also a single phase at500" and its X-ray photographs also show streaks when it is quenched toroom temperature. The orientations of these streaks show that the planesof precipitation are the (111) set, and this fits in well with the structurethat is ultimately precipitated. This structure is that of AgsAl which hasbeen shown by A. Westgren and A. J. Bradley% to be close-packed hex-agonal with a random distribution of silver and aluminium atoms. It cantherefore fit on to the cubic aluminium structure since both are formed ofsimilar close-packed planes of atoms ; the relative orientations agree withthis suggestion.Barrett and Geisler think that there may be other ex-planations of the streaks, but G. D. Preston26 has published more datawhich tend to, confirm that given here.C. Samans26 has discussed the changes in the aluminium-copper alloyand has shown how the " calcium fluoride " structure can be derived fromthe plates of copper atoms formed initially.The alloy Cu,FeNi, has been shown by A. J. Bradley 27 to dissociate ina way which has some similarity to these other cases. In equilibrium a troom temperature this alloy contains almost equal amounts of two face-centred cubic structures; 28 this state is formed by the splitting-up of asingle face-centred cubic structure at about 800".At temperatures belowthis, diffusion is not very rapid and the process of change is slow, so thatby quenching the alloy after different times of annealing a t about 700" itcan be preserved in different stages of decomposition. X-Ray powderphotographs of the alloy quenched from 800" show, of course, only one setof spectra, while those of the alloy annealed a t 650" for one week showtwo sets due to the two phases. The intermediate structure, however,shows some lines (e.g., 222) that are double, as for the two-phase state,but others (e.g., 311) that have more comp0nents.2~ The explanation pro-posed by Bradley is that in this intermediate state two tetragonal structuresexist with a common (001) face.These two structures are nearly cubicbut one has an axial ratio greater than unity, the other, less. In this waythe two structures preserve their respective atomic volumes and yet retainmuch of their relation to the parent cubic lattice.A similar state of affairs has been found by electron diffraction for thedeposition of aluminium on platinum.30 The aluminium is constrained toadopt the slightly smaller spacing of platinum, but it preserves its atomic23 J . Appl. Physics, 1940, 11, 733.26 J . Sci. Iwtr., 1941, 18, 154.26 Amer. Inst. Min. Met. Eng., 1940, 137, 85.27 Proc. Physical Soc., 1940, 52, 80.28 W. Koster and W. Dannohl, 2. Metallk., 1935, 27, 220; A. J. Bradley, W. F. Cox,28 A. J. Bradley, W. L. Bragg, and C. Sykerr, J.Iron Steel Inst., 1940, 141, 113.so Proc. Roy. SOC., 1933, A, 141, 398.24 Phil. Mag., 1928, 6, 280.and H. J. Goldschmidt, J . Inst. Metals, 1941, 6'9, 189.REP.-VOL. XXXVIII. 98 CRYSTALLOGRAPHY ,volume by becoming tetragonal with an axial ratio greater than unity.This tetragonal aluminium can, however, exist only as unstable thin films.This is oneof the few elements whose structural history has never been satisfactorilye~tablished,~~ and specimens that have been heat-treated in the solid stateusually contain two phases, face-centred cubic and close-packed hexagonal.The hexagonal phase gives a remarkable mixture of sharp and “ fuzzy ’’diffraction lines, and the following explanation of this has been given by0. S. Edwards, H. Lipson, and A.J. C. Wilson.32Close-packed structures can be considered as composed of layers ofclose-packed atoms, each layer fitting neatly on the one below. Con-sideration of the figure shows that there are two possible positions in whichthe second layer can fit on the firstone, A ; these two positions are shownas small circles and crosses respect-ively. Suppose the second layer goesinto the B positions; then the thirdlayer can take either the first position, 0;o;o;o A , or the position C. A close-packedstructure can be built up of anyrandom arrangement of types ofplanes A , B, and C, the only limit-ation being that no two successiveplanes shall be the same. In practice,however, only two arrangements areof importance : ABCABCA . . .whichis cubic, and ABABABA . . . . which is hexagonal.Hexagonal cobalt approximates t o this second sequence, as is shownby the positions of lines on powder photographs. how-ever, that mistakes occur occasionally, the sequence changing fromABABABA , . . to, say, ACACACA . . . Some of the reflections, suchas the orders of 0001, will not be affected by the resultant faults; theseremain sharp; but all others will be blurred t o an extent depending onthe average size of the regions of perfect sequence and on the orientationof the reflecting planes.Oscillation photographs of cobalt33 bring out the peculiarity of thestructure very markedly. The sharp reflections are represented by ordinaryspots; the others are drawn out into streaks which nearly merge into oneanother forming long lines on the photographs with periodic maxima andminima.It is interesting that Barrett and Geisler 23 have suggested this samedefect as a possible cause of the streaks 011 their silver-aluminiumphotographs.Imperfections of another sort have been found in cobalt.0 ~ , o , O ,0 0;o;o;O+*@+O+O+FIU.1 .It is31 A. E. van Arkel, “ Reine Metslle,” 1939, p. 327, Berlin.32 Nature, 1941, 148, 166.s3 0. S. Edwards and H. Lipson, Proc. Roy. Xoc., 1942, A , (in press)POWETITi : INORGANIC! STRUCTTJRRS. 99The general use of X-rays in the study of the properties of metals andalloys has been well summarised by several recent publications. On thedetermination of equilibrium diagrams there are papers by A. J.Bradley,W. L. Bragg, and C. Sykes,u and by W. H~me-Rothery,~~ both summarisingseveral years’ work by their respective schools. The report of a conferenceon “ Internal Strains in Metals,” 36 organised by the Physical Society,contains many papers on X-ray investigations, particularly by W. A. Woodand by G. W. Brindley on distorted lattices; it also contains, in greaterdetail, much of the material in the present report. Finally, the Instituteof Physics has collected together a number of papers on the application ofX-rays to industrial problems,37 and on particular branches of X-raytechnique.38 These papers taken together form a useful review of thepresent state of development in the use of X-rays for the examination ofmetals. H. L.4. INORGANIC STRUCTURES.In a general discussion of the presentation of crystal chemistry, A.F.Wells 1 suggests that the most satisfactory classification of crystal structuresfor this purpose is one which is primarily geometrical, and based as far aspossible on observed interatomic distances. In a classification of thekind described, the first division, following Weissenberg, is into four groups,in which the complexes that may be distinguished in the structure arefinite, or infinite in one, two, or three dimensions. The recognition of thecomplex rests largely on interatomic distances, supported by the propertiesof the crystal. Sub-divisions are then made according as there are in thecrystal, structural units all of one kind or of more than one kind; furthersub-divisions arise according as the complexes are joined by van der Waals,hydrogen, or ionic bonds.The recognition of a particular type of geo-metrical complex does not imply any special bond type-inert gases, metalsand alloys, and ionic and homopolar crystals are all classed under crystalscontaining 3-dimensional complexes-but the scheme is designed to allowa discussion of the nature of the bonds in a given structure without theissue having been prejudged.Critical reviews of the classification of structures according to bond typeand of the sub-division of ionic structures according to electrostatic bondstrength are included, and the treatment of these topics in recent textbooksof crystal chemistry is discussed. In the detailed classification given, thechlorite minerals are by accident omitted as the example of a class contain-ing infinite two-dimensional ions of opposite charge.In another paper 2finite complexes in crystals are classified according to the number ands4 J . Iron Steel Inst., 1940, 141, 63.35 Research Reports of the British Non-Ferrous Metals Research Association,38 Proc. Physical Xoc., 1940, 52, 1.38 Ibid., p. 126.1941, No. 562.37 J . Sci. Inetr., 1941, 18, 69.Phil. Mag., 1941, [vii], 32, 106.2 A. F. Wells, ibid., 1940, [vii], SO, 103100 URYSTALLOORLPF€Y.arrangement of the atoms of one kind only, this being in general the moreelectropositive atom.K. Fajans3 discusses the effect of polarisation of ions on the departureof interionic distances from additivity in the oxides, fluorides, and hydridesof the alkali metals and the oxides and fluorides of calcium, strontium,and barium.The difference between the metal-oxygen and the samemetal-fluorine ion separations increases in the alkali-metal salts fromlithium to potassium, ie., in the order of increasing radius and decreasingfield strength of the alkali ion; the same is true for the difference betweenmetal-hydrogen and metal-fluorine distances, and a similar result is foundin the comparison of the oxides and fluorides of the doubly charge cationsCa**, Sr", and Ba". Since 0- and H- are more polarisable than F-, it isconcluded that the decrease in size of anion in the field of the cation con-tributes distinctly to the deviations from additivity.Further, since thedifference between the hydride and fluoride separations has a maximumfor the rubidium compounds, there is a second effect which is interpretedas a stronger " loosening " action of F- compared to H- on the more easilypolarisable of the cations.V. Schomaker and D. P. Stevenson propose a revision upwards of thecovalent single-bond radii of fluorine, oxygen, and nitrogen. They suggestN = 0.74 (0*70), 0 = 0.74 (0*66), F = 0.72 (0.64) ; the figures in parenthesesare those of L. Pauling and M. L. hug gin^.^ With the new values, whichare based on electron-diff raction results for hydrazine,6 hydrogen peroxide,sand fl~orine,~, many bond lengths which formerly obeyed the additivityrule are now less than the sum of the covalent radii r, + T,.The bondlength r, is obtained from the expressionTAB = rA + rB - P ( x A - xB);p = 0.09, xA and X, are the values of the Pauling electronegativities of thebonded atoms. It is inferred that the deviation from addivity representedby - p(x, - x,) is associated with the extra ionic character of the bondA-B as compared to the ionic character of the normal covalent bond betweenlike atoms.W. Hume-Rothery and G. V. Raynorg discuss the apparent sizes ofatoms in metallic crystals with special reference to aluminium and indium,and the electronic state of magnesium.Recent Structures.-From X-ray diffraction by Liquid and plastic sulphurat temperatures from 124" to 340" N. S. Gingrich10 has obtained atomicdistribution curves. The nearest neighbour for an atom in plastic sulphuris at 2.08, and at approximately 2-07 A.for liquid sulphur a t all temper-atures. In plastic sulphur each atom has two near neighbours, and theJ J . Chem. Physics, 1941, 9, 281. J. Amer. Chem. SOC., 1941, 63, 37.2. K T ~ s ~ . , 1934, 87, 205.P. A. Giguere and V. Schomaker, quoted in (4).M. T. Rogers, V. Schomaker, and D. P. Stevenson, ibid., 1941, 63, 2610.Proc. Roy. SOC., 1940, A , 177, 27.7 L. 0. Brockway, J . Amer. Chem. SOC., 1938, 60, 1348.lo J. Chem. Physics, 1940, 8, 29P0WEI;L : INORQANIC STRUOTUBHS. 101estimated number of neighbours for liquid sulphur is 1.7. This showsthat liquid sulphur is not a close-packed liquid, such as mercury. Also,if all the liquid consisted of closed rings, 2 near neighbours should be foundand the observed 1.7 means that an average of 30% of the atoms have oneneighbour only. The physical properties preclude the presence of S,molecules, but if the S, rings, that exist in the solid, break into chains onmelting, this gives approximately the right proportion of atoms with onlyone near neighbour.The disrupted rings may join to form irregular chainswith larger numbers of atoms.A. H. White and L. H. Germer 11 have made electron-diffraction experi-ments on extremely small crystallites of carbon deposited on silica bypyrolysis of methane, and conclude that this carbon-black consists ofpseudo-crystals, in each of which carbon atoms are hexagonally arranged,as in graphite, but successive atomic layers are displaced so that no regu-larities exist other than the uniform separation of the planes, and theregular arrangement of atoms in each of these.The spacing betweenplanes is found to be 3.6, A., 9% larger than the spacing, 3-35 A., in graphitecry st als.The unit cell and space-group of jamesonite, 4PbS,E’eS,3Sb2S,, havebeen determined, from single- crystal photographs.12 Powder photographsagree with those previously published, and it is shown that a previousattempt l3 to obtain the dimensions of the large monoclinic cell failedowing to the inherent limitations of the powder method. By electron-diffraction methods I. H. Usmani l4 finds that copper sulphide film formedelectrolytically and film formed by direct action of hydrogen sulphide oncopper crystals give structures which do not agree with the structure ofeither cuprous or cupric sulphide as given by X-ray studies.A re-examination of lead monoxide by W.J. Moore and L. Pauling l5has proved that the structure of the red tetragonal form is that previouslyreported by R. G. Dickinson and J. B. E’riauf,l6 and not that of G. R. Leviand E. G. Natta.17 In the structure now confirmed each oxygen has fourlead atoms arranged tetrahedrally around it and each metal atom is bondedto four oxygens which form a square to one side of it. It is suggestedthat the orbital arrangement of PbII (and of SnII) is that of a square pyramidwith four bonds directed from the metal within the pyramid towards thefour corners of the base, and a fifth orbital occupied by a stereochemicallyactive unshared pair, directed towards the apex.This stereochemical typeis in contrast with the trigonal bipyramid arrangement found for manyother compounds where the valency group is a decet, and it would be ofgreat interest to know whether it persists in finite complexes, or whether,like the trigonal prismatic arrangement for six bonds, it is confined to giantmolecules as in this structure. Stannous oxide has a similar structure tol1 J. Chm. Physics,, 1941, 9, 492.J. E. Hiller, 2. Krist., 1938, 100, 128-l6 J . Amer. Chem. SOC., 1941, 63, 1392.le Ibid., 1924, 46, 2461.l2 L. G. Berry, Min. Mag., 1940,25,597.l4 Phil. Mag., 1941, [vii], 82, 89.l7 N w o ah., 1926, 8, (3)102 CRYSTAT&OGRAPHY.lead momxide, but palladous and platinous oxides, though tetragonal,have a structure quite different in its essentials, which are that it is a com-promise arrangement whereby each metal atom has four neighbours at thecorners of a rectangle, O-M-0 = 82" and 98", and each oxygen has fournearly tetrahedral neighbouring metal atoms, M-O-M = 98" and 116";Tellurite, Te02,18 has a structure resembling that of brookite (TiO,),each tellurium being surrounded octahedrally by six oxygens.The relativedisposition of octahedra is the same as in brookite but is greatly deformed.In lithium hydroxide monohydrate19 each lithium is at the centre of atetrahedron of oxygens and two such tetrahedra share an edge in a reflec-tion plane while the upper and lower corners are shared with tetrahedrain the unit cells above and below, resulting in unending chains of pairedtetrahedra along one axis of the crystal.These chains are linked sidewaysby hydroxyl bonding. Between cornera of adjacent tetrahedra the 0-0distance is 2.68, and there are therefore hydroxyl bonds here, two reachingfrom each oxygen to other oxygens. The oxygens of the shared tetra-hedron edges in the reflection plane are shown to be in hydroxyl groups,and the oxygens of the upper and lower tetrahedron corners are in watergroups. The tetrahedra are linked sideways by hydroxyl bonding betweenthe hydroxyls and water.Li-O,, 1.95, Li-OnB,o 1.97, OOH-OH,O 2.68, OO,-OoH 3.74, OH,O-OHInO 3.48 A .The compound20 CoSe, has metallic properties and the iron pyritestype of structure.The inter-atomic distances now found are Co-Se 2.43 -j= 0.01, Se-Se 2.49 & 0.04.The latter value is a slight correction of an earlier reported value and isstill 0.21 A. greater than the Se-Se distance in crystalline selenium.The a-modification of iodic acid is orthorhombic. A structure deter-mination by M. T. Rogers and L. Helmholz 21 shows the existence of dis-crete 10, groups with 1-0 = 1.80 and 1-81 A,, and the 0-1-0 angles 96",98", and 101". Three oxygen atoms in positions approximately opposed tothe three bonded atoms are a t distances 2-45, 2.7, and 2-95 A. These com-plete a distorted octahedron of oxygen around the iodine, with three strongand three weaker bonds. In addition the HIO, molecules are held togetherby hydrogen bonds, the hydrogen atom of the hydroxyl group preferringin this structure to form two weak bonds (the bifurcated type) 22 to twoother oxygens, rather than one strong bond to one only of the availableoxygen atoms.Electron diffraction of nitric acid agrees with a planar model of thein~lecule.~~ There is an NO, group with N-0 = 1-22 for both distancesPd-0 = 2.01 0.01, Pt-0 = 2.02 & 0.02.Observed interatomic distances are :The cube edge has a = 54345 &- 0.005.T.Ito and H. Sawada, 2. Krist., 1939, 102, 13.19 R. Pepinsky, ibid., p. 119.fo B. Lewis and N. Elliott, J. Amer. Cheni. SOC., 1940, 62, 3180.*l Ibid., 1941, 68, 278.24 See Ann. Reports, 1940, 87, 193; 1939, 36, 181.2s L. R. Maxwell and V. M. Mosley, J. Chem. Physics, 1940, 8, 738POWELL : INORQ-ANTC STRUCTURES.103and angle 0-N-0 = 130", which is also found for nitrogen dioxide. Thethird oxygen is a t 1.41 & 0.02 A. from the nitrogen atom and equidistantfrom the other oxygen atoms. Methyl borate= has a planar BO, groupwith B-0 = 1.38 0.02, 0-C = 1-43 & 0.03 A., and angle B-O-C =113" & 3". Trimethyltriboranetrioxan 24 has the structure (I), which iscompletely planar, except for the hydrogen atoms.BB-O = 1-39 & 0.02 A.B-C = 1.57 & 0.03 A. / \(1.1 Q $)B B /O\ AA B B = 112" & P oCH, 0 CH,A1B,H12,25 by electron diffraction, has aluminium bonded to BH4 groupsa t angles of 120", making the molecule planar except for the hydrogen.The boron atoms are located near the centres of trigonal bipyramids formedby the four hydrogen atoms of each BH, and the central aluminium :A1-B = 2.14 -J-- 0.02, B-H = 1-24 & 0.04.Hexamethyldilead 26 gives byelectron diffraction Pb-Pb = 2.88 & 0-03 and Pb-C = 2.25 & 0.06 andhas a tetrahedral arrangement of the lead bonds. Electron-diff ractionmethods give values for interatomic distances in a large number of halidesof elements of Groups 11, IV, and VI. The cadmium halides27 are prob-ably linear, and Cd-C1 = 2.235 & 0.03, Cd-Br = 2.39 rfi 0.03, Cd-I =2-56 & 0.03. The halides of bivalent tin and lead are not linear : Sn-C1 =2.42 & 0.02, Sn-Br = 2.55 & 0902, Sn-I = 2.73 & 0.02, Pb-C1 = 2-465 &0.02, Pb-Br = 2.60 & 0.03, Pb-I = 2.79 &- 0.02. In the tetrahalides 28the distance from the central atom to halogen is for C-Br = 1-94 & 0.02,G I = 2.15 -J= 0-02, Si-Br = 2.14 & 0.02, Si-I = 2-43 0.02, Ge-Br =2.29 0.02, &-I = 2-50 & 0.03, Sn-Br = 2.44 & 0.02, Sn-I = 2.64 &0.04, Ti-C1 = 2.18 ,t0-04, Ti-Br = 2.31 & 0.02, Zr-C1 = 2.335 & 0.05,There is some uncertainty regarding the value given for Se-C1 in thetetrachloride, which is not quoted here.The discussion given on thedepartures from additivity in these and other interatomic distances mayrequire some modification in view of the proposed change in the standardcovalent radii referred to above. For thionyl bromide, D. P. Stevensonand R. A. Cooley 29 find S-Br = 2.27 & 0.02, Br-0 = 3-05 & 0.03, andBr-S-Br = 96" 2".Among structures containing complex ions is that of tetraphenylarsoniumiodide,m where the arsenic bonds are strictly tetrahedral : As-C = 1-95 A.Th-C1 = 2.61 -+ 0.03.24 S.H. Bauer and J. Y. Beach, J. Amer. Chern. SOC., 1941, 63, 1394.26 Idem, ibid., 1940, 62, 3440.26 H. A. Skinner and L. E. Sutton, Trans. Faruday SOC., 1940, 36, 1209.2 7 M. 1%'. Lister and L. E. Sutton, ibid., 1941, 37, 406.28 Idem,, ibid., p. 393.30 R. C. L. Mooney, ibid., p. 2965.29 J . Arner. Chem. SOC., 1940, 62, 2477104 CRYSTALLOGRAPHY.Details of the phosphorus pentachloride structure?l which is roughly ofthe cEsium chloride type with PC14+ and Pc16- ions, are the dist>ancesP-Cl (PCI,) = 1.98, P-Cl (PCl,) = 2-06 A. Barium fluosilicate and fluo-germanate32 have a l-molecule rhombohedra1 cell in a structure with aformal resemblance to caesium chloride, containing Ba++ and a nearlyregular octahedral complex anion.Bunsen’s salt 33 has the constitution(NH,),Fe(CN),Cl,, with a regular octahedral ferrocyanide ion and distinctC1- ions. The structure of hexamethylisonitriloferrous ~ h I o r i d e , ~ ~Fe( CNCH3),C12,3H20, gives a clue to the constitution of the ferr~cyanides.~~The hexagonal structure gives an accurate view of the octahedral complexcation with the atoms linked in the order FeCNC : Fe-C = 1.85, C-N =1-18 A. The iron-carbon distance is approximately that calculated for a50% single-double bond character of the link, as in L. Pauling’s suggestedformula 35 for the ferrocyanide ion ; the pure single- and double-bond lengthswould be 2.0 and 1-79 A., respectively.A bend of 7” a t the nitrogen atombrings the methyl groups out of line with an otherwise linear sequenceFeCNC, and provides additional evidence for the partial double-bondcharacter, since, although the form Fe-CEN-C is naturally linear, theform FG(=N\C would bend a t the nitrogen atom.Further work by J. L. Hoard and W. J. Martins6 on complex niobateshas led t o interesting results. They find that several salts which areobtained from but slightly different aqueous solutions contain complexniobate ions of different stereochemical types. K,HNbOF, is an aggregateof K+, HF2-, and octahedral [NbOF,]= ions ; &NbOF,,H,O also containsthe octahedral complex. I n GNbF, the [NbF,]= ion has the arrange-ment of seven bonds derived from the trigonal prismatic type for six bondsby the addition of an extra fluorine over one of the prism faces.s7 I nK,NbOF,, however, the group [NbOF,] has the alternative codgurationfor a 7-co-ordinated complex, derived from the octahedron by addition ofthe seventh atom over the centre of an octahedron face, a configurationwhich is found in K,ZrF,.3* It appears therefore that these two 7-co-ordinated types are about equally stable, and it would not be surprisingif both the [NbF,] = and [NbOF,] (or [ZrF,] =) groups should be found incompounds where they have the opposite configurations to those so farfound for them.Apart from the case of lead and tin monoxides referred to above, thestereochemical type for a group of ten valency electrons is usually derivedfrom the trigonal bipyramid, and a particularly interesting case is thatS1 D.Clark, H. M. Powell, and A. F. Wells, in publication.32 J. L. Hoard and W. B. Vincent, J . Amer. Chem. XOC., 1940, 62, 3126.33 H. M. Powell and G. W. R. Bartindale; see H. Irving and G. W. Cherry, J . ,34 H. M. Powell and U. W. R. Bartindale, in publication.s6 “ Nature of the Chemical Bond,” 1939, p. 235.30 J . Amr. Chem. Soc., 1941, 63, 11.mi^ G. C. Hampaon and L. Pauling, J . Am?. C h . SOC., 1938, 60, 2702.1941, 25.37 See Ann. Reports, 1939, 36, 170POWELL : INORGANIU STRUOTURES. 105where there are four attached groups. The arrangement found for [IO&?,]-and suggested for TeC1, 39 has now been observed by J.D. MoCullough andG. Hamburger 40 for diphenylselenium bromide. The molecule approx-imates very closely to a trigonal bipyramid with the two bromine ahmaopposed to each other a t the apices, and the phenyl groups in two of thethree equatorial positions : Br-Se-Br = 180" & 3", C-Se-C = 110" rt: lo",Se-Br = 2.52 & 0.01, Se-C = 1-91 -J= 0.03 A.Accurate quantitative methods of structure determination have beenapplied by S. H. Chao, A. Hargreaves, and W. H. Taylor41 to a typicalorthoclase, and confirm the essential accuracy of the felspar structurespreviously determined by qualitative methods. Co-ordinates for all atomsin the cell are determined to an accuracy probably better than 0.1 A. Thecell contents, 4KAlSi,O,, are treated as containing two groups of atoms" SSi, " and " 8SiII," which really oomprise l2Si and 4A.l atoms.In $hetetrahedral group of oxygen around SiI the Si-0 distances are within thelimits 1-66-1-70, and for SiII the limits are 1.57-1.60 A. It is assumedtherefore that while the SiI1 contain silicon only, the 8Si, with the slightlylarger average size of tetrahedron contain 4Si and 4A1 atoms, with a,randomdistribution among the atomic sites. The potassium ion is surrounded bynine oxygens in a group of rather irregular shape.S. H. Chao and W. H. Taylor42 by a detailed examination find thatthere are different types of lamellar structure of the soda-potash felsparsaccording as the proportion of soda felspar is less or greater than ca.30%.The low-soda type comprises monoclinic potash felspar with triclinic sodafelspar lamellze in mutual orientation characteristic of pericline twins, andthe high-soda structure comprises monoclinic potash felspar with triclinicsoda felspar lamella orientated in accordance with the albite twin law.The same authors 43 find for the plagioclase felspars of known compositionsranging from nearly pure soda felspar to nearly pure lime felspar, thatX-ray observations are consistent with the existence of two separate iso-morphous series. One, with the albite structure, extends from pure sodafelspar to at least 22% lime felspar, the other with the anorthite structureextends from pure lime felspar to 20-30~0 soda felspar. In addition,there is a group of intermediate plagioclases recognisable by a characteristicarrangement of subsidiary layer lines in c-axis photographs.Experimentalwork, but as yet no complete structure, has been reported for tricalciumaluminate 3Ca0,A120,.44 The cell is ecubic, a = 15.235 A. Libethenite,Cu,( OH)P0,,45 has deformed PO, tetrahedra, a deformed octahedral arrange-ment of four oxygens and two hydroxyls around some copper atoms, and a,pseudotrigonal-bipyramal grouping of four oxygens and one hydroxyl3Q Ann. Reports, 1940, 37, 181.41 Min. Mag., 1940, 25, 49842 Proc. Roy. Xoc., 1940, A, 174, 57.43 Ibid., 1940, A, 176, 76.44 L. J. Brady and W. P. Davey, J . Chem. Physics, 1941, 9, 663.46 H. Heritsch, 2. Krist., 1939, 102, 1.40 J . Arner. Chern. SOC., 1941, 63, 803106 CRYSTALLOGRAPHY.around other copper atoms, similar to the corresponding five-co-ordinatedgroups in andalusite, adamine, and 01ivenite.~~From a study of absorption and fluorescent spectra of yttrofluorite,N.Chatterjee 47 concludes that solid solutions of yttrium fluoride in calciumfluoride are of the substitution type, since the interstitial type would pro-duce greater disturbances than are observed in the spectra. The sub-stitution lattice persists up to 50% of YF,, but above this other crystaltypes develop. A. L. Greenberg andG. H. Walden 48 studied the system KMnO4-KC10,-H,O by equilibriumand X-ray methods. I n the continuous series of solid solutions of thesalts, Vegard’s additivity law is followed for the a and the c unit cell dimen-sions but not for 6 .The system NH,Cl-MnCI,-H,O shows three solid-solutionseries. I n one of these, interstitial inclusion of manganese in the ammoniumchloride lattice is accompanied by random substitution of water for NH,+to maintain electrical neutrality. The compound 2NH,C1,MnC12,2H,0 issimilar in structure to the corresponding cupric compound. It has beenfound 49 that the alkali sulphates a-K2S0,, cr-Na,SO,, and glaserite,(K,Na),SO,, constitute an isomorphous series of simple hexagonal structurewith the alkali alkaline-earth phosphates MIMIIPO,, and with the calciumphosphate-silicates, Ca,( SiO,,PO,), and the series may be expected toinclude other XO, groups.This trifluoride itself is not cubic.H. M. P.5. ORGANIC STRUCTURES.Very few new structures have been fully determined by accurate methodssince the last Report.A detailed analysis of the cis-form of azobenzenehas now been completed and the results of the Fourier analysis confirmin general the conclusions reported in the earlier analysis.2 The maininterest of this structure lies in the dimensional effects due to suppressionof resonance. The molecule is unable to attain a planar configuration,and hence structures of the type (I) cannot make any large contributionto the normal state. This is reflected in ameasured C-N distance of 1.46 A. (&O-03) ascompared with a corresponding distance of1.41 A. in the molecule of the t~ans-form.~There is also apparently a small distortionin the normal valency angles (N-C-C) in adirection which permits the two benzene rings to become as nearly coplanaras possible, while the approach of the non-bonded carbon atoms is main-tained at 3.3-3-4 A.//N-N\(1.1//-I \--! .L//46 See Ann.Reports, 1937, 34, 166.J. Chem. Physic8, 1940, 8, 645.49 M. A. Bredig, J . Amer. Chem. SOC., 1941, 63, 2533.G. C. Harnpson and J. M. Robertson, J., 1941, 409.Ann. Reports, 1939, 36, 183.J. J. de Lnnge, J. M. Robertson, and (Miss) I. Woodward, PTOC. Roy. Soc., 1939,4 7 2. Krist., 1940, 102, 246.A, 171, 398ROBERTSON : ORGANIC! STRUCTURES. 107A very detailed study by electron-diffraction methods has been carried out011 1 : 3 : 5-tribromobenzene, 4 : 5-dibromo-o-xylene, 5 : 6-dibromohydrindene,and 6 : 7-dibromotetralin with a view to observe the effect of strain onthe benzene ring (Mills-Nixon effect).In structures as complicated asthese it is not possible in any sense to measure all the interatomic distancesby electron diffraction, but the method consists rather in testing certainspecified models. It is concluded that the Mills-Nixon effect is concernedprimarily with changes in the contributions of the excited states of themolecule (which should lead to small but not easily measurable dimen-sional changes) and not with the “freezing” of the double bonds of thering into a particular Kekul6 structure.An interesting quantitative study of a fibre diagram has been recordedfor polyvinyl alcohol, [-CH,*CH( OH)-],.5 About 30 reflections have beentaken into account, and although the agreement between the measuredand the calculated intensities is poor, yet the essential outlines of the struc-ture appear to be clear.There are two chain segments (CH,*CH*OH) inthe unit cell (periodicity along the fibre axis, 2.52 A.). The long chainsof the molecule are so oriented that pairs of chains are linked throughhydroxyl bonds.Preliminary X-ray studies have been recorded for a number of com-pounds. Amongst these may be mentioned an extensive study of thianthren,selenanthren, phenazine, diphenylene dioxide, and related compounds byR). G. Wood and co-workers.sa 79 8 The structural work on these com-pounds should be of interest in view of existing work on their dipolemoments,g* 10 and the unsuccessful attempts which have been made toaccomplish their optical resolution,ll which have been attributed to in-sufficient rigidity in the molecule.The present work brings to light certaininteresting relationships in the crystal structures of these compounds,which do not appear to be too complicated for detailed analysis. Somepossible structurm are described, but as X-ray intensity measurementsare not even mentioned, it would be quite unprofitable for us to discussthese at present.The structure of melamine, C,N,H,, has been discussed by (Miss) I. E.Knaggs and (Mrs.) K. Lonsdale,12 the results being based on X-ray andmagnetic measurements. The amide structure is based on the cyanuricring, and is of a layer type similar to that of other compounds in this class.Amino-groups appear to be connected to the ring nitrogens of adjoiningA. Kossiakoff and H.D. Springall, J . Amer. Chem. SOC., 1941, 63, 2223.(Miss) R. C. L. Mooney, ibid., p. 2828.13. C. Wood and J. E. Crackstone, Phil. Mag., 1941, 31, 62.R. G. Wood, C. H. McCale, and G. Williams, ibid., p. 71.R. G. Wood and G. Williams, ibid., p. 115.G. M. Bennett and S. Glasstone, J., 1934, 128.lo (Miss) I. G. M. Campbell, (Mrs.) C. G. LeFBvre, R. J. W. LeFBvre, and E. E.l 1 G. M. Bennett, (Miss) M. Lesslie, and F. $2. Turner, J., 1937, 444.l2 Proc. Roy. SOC., 1940, A , 177, 140.Turner, J., 1938, 404108 CRYSTALLOURAPHY.molecules by means of weak hydrogen bridges. The results of more detailedX-ray measurements will be of interest.Sorbic acid13 has been analysed to a first approximation by means ofabsolute intensity measurements, and the orientation of the molecules isconfirmed by magnetic measurements, and by the shape and size of thediffuse spots occurring on well-exposed Laue photographs (see Section 2 ofthis Report).The magnetic anisotropy due to resonance in the conjugatedcarbon chain is shown to be about half as large as that in the benzene ring.The molecules, which are linked in pairs by hydrogen bonds, appear to liein rather favourable positions for accurate interatomic-distance measure-ments, but detailed work in this direction has not yet been completed.Symmetry determinations on a number of substituted stilbene anddibenzyl compounds l4 have been carried out, with results of importancein structural chemistry.A preliminary account of the X-ray analysis ofcalycanine, CI6Hl0N2, and of a series of substituted diphenyls,15 and a fullaccount of the structure of dl-alanine l6 by accurate methods have beenpublished, but details are not yet available.Sterok.-It is only possible to refer very briefly to a, comprehensiveaccount of the crystallography and chemistry of some eighty sterol deriv-atives which has now been published by J. D. Bernal, (Miss) D. Crowfoot,and I. Fankuchen.17 Following Bernal’s original X-ray studies,l8 whichgave a clue to the structural formula of cholesterol, the present work hasbeen carried on parallel with much of the chemical work of the last eightyears. The derivatives described belong mainly to the cholesterol and theergosterol series, but include calciferol and other photo-derivatives ofergosterol, and some higher plant and animal sterols.Crystallographicand optical data for all these compounds are classified in a number of con-venient tables, and a detailed study is made of the effect of substituentson the crystal structures. More detailed analyses, involving intensityobservations and Patterson projections on the (010) planes, have been madefor cholesterol chloride, bromide, and cholesteryl chloride hydrochloride,with results which confirm the earlier deductions regarding the shape andsize of the sterol mo1ecules, and give some indication of the arrangementof the carbon and halogen atoms in the ring system. No exact structuralanalysis determining the position of every atom in the molecule has yetbeen attempted.The present paper, however, is an essential preliminaryto such an undertaking, because it indicates those compounds which arecrystallographically simple enough to make such an attempt feasible. Fromthis point of view alone the survey is of extreme value, because many of the13 (Mrs.) K. Lonsdale, J. M. Robertson, and (Miss) I. Woodward, Proc. Roy. SOC.,l4 C. H. Carlisle and (Miss) D. Crowfoot, J., 1941, 6.l5 A. Hargreaves and W. H. Taylor, J. Sci. Instr., 1941,18, 138.l6 H. A. Levy and R. B. Corey, J . Amer. Chem. Soc., 1941, 63, 2095.l7 Phil. Tram., 1940, A, 239, 135.l8 Nature, 1932, 129, 277.1941, A, 178, 43ROBERTSON : ORaANIC STRUOTIJRES. 109outstanding structural problems of the sterols can only be solved by suchexact analyses.Protein Structures : Keratin and Myosin.-Perhaps the most hopefulapproach to any detailed picture of protein structure by X-ray methodslies in the study of the fibrous proteins keratin and myosin.Recently,Astbury and his co-workers 19* 2** 21 have given a new impetus and orientationto this field by their revised discussion of the nature of the intramolecularfold in a-keratin and a-myosin. The earlier theory of the transformationwhich takes place when these fibres are stretched, involving the openingof hexagonal rings (incidentally, this led to D. Wrinch's cyclol hypothesis)has been abandoned in favour of a reversible intramolecular fold of thegeneral type (11).This kind of fold permits the close packing of the side chains (R) in asimple, regular pattern, alternately on one side and the other of the planeU = Side chain up.D = Side chain down.of the fold.The model may, in fact, be reached bydeduction from the principle of close packing, whichis necessarily involved on account of the very Uniformdensities of proteins in general. The various ex-perimental and structural conditions, such as thelong-range elastic properties, and the fold repeatdistance of 5.1 A., then become consequences of theclose packing of the side chains. By means of sucha model it is possible to account for the great rangeof chemical constitutions in the keratin-myosingroup, merely by interchanging side-chain residues ;moreover, the fundamental structural similarities,as revealed by X-rays, are explained by the invariabIep ol ypep t i de skeleton.More detailed analysis of the structure is now ofgreat importance, and the data appear to be availablein certain new and very perfect photographs byMacArthur22 on porcupine quill.In these photo-graphs the striking feature that a large fibre axisperiodicity and its dominant orders are foundexpressible in 2m 3" terms of the probable amino-acidlength promises to be of great significance in elucidat-ing the actual sequence of amino-acid residues.Plant-virus Preparations.-J. D. Bernal and I.Fankuchen 23 have now published a very full preliminaryaccount of their X-ray crystallographic work ontobacco-mosaic virus, enation-mosaic virus, aucuba-mosaic virus, cucumberW. T. Astbury and (Miss) F. 0. Bell, Nature, 1941, 147, 696.2o W. T. Astbury, Chem. and Ind., 1941, 40, 41, 491.21 W. T. Astbury, J., 1942 (in publication) (Lecture to the Chemical Society, Nov.s2 Wool Rev., 1940, 9; unpublished work.20th, 1941).28 J . Oes. Phg&l., 1941,26, 111110 CRYSTALLOGRAPHY.virus, potato virus, and tomato bushy-stunt virus. The tobacco-mosaic viruspreparations were studied in most detail, and the recorded data refer chieflyto them. These preparations have a remarkable capacity for formingdoubly refracting aggregates. In dilute solution they exhibit flow orient-ation and other peculiarities indicating the presence of long, thin particles.In concentrated solution the orientation is spontaneous, and in fact anysmall region behaves like a uniaxial positive crystal. X-Ray investigationshows that even in solution the particles are equidistant, the distancebetween them depending on the concentration. At concentrations of 30?/,and over, gel-like properties are found, and the preparations become stifferas the water content decreases, but there are no abrupt transitions. Theforces maintaining the particles equidistant and parallel in the gel areattributed to the ionic atmospheres surrounding them, and there is no doubtthat the results obtained in this field will have wide extensions to othercolloid systems.The arrangement isSO perfect that each specimen is in fact a two-dimensional single crystal.It is concluded from the evidence that the virus preparations consist ofparticles of about 150 A. in diameter and with a minimum length of 1500 A.24These extraordinary particles are in a sense intermediate between themolecule and the crystal. With regard to inner structure there is evidenceof the existence of sub-units of approximately 11 A . ~ fitted together in ahexagonal or pseudo-hexagonal lattice of dimensions a = 87 A . , c r= 68 A.The particle itself seems to be virtually unchanged by drying, and so mustcontain but little water. In the case of bushy-stunt tomato virus there isevidence of spherical rather than long particles, and so it seems likely thatthe elongated particle form has no essential biological significance.In the dry gel the interparticle distance is 152 A.J. M. R.H. LIPSON.(Mrs.) K. LONSDALE.H. M. POWELL.J. M. ROBERTSON.za G. A. Kausche, E. Yfankuck, arid H. Rusktt, Nuturwiss., 1939, 27, 292