Crystal structure and chemical constitution of basic...

The average difference for 20 wave-lengths is 0*00006. In the last two wave-lengths, however, the differences are much larger, and appear to indicate a real deviation from the formula as the limits of absorption are approached.

My thanks are due to Prof. Lowry for suggesting the subject of research, providing and describing the material, and communicating the paper, and by no means least to my wife for drawing the curves for checking the work.

Basic Beryllium Acetate and Propionate. 437

Crystal Structure and Chemical Constitution o f Basic BerylliumAcetate and Propionate.

By Sir William Bragg, F.R.S., and Gilbert T. Morgan, F.R.S.

(Received October 6, 1923.)

The co-ordination theory of molecular structure postulates the existence for each valent chemical element of an integer representing the maximum number of associating units which can be grouped around the atomic sphere of the element. This so-called “ co-ordination number ” varies with the atomic volume of the element, and although in the case of elements of medium atomic volume the number is generally six, yet with elements of small atomic volume this number is diminished to four or even less.

This conception of co-ordination was elaborated by Alfred Werner into a comprehensive theory of chemical combination and molecular structure long before the electronic constitution of chemical atoms had been revealed, but nevertheless the co-ordination theory has been shown to accord with many of the new facts since discovered by infra-atomic investigations and by X-ray analysis.

Considerable success has already attended the application of X-ray measurements to the‘study of the crystals of co-ordination compounds. Last year Wyckoff showed in this way that in the hexamminonickel halides, NiX2, 6NH3 (where X = Cl, Br or I), all six of the ammonia molecules must be alike and related in the same manner to a central nickel atom to which they approach more closely than to any other atoms of the compound. This investigator concludes that it is entirely appropriate to write the formula of these compounds in the customary co-ordination manner [Ni, 6NH3] X2

VOL. civ.—a. 2 i

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438 Sir W. Bragg and G. T. Morgan

(‘ J. Amer. Chem. Soc.,’ vol. 44, p. 1244 (1922)). I t must be admitted that this experimental evidence is a remarkable confirmation of the validity of Werner’s conception of the co-ordination sphere.

By the X-ray measurements of crystals of ammonium platinichloride, Wyckoff and Posnjak {ibid., vol. 43, p. 2308 (1921) ) showed that this salt has a structure like that assumed for calcium fluoride if platinichloride radicals,PtCl6, replace the calcium atoms, and if NH4 groups are introduced in place of fluorine atoms. These results furnish a physical demonstration of the existence as separate entities of the three co-ordination nuclei of this platinichloride [NH4]2 [PtCl6]. Wyckoff has also by X-ray analysis thrown light on the constitution of caesium dichloroiodide, a substance in which co-ordination occurs round the anion Cs[ICl2] {ibid., vol. 42, p. 1100 (1920)). Caesium dichloroiodide has also been investigated by G. L. Clark (‘ Proc. Nat.Acad. Sci.,’ vol. 9, p. 117 (1923)), who has determined the crystal structure of this salt and of its analogues, in which iodine and bromine replace the chlorine atoms. His results indicate that the halogen atoms constitute a

psingly acting group as in the formation of complex ions in solution.In connection with co-ordinated anions reference should be made to the

work of W. L. Bragg on the isomorphous pair, NaN03, and CaC03, which on the ordinary theory of fixed valency constitutes one of the anomalies of Mitscherlich’s principle of isomorphism. The X-ray analysis of the crystals affords experimental support to the co-ordination hypothesis which attributes a similar structure to the two isomorphs,

Nar 0 i

NLo oJwith the same spatial arrangement of the two pairs of four atoms which make up the two anions (‘ Roy. Soc. Proc.,’ A, vol. 89, p. 468 (1914)).

In all probability, atomic volume and co-ordination number are both closely connected with the electronic structure of atoms, for elements of simple structure possess relatively small atomic volumes and low atomic numbers.

Palling within this category are the elements of the first short period of the natural classification, namely, the series beginning with lithium and ending with fluorine. These elements exhibit co-ordination numbers which do not usually exceed four and are sometimes less. A possible exception to this generalisation occurs in the borotungstates [B (W20 7)6] X '9, complex compounds of boron, in which this element appears to have a co-ordination number of six, although



in the great majority of its inorganic and organic derivatives boron has a co-ordination index of four (Pfeiffer, ‘ Zeitschr. anorg. Chem.,’ vol. 105, p. 26 (1918); Werner, ‘ Neuere Anschauungen auf dem Gebiete der Anorganischen Chemie,’ p. 148 (1920)).

The case of the neighbouring element, beryllium, which merits further attention, is the subject of the investigation described below.

Among the simpler inorganic compounds of this metal one finds the following : BeCl2, 4H20 ; Be(C104)2, 4H20 ; BeS04, 4H20 and K 2BeF4, the compositions of which suggest that the co-ordination number is four. A similar indication is furnished by the organic compound, beryllium acetylacetone, Be(C5H70 2)2, in which each diketonic complex, C5H70 2 or CH3.CO.CH. CO.CHg, functions as two associating units.

One of the outstanding features of the chemistry of beryllium is the tendency for this metal to form basic salts with the volatile inorganic and organic acids. Of the basic beryllium salts with acids of the acetic series a remarkable group exists, the members of which have the general formula Be40(RC02)6, where R is either hydrogen in the basic formate or an alkyl group in any of the homologues of this salt. The higher members of the group are oily, the lower members, with the exception of the basic formate, have been obtained crystalline and are amenable to X-ray analysis.

These organic beryllium compounds are practically insoluble in water, but with the exception of the formate, they dissolve readily in anhydrous organic solvents. In their solubility in non-ionising media these compounds differ essentially from the majority of normal metallic salts of organic acids. Moreover, when solid the beryllium derivatives have definite melting points, and they can be distilled without decomposition under diminished pressure. The vapour density of the basic acetate has been determined (Urbain and Lacombe, ‘ Compt. Rend.,’ vol. 133, p. 875 (1901); vol. 134, p. 773 (1902) ), and it agrees closely with the value calculated for the undissociated compound of formula Be40(CH3.C02)6. Throughout this group of beryllium compounds there is no sign of either hydrolytic or ionic dissociation except under conditions involving complete disruption of the molecule.

These remarkable physical and chemical properties do not lend support to the formulation of basic beryllium acetate as a molecular compound, 3Be(CH3.C02)2, BeO, but they point to a unitary structure for the basic acetate similar to that ascribed to non-ionised organic compounds.

At the Australian meeting of the British Association (Melbourne, 1914) one of us (G.T.M.) suggested for basic beryllium acetate a plane formula

2 I 2




440 Sir W. Bragg and G. T. Morgan.

(fig. 1), in which this unitary conception of the structure is combined with the hypothesis of a maximum co-ordination number, four, for beryllium.

Further consideration of the problem, however, led to the view that in this extremely stable compound a more symmetrical arrangement is probably present. This greater symmetry would be attained by placing the unique oxygen atom at the centre of a tetrahedron, the four beryllium atoms at or near the vertices of this solid, with the six acetate groups spanning the edges of the tetrahedron as in fig. 2.*

This symmetrical arrangement of the constituent parts of the basic acetate

Fig. 1.

Fig. 2.

molecule is capable of simple interpretation in terms of the electronic theory

of valency.Each of the four beryllium atoms possesses two peripheral or valency

electrons making eight in all, and when the four bivalent atoms enter into combination with the other constituents of the acetate molecule the distribution of these eight electrons completes the octet of the unique oxygen atom, and also completes those of the oxygen atoms of the six acetate radicals. In this distribution of electrons two pass to the unique oxygen atom and one to each of the six acetate groups.

* In the course of private discussion it was found that this idea had occurr independently to Mr. A. Berry (Aberystwyth) and to Dr. J. D. Main Smith (Burning am) on the basis of co-ordination and to Mr. T. V. Barker (Oxford) on crystallograpb grounds. Compare also Sidgwick, Nature,vol. I l l , p. 808 (1923).



441

Dealing first with the unique oxygen it will be seen that this atom now having eight peripheral electrons resembles neon in its outer form, and is, therefore, nonvalent in a chemical sense. But it differs from the inert gas in being negatively charged (— 2e) and is accordingly able to exert an electrostatic force attracting to its centre positively charged ions. The four beryllium ions having lost their peripheral electrons now resemble helium as regards their outer form, and like this inert gas they are nonvalent, although they differ from helium in being positively charged (-j- 2e). Hence these four beyllium ions are attracted by the unique oxygen, and their mutual repulsions cause them to assume a tetrahedral arrangement round the sphere of influence of this central atom.

The four positively charged beryllium atoms can themselves attract negatively charged ions. The twelve oxygen atoms of the six acetate groups having completed their respective octets are also nonvalent like neon, but differ from this gas in being negatively charged. Three of these acetate oxygens are attracted to each beryllium ion making with the central oxygen atom subsidiary tetrahedral arrangements round each beryllium atom. The carbon nuclei of the six acetate groups become arranged symmetrically along the edges of the main tetrahedron as shown in fig. 2. I t is to be expected that the acetate group will undergo modification of its internal structure in consequence of its inclusion in the molecule and in the crystal.

This tetrahedral structure is highly developed in the molecule of the basic acetate inasmuch as no less than eleven tetrahedra can be distinguished as follows: The central tetrahedron round the unique oxygen atom, fourtetrahedra round the beryllium atoms and the six tetrahedral carbon atoms of the six methyl groups.

These speculations concerning basic beryllium acetate have now been put to the test of X-ray analysis with the result that the earlier plane formulation (I) has been disproved, whereas the tetrahedral configuration (II) has been fully substantiated.

The completely symmetrical arrangement of acidic groups along the edges of the tetrahedron is possible only in the case of the basic formate or acetate, and this symmetrical distribution of the component atoms of the acetate group manifests itself in the crystalline structure of basic beryllium acetate which belongs to the cubic system.

When, however, the acetate groups are replaced by propionate complexes, as in basic beryllium propionate, the symmetrical arrangement is no longer possible. This decrease in molecular symmetry is also revealed in the

Basic Beryllium Acetate and Propionate.




crystallographic examination and X-ray analysis of the crystalline propionate which belongs to the monoclinic prismatic class.

The same loss of molecular symmetry is apparent in the basic beryllium acetate-propionate which, however, has so far been examined only in powder form owing to the difficulty of obtaining large crystals.

The Co-ordination Formula for the Acetate Group.

The results of X-ray analysis confirm in a remarkable manner the formulation of the acetate group on a co-ordination basis.

Although the presence of a carbonyl group in ethyl acetate (III) is revealed by the action of zinc alkyls or the Grignard compounds, these reagents fail to detect carbonyl in the molecule of acetic acid or its metallic salts (IV). This difference is in all probability associated with a definite change of chemical structure. In the esters the carbonyl group retains its integrity and unsaturated character as in III, but in the free acid or in the

H

III. IV.

basic beryllium acetate, the original difference between liydroxylic and carbonyl oxygens has been smoothed out so that the co-ordinated acetate group has now become symmetrical in the sense demanded by X-ray analysis.*

Preparation of Basic Beryllium Acetate and its Homologues.

The beryllium hydroxide employed as the starting point in these preparations was obtained from American beryl, which was decomposed by sintering with sodium silicofluoride (Copaux, ‘ Compt. Rend.,’ vol. 168, p. 610

c-c<a "o

0—C„H2aa5 H

H C

. o -<

H x 0

* According to some recent experiments made by Mr. Astbury the X-ray analysis of aluminium and gallium acetylacetones proves that there is a plane of symmetry in the molecule and in the acetylacetone group, which points strongly to a smoothing out of differences between carbonyl and enolic oxygen and leads to the establishment in these compounds of a symmetrical acetylacetone group,

CH

Me MeC = 0

* ° vC -O —Me Me

as predicted by one of us and H. W. Moss from the co-ordination view-point in 1914 (Trans. Chem. Soc., vol. 105, p. 193 (1914)).



443

(1919)), the purification of beryllia from alumina being effected by separating the soluble sodium beryllium fluoride from the sparingly soluble sodium aluminium fluoride and by the solubility of beryllium in aqueous ammonium carbonate.

Basic beryllium acetate Be40(CH3.C02)6 was prepared by dissolving beryllium hydroxide in 50 per cent, acetic acid, evaporating to dryness and crystallising the residue repeatedly from chloroform until the melting point remained constant at 285—286v (the highest recorded value being 283—284 (Lacombe, ‘ Compt. Rend.,’ vol. 134, p. 773 (1902)).

The purity of the specimen was confirmed by analysis. [Found C = 36-06, H = 4-44, Be — 8-73. C12H 180 13Be4 requires C = 35-43, H = 4-43, Be = 8 - 9 6 per cent.]

Basic beryllium propionate, Be40(CH3.CH2.C02)6, prepared by dissolving beryllium hydroxide in propionic acid, was crystallised from light petroleum (b.p. 80-100°) until the melting point remained constant a t 133-135°; the melting point for this compound has hitherto been recorded as 119-120° (Lacombe, loc. cit.).

The following analytical data confirm the purity of the specimen [found C = 43-94, H = 6-11, Be = 7-59, 7 -56. C18H30O13Be4 requires C = 44-04, H = 6-12, Be = 7 - 4 3 per cent.]. The yield of propionate was 82 per cent. For the following measurements, large crystals were prepared by allowing a solution of the compound in propionic acid to evaporate spontaneously.

Basic beryllium acetate-propionate, Be40(CH3.C02)3.(CH3.CH2.C02)3, was prepared by heating in a reflux apparatus for 1 | hours 14 gms. of basic propionate and 25 c.c. of acetyl chloride. The greater part of the propionate was then transformed into the mixed compound. Excess of acetyl chloride was removed in vacuo at the ordinary temperature. The product was separated from basic acetate and unchanged propionate by repeated crystallisation from light petroleum (b.p. 80-100°). The mixed acetate-propionate separated in felted masses of needles sintering at 139° and melting at 140-142°, the recorded value being 127° (Tanatar and Kurowski, £ J. Russ. Phys. Chem. Soc.,’ vol. 39, p. 936 (1907)). The purity of the product was confirmed by analysis : [Found C = 40-33, H = 5-57, Be = 8-13. C15H240 13Be4 requiresC = 40 • 15, H = 5 • 35, Be = 8 - 1 2 per cent.].

The foregoing preparations were employed in the X-ray measurements recorded below.

A crystallographic examination of the basic beryllium acetate made by ^ r- T. Y. Barker confirmed the fact that this compound is a regular octahedron

Basic Beryllium Acetate and Propionate.



444

geometrically and singly refracting optically. There is no doubt that it belongs to the cubic system, and in regard to symmetry there is no evidence against it belonging to Class 32. The natural etch figures are similar triangles on all the faces, and not tetrahedrally segregated. There is a moderately well developed octahedral cleavage. The specific gravity determined on clear splinters is 1 • 39 correct to the second decimal place.

\Part II. Analysis by X-Rays.

Basic Beryllium, Acetate.

The structure of basic beryllium acetate is of peculiar interest on account of its own characteristics as well as on account of its chemical significance. It resembles the structure of the diamond, the acetate molecule replacing the carbon atom. This will be clear if the figures in the first table are considered in connection with the following rules of interpretation of X-ray results.

In the simple cubic lattice the crystal unit of pattern is a cube formed by joining together eight neighbouring points of the lattice. A point may represent any group of atoms, but every point in any one crystal must represent a group identical in all respects including orientation. The unit cell then includes, as it should do, the substance of one group. The general form of the (Imn) plane referred to the edges of one of the cubes as axes is lx -f- my -|2 pa, where 2 a is the edge of the cube and p is any integer. The perpendicular from the origin on the plane is 2paf\/(l2 -f- m2 -j- n2), and the spacing is 2 a/<y( l2 -f- m2 -j- n2). By giving Imn all possible integral values we form a list of all possible spacings : the largest spacings being usually the most interesting and the most easily observed. Such a list is given by Hull (‘ X-Ray Studies,’ p. 189).

The face centred cubic lattice may be derived from the simple lattice by placing an additional point, representing the same group as before, at the centre of each cube face. The cube now contains the substance of four groups. The crystal unit cell is not now the cube itself, but the equal-sided rhomb formed by joining together the six face centres and any two corners of the cube which lie a t opposite ends of a diagonal (‘ X-Rays and Crystal Structure,’

P- 57)-I t is still convenient to use the cube as a frame of reference ; so that we may

draw up a list of the spacings of the new lattice by striking out of the former list such as are no longer possible. The original list is to be reduced because the introduction of the new points into the old lattice has halved a number of

Sir W. Bragg and G. T. Morgan.



the original spacings. I t can be readily shown that the spacings of the simple cube are unaffected by the addition of the new points if the indices of the corresponding planes are all odd or all even. For if + for example, were odd, then the result of substituting the co-ordinates of a face centre such as (a a o)in the expression lx -j- my -f- nz would be to make it an odd multiple of

a, whereas for all the old points the expression = 2 an even multiple of a. The (111) and (113) spacings are still in the list, but not the (100), nor the (112). So also the (224) is to be found, in the sense that though the first order reflection from (112) has gone, the second order remains. The halving of all but the “ all-odd ” or “ all-even ” spacings is the characteristic feature of the face-centred lattice, and is to be found, for example, in rock salt.

The diamond arrangement is obtained from the face-centred cube lattice by giving the latter a translation along a cube diagonal equal to a quarter of the length of the diagonal, thus adding four more points to the cube content. The arrangement is then such that every point is a t the centre of gravity of four others. If all these points are supposed to have equal effects on the X-rays we have a new set of spacings, which, it may be shown, can be obtained from the face-centred set by striking out all spacings for which

l m n — Ap -\- 2, where p is an integer. A full list has been given by Hull ( loc.cit.).

I t is not to be expected, however, that this set will ever be found to exist, because the diamond arrangement is not a true lattice. There are two kinds of representative points in i t : the two kinds of group which are represented face opposite ways and may therefore be expected to differ in their action on X-rays. The differences in the case of diamond itself are very small, and were overlooked in the early measurements. Later it was shown that a (222) reflection really existed, though (222) is one of those for which the sum of the indices = 4p -f- 2.

We ought not, in fact, to look upon the new set of points, the addition of which has led to the diamond arrangement, as due to a translation from the original set, and consequently there is no “ diamond lattice,” correctly speaking. I t happens that in the case of diamond the spacings actually found are very nearly those of the “ diamond arrangement ” ; which suggests that the carbon atoms are very nearly but not quite spherical. I t is natural to suppose that the departure from the sphere is towards the regular tetrahedron; and, further, that the tetrahedron which represents the behaviour of the carbon atom is placed so that the centre of each of its four neighbours lies on one of the lines joining the centre of the tetrahedron to a vertex. Two





neighbouring tetrahedra face each other, vertex to vertex. There are two kinds of orientation of the one kind of tetrahedral atom : they face each other along any diagonal of the cube. One kind is the reflection of the other, with a necessary shift, in any cube face. Every tetrahedron edge is parallel to a diagonal of a cube face.

We are led to consider how the list of face-centred spacings is to be modified if the points of the diamond arrangement represents tetrahedral atoms or groups of atoms which differ in orientation and in nothing else. We may represent a tetrahedron by its vertices.

If we imagine ourselves to be looking through the cube in a direction parallel to one edge, we should see the arrangement of points represented in the figure, the two nearer corners of each tetrahedron being represented by dots and the two further by small circles. The unseen edge of each tetrahedron

Fig. 3,

is represented by a dotted line. The various centres of tetrahedra do not all lie in the same plane. The centres of those marked Ax may be considered to be in the plane of the paper. Those marked lb lie in a plane which is at a depth a/2 below the paper ; the next are the A2 tetrahedra at a depth a ; the next those marked B2 lying at a depth 3a/2. The next are immediately behind those marked A15 and are at a depth 2a.

If the sides of the square are taken as axes of and y, the axis of being perpendicular to the plane of the paper, then ( ) or lx + my = 2 representsa series of parallel planes perpendicular to the paper and passing through the corners of the square and similar corners : are integers and the side of



the square is equal to 2a. When l and m are both even, the plane is an •“ all-even ” plane, and its spacing is found in the face-centred list.

To all ( Imo)planes, when acting as reflectors of the X-rays, an A tetrahedron is indistinguishable from a B tetrahedron ; the perpendiculars upon a plane from the four corners of an A are the same as those from a B, provided the perpendiculars from the centres are the same.

The co-ordinates in the plane of the paper of the centres of the B tetrahedra are of the form Jiaj2, hal2, where h and are both odd numbers. Such points will lie on “ all-even ” planes lx + my — only if + is a multiple of four.Thus if l, m are any integers we have the various planes of the form (Imo) which belong to the simple cubic lattice : if l and m are even the number is reduced to those that belong to the face-centred cubic lattice, and if the B points are put into the lattice we must further strike out the planes for which l-j— mis not a multiple of four.

In the last case the planes that are removed from the face-centred list are, so far as the table goes, (200), (420), (600), (640). This sifting applies only to planes passing through an edge of the cubic cell: for the tetrahedra are not equivalent to each other for any other set of planes. For instance, 222, 442 and 622 remain. If the A and B points represented spheres and not tetrahedra these last would disappear also, and we should have the diamond arrangement. In diamond they do very nearly disapppear, because the carbon atoms are almost spherical. In the basic beryllium acetate the molecules represented by the A and B points have a pronounced tetrahedral character and the spacings in question are all found.

The table shows in the first column a list of planes whose spacings are to be found in the face-centred lattice : for each of them l, m, n are all even or all odd. All the spacings are included down to (800), together with (555) and (888). In the second column are placed the spacings of these planes, calculated for a cubic cell of side 15 • 72 ; which cell will contain eight molecules, the number required by the hypothetical arrangement of the acetate. This number also gives the correct value of the specific gravity, viz., 1*39.

The third column shows which of these spacings are to be found in the diamond arrangement; those remaining are marked with a cross, and those disappearing with a dash. The fourth column shows the tetrahedral set, which includes some that are not found in the third column.

The fifth column shows the lines which have been observed by the ionisation spectrometer, the figures denoting relative approximate values of intensity. In some cases the spacings have been measured and the results are shown





in the sixth column. The seventh and eighth columns show the results found by the powder method,* and the relative strengths of the lines on the photographic plate ; w, m and s stand for weak, moderate and strong respectively, v denotes “ very.”

Plane. Calculatedspacing. “ Diamond.” “ Tetra

hedron.”Ionisation

spectrometer.Powdermethod.

i. i i . h i . IV. V. VI. VII. VIII.

i n 9 0 7 X X 300 9 0 7 9-07 v.s.200 7-86 —- — 0220 5-58 X X Trace311 4-75 X X 35 4-73 4-81 m.w.222 4-53 — X 110 4-53 4-57 m.w*400 3-93 X X 220 3-92 3-96 v.s.331 3-62 X X 55 3-60 3-64 s.420 3-52 — — 0422 3-22 X X 55 3-23 v.s.511 3-03 X X Trace 2-97 v.w.333 3 03 X X Trace 2-97440 2-79 X X 55 2-81 2-81 m.s*531 2-67 X X 2-67 m.442 2-64 — X 2-60 v.w.600 2-62 — — 0620 2-49 X X 20 2-51 s.533 2-40 X X 2-39 m.s.622 2-37 — X 2-39444 2-27 X X 35 2-28 m.w*711 2-21 X X 2-22 v.w.551 2-21 X X 2-22640 2 1 8 — —642 2-10 X X 2 1 1 v.w.553 2-05 X X 2 0 5 v.w.731 2-05 X X 2-05800 1-97 X X 35 1-95 v.w.555 1-81 X X 45888 1 1 3 X X 10

The above table shows that all the spacings observed by either method belong to the tetrahedral arrangement; and that nearly all the spacings of the arrangement have been found. The general argument is in no way affected by the absence of a spacing that might be found, though it would be by the presence of a spacing that ought not to be found.

The tetrahedron of our hypothesis is regular ; and therefore also the molecule of the crystal which we have shown to possess the tetrahedral arrangement is regular. The arrangement of the atoms within the molecule must be such as to give this degree of symmetry. The unique oxygen atom must be at the centre of the tetrahedron, and the four beryllium atoms must lie on the four lines joining the centre to the vertices. An acetate group, oxv

* We are indebted to Dr. Shearer for taking the photographs.



more correctly, the atoms composing an acetate group, must be disposed so as to be symmetrical about a plane perpendicular to each tetrahedron edge and passing through the opposite edge.

The intensities of the various reflections have not been considered so far ; nor can we interpret them fully since the molecule is complex. The most remarkable points are the great weakness of 220 and the strength of 400 and 222. All these facts fit in with a hypothesis that the influential centres of dispersion are crowded towards the corners of the tetrahedra, the tetrahedra being supposed to be in contact with their neighbours at their corners.

Basic Beryllium Propionate.

This crystal does not appear to have been measured previously. The X-ray measurements show that there are two molecules in the unit cell, and that it probably belongs to the monoclinic prismatic class. The specific gravity is 1*25. The dimensions of the cell are very nearly as shown in fig. 4.


Fig. 4.

a = 16-00, b = 9-76, c = 9-15, (i = 116° 7'. If dOa 90°, cd — very nearly. Assuming these dimensions the calculated values of the spacings of the various planes are as shown in the second column of the following table. The observed values are shown in the third, and the agreement is satisfactory. A number of the angles between the various planes were also measured, and found to agree within a few minutes with the calculated values.

As in naphthalene the second molecule is at the centre of the ab face, or very nearly so. If the molecule has a centre of symmetry its position can be exactly defined with reference to the first. I t is at once the reflected molecule, and must therefore lie on some line parallel to the b axis and passing through the centre of a face of the cell, and at the same time it is the digonal molecule, and therefore lies on a plane perpendicular to the 6 axis and passing through a



450 Basic Beryllium Acetate and

face centre. The halving of the (010) spacing proves the la tte r : the 9-76 spacing was not found by the spectrometer, nor was it recorded on a photograph which Dr. Shearer was good enough to make by the powder method. The former would follow from the halving of all (l, o, n) spacings for which l is odd. The table shows that (100) is halved, and that (001), (20l), (401) are not, which is so far correct. The 101 and 101 spacings if not halved would be 6'08 and 9-07 respectively. Dr. Shearer’s plate shows a line of moderate strength which might be the latter, but which also might be due to the /1-ray reflected by the (001) plane ; and also a weak line which gives a spacing 6-00. If these are real then the second molecule is not quite the reflection of the first, while at the same time it is derived from the first by a rotation through 180° and a shift. This implies that the molecule of the propionate has actually no symmetry a t all, though it is approximately centrosymmetrical. The 101 and 101 spacings were not found by the ionisation spectrometer, but a more careful search might be made.

However, the chief point of interest, which is perfectly clear, is that the addition of the CH2 to the acetate has completely destroyed the symmetry of the molecule as it is built into the crystal. This may be due to the fact that the atoms of the propionate group can no longer be arranged in a symmetrical manner with respect to a plane.

Plane.Spacing

calculated.Spacing

observed.Intensity. Lines found by

Shearer.

100 14-36 7-18 m.010 9-76 4-88 m.001 8-23 8-23 v.s. 9-00 to. 001/3 or 10Io !021 4-19 4-21 m. 8.15 «. 001, 110.043 1-82 1-82 v.w. 7-79 to.to.041 2-34 2-34 ■w. 7-26 m.s. 100, 201.201 7-20 7-20 s. 6-70 to. 111.401 4-00 4-00 s. 6-00 w. 101?110 8-07 8-08 s. 5 • 42 to. ?120 4-62 4-66 s. 4-26 to. 021, 121.310 4-30 4-29 v.w. 3 -95 s. 401.111 5-16 5-16 v.w.222 2-58 2-58 w.111 6-64 6-61 m.112 3-35 3-34 m.s.112 4-10 4-05 m.121 3-80 3-81 m.121 4-30 4-32 m.411 3-70 3-70 w.421 3-09 3-11 v.w.



An Experiment on the Origin of the Earth's Magnetic Field. 451

Basic Beryllium Acetate-Propionate.

The powder photograph shows a set of spacings which are not those of a crystal of high symmetry; but might belong to a structure analogous to the propionate. In the absence of crystals of sufficient size it was not possible to use the spectrometer.

The beryl employed in these experiments was obtained by the aid of a grant from the Department of Scientific and Industrial Research ; it was worked up into the purified organic compounds with the assistance of Messrs. T. J. Hedley and C. R. Porter.

An Experiment on the Origin o f the Earth's Magnetic .By H. A. W il so n , F.R.S., Rice Institute, Houston, Texas, U.S.A.

(Received July 30, 1923.)

Of the many suggestions which have been made as to the origin of the earth’s magnetic field, perhaps the most promising is that it may be due to a slight modification of the laws of electrodynamics from the commonly accepted form.* Electrically neutral matter is believed to consist of an intimate mixture of enormous amounts of positive and negative electricities, the electric and magnetic effects of which are usually supposed to balance each other. If the balance were not quite exact then small residual effects would be expected, among which gravitation and the earth’s magnetic field might be included.

On such an hypothesis we might expect moving matter to produce a magnetic field similar to the field due to moving electricity, and we should expect some relation between the magnetic field due to moving matter and its gravitational action.

The gravitational unit of matter (about 4,000 grams), which attracts an equal mass at one cm. distance with a force of one dyne, might be expected to produce a magnetic field of the same order of magnitude as the electrostatic unit of electricity defined in the same way.

* “ A Critical Examination of the Possible Causes of Terrestrial Magnetism,” by Sir A. Schuster, ‘ Proc. Lond. Phys. Soc.,’ p. 121 (1912); “ Unsolved Problems of Cosmical Physics,” by W. F. G. Swann, ‘ Journal of the Franklin Institute,’ April (1923).



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