NEWS

not exceeding the one thousand millionth of a milligramme

CRYSTALLOGRAPHY has made such remarkable progress during the last few months, and the position at the present moment is so interesting, that it was considered opportune to review it in a Discourse from this historic lecture-table. For, firstly, the descriptions of the crystals of all the ten thousand substances which have ever been subjected to goniometrical measurement have been collected together and classified within the four volumes of a monumental work by Professor von Groth, of Munich. Secondly, this immense labour has been paralleled by the construction, by Professor von Fedorow, of St. Petersburg, of a tabular record of the main crystal elements of all these substances, arranged in a simplified form and with the assurance, which has entailed untold labour to achieve, that they relate to a truly comparative orientation; so that this table is the index to and basis of a new method of "Crystallo chemical analysis," which enables a trained investigator to identify any well-c. ystallised substance from the result of a brief goniometrical examination. And, thirdly, the whole of these invaluable results have been placed on a firm experimental basis; for the internal structure of crystals, as imagined in all its wonderful details by the greatest geometrical and mathematical minds amongst us, has been revealed on the photographic plate as the result of direct experiment with the excessively minute and all-penetrating wave-motion, or corpuscular energy, of the X-rays.

For

It is easy to prove that a crystal has an organised structure. The fact is at once revealed by the influence of rapidity or slowness of growth on its character. example, if a little benzoic acid be melted on a glass plate over a spirit lamp and the plate allowed to cool rapidly in the air, on the object-stage of the projection polariscope, using crossed Nicols, the dark field is almost immediately illuminated by crystals beginning to grow at the margin of the liquid film, and from each bright spot a crystal-needle darts, with fiery tip, like a lightning flash, into the centre of the field, until the whole picture is an interlacing mass of acicular crystals brilliantly coloured in the polarised light, which renders them more visible, the colour being due to double refraction caused by structure. Again, if we crystallise a substance from solution, say potassium bichromate, it entirely depends on whether the degree of supersaturation is slight, the "metastable" condition of Miers and Ostwald, or excessive, the "labile" condition, as to what kind of crystal we obtain. From the metastable solution perfect little single crystals are started into slow growth, by the advent of germ-crystals of the same or an isomorphous substance from the air; while the labile solution spontaneously and rapidly crystallises in the beautiful feathery forms illustrated on the screen. monium chloride and metallic silver afford us even more beautiful screen pictures of arborescent crystallisations, and nothing can exceed the beauty of snow crystals, an example of rapid crystallisation of water vapour. For goniometry these labile forms are useless, but, neverthe: less, they teach us much concerning the structure of crystals. For in them the skeleton, or inner framework and plan of architecture of the crystal is revealed.

Am

It is hard to realise the clearly proved fact that our atmosphere teems with excessively minute crystals-for they possess the complete organisation of a crystal-often A Discourse delivered at the Royal Institution of Great Britain,

March 14, 1913.

slightly supersaturated solution of a crystalline substance of like structure, of calling forth its power of crystallising. In order to be able to exercise this remarkable power, however, the germ crystal must be isostructural in a very strict sense, if not identical with the substance which is set crystallising. Not only must its symmetry be similar, but the dimensions of its structural units-the "bricks" of the crystal edifice so to speak-must be all but identical. The conditions are, indeed, similar to those required for substance on another, so admirably investigated by the facile formation of parallel growths of one crystallised Barker.

Perhaps the most striking cases for the purpose of illustration are those of the rhombic alkaline sulphates, selenates, perchlorates, chromates, or other of the well crystallised salts of the alkali metals potassium, rubidium, and cæsium, and of the base ammonium which is so extraordinarily capable of replacing them. In any such group of salts the periodic law of Newlands and Mendeleeft is most beautifully illustrated by the regular progression of all the properties of the crystals of the three metallic salts, corresponding to the progression in the atomic weights of the metals, the salt of rubidium, the metal of intermediate atomic weight, having invariably intermediate properties, both morphological and optical. showing the gradual slight change in the prism angle of the crystals of the sulphates will make the point clear, the slope of the prism face of rubidium sulphate being intermediate between the greater slope of that of the potassium

A slide

FIG. 1.-ROTATION OF THE OPTICAL ELLIPSOID of Double SULPHATES ON REPLACING POTASSIUM BY RUBIDIUM OR CESIUM.

salt and the lesser slope of that of the cæsium salt. The variation of the position of the optical ellipsoid in the three monoclinic double sulphates of the 6H20 series containing these three salts is also illustrated by a slide in which the ellipsoid can be rotated through the three positions (the rotation being also indicated in Fig. 1 by the dotted ellipses). In a similar manner the dimensions of the structural units-the molecular volume (that of the "brick" regarded as a molecule) and its expression in the three dimensions of space (which we now have a means of determining, by combining the density and the crystallo. graphic axial ratios, in what are known as topic axial ratios)-vary in regular progression as functions of the atomic weight. Hence, the structural dimensions of the two extreme members of any of these groups of salts, the potassium and the cæsium salts, are most divergent, and Barker has shown that while the rubidium salt will in general form parallel growths with either the potassium or

the cæsium salt, the potassium and cæsium salts themselves will never form satisfactory parallel growths on each other, clearly owing to the disparity in the dimensions of their structural units. Further, the ammonium salt of any group has the interesting property of forming crystals of which the molecular volumes and topic axial dimensions are almost identical with those of the rubidium salt of the same group. Now it is most important and conclusive that the ammonium and rubidium salts form the best of all parallel growths; they form, indeed, no longer merely parallel growths, but zonal growths and complete overgrowths. They also form excellent mixed crystals, and in every way which has yet been experimentally tested they show the nearest approach to true iso-structure. Some photographs of such parallel growths of rubidium and ammonium salts will render the matter clear. There is also one case of iso-structure investigated by Barker, that of calcite and sodium nitrate, which is of particular interest; for these are not chemically isomorphous substances, but they happen to have similar rhombohedral symmetry and almost identical molecular volumes and topic axial dimensions. The slide of parallel growths of sodium nitrate on calcite (reproduced in Fig. 2, by the kindness of Mr. Barker) is very striking and conclusive. Barker has

FIG. 2.-PARALLEL GROWTHS OF SODIUM NITRATE
ON CALCITE.

recently suggested the nature of the structure of these two substances, two molecules forming the unit "brick," as represented in the slide exhibited. In the case of potassium sulphate and its analogues, one molecule forms the "brick," as represented by the next slide.

It has thus been proved experimentally from the morphological side that a crystal has a definite structure, and that its unit "bricks" or "cells" have definite and measurable dimensions. But the fact is equally well demonstrated optically. We have only to pass a beam of light through a 60° prism of a non cubic crystal, for instance, quartz, to see at once the radical difference of effect from that given by glass or other non-crystallised substance. For instead of the usual single spectrum produced by glass the quartz prism refracts two distinct spectra, unless it happens to have been cut so that the light traverses the unique direction of single refraction, coincident with the axis of the natural quartz crystallographic prism, which is also the axis of trigonal (threefold) symmetry. The quartz prism used in the experiment is cut at right angles to this direction, so that the axis and refracting edge of the cut prism are parallel to the natural axis, and the separation of the two spectra, corresponding to the two refractive indices of quartz, is thus at a maximum. Moreover, on placing a

Nicol prism in the path of the refracted rays, we observe that the light producing the two spectra is oppositely polarised, one spectrum extinguishing when the Nicol has its vibration plane vertical, and the other when the Nicol is rotated so that the vibration plane is brought into the horizontal position. The crystal thus possesses a structure, which is capable of separating a beam of ordinary light into two beams, having definite and perpendicularly different vibration directions.

Again, we see proof of structure if we cut a plate out of the crystal and examine it in a converging beam of polarised light, especially if the crystal, say one of calcite, be cut perpendicularly to the singular axis (or to the bisectrix of the two such axes in the cases of biaxial crystals) of single refraction. A beautiful interference figure is produced, composed of spectrum-coloured rings and a black cross, that is, a figure symmetrical about the axis of trigonal symmetry and of single refraction. This evidence of structure can be most wonderfully reproduced by glass, it we strain the glass by heating and rapid cooling about a cylindrical axis, but an ordinary unstrained piece of glass affords no such effect at all. It is clear, therefore, that the calcite crystal has a symmetrical structure about the axis of single refraction. Similarly, the beautiful biaxial interference figure exhibited on the screen, afforded by a plate of rhombic potassium nitrate, is symmetrical about the centre of the double-looped figure of spectrum-coloured lemniscates.

Sufficient evidence will now have been brought forward that a crystal is endowed with a definitely organised structure. In the crystal of a pure substance we are dealing with a chemical element or compound, and if with the latter it may be of any grade of complexity, from a very simple binary compound to a most highly complicated one composed of a large number of atoms. If the crystal be that of an element the structure is obviously composed of the similar atoms of that element, while if it be a compound we have a structure composed of atoms of as many kinds as there are chemical elements present combined in the substance, and in the same relative proportion as is expressed by the chemical formula of the substance. In the case of a compound, moreover, the structure may also be considered to be that of the molecules of the substance, for they or a simple arrangement of a small number (group) of them form the grosser units of the structure, whilst the atoms are the ultimate units.

Suppose we now represent this molecuar or poly. molecular grosser structural unit by a point, and that such point be analogously situated within each unit. The essence of crystal structure then is that these points are so arranged in space that if they are joined along the three directions of space by imaginary lines the latter form a "space-lattice" (German, "Raumgitter "), each unit cell of which may be conceived to be the brick" already alluded to, and the domicile of the chemical molecule or group of molecules (indeed, it is immaterial whether the points are considered as placed at the corners or in the centres of the cells), or, in the case of an elementary substance, of a group of similar atoms. We may therefore define a crystal as follows:

"A crystal of any definite chemical substance consists of a homogeneous arrangement of grosser units of matter, each consisting of one chemical molecule or a small group of molecules of the substance, and the kind of arrangement is such that these grosser units are all identically (sameways, parallel-wise) orientated, and that their analogously chosen representative points, one from each such grosser unit, form a space-lattice (Raumgitter)."*

Since this lecture was delivered (March 14, 1913), and printed by the Royal Institution, a paper by Prof. Theodore W. Richards, of Chemical Society for April, 19.3 (vol. xxxv., p. 381), in which he shows Harvard University, has appeared in the Journal of the American that his theory of compressible atoms leads to "crystal units" of precisely the molecular or polymolecular character described in this lecture. He supposes such crystal units to be the entities necessary to points of the crystal space-grating, assumptions with which the relieve metastable supersaturation, and their centres to form the lecturer obviously fully concurs.

There are fourteen kinds of space-lattices, slides of several of which are exhibited on the screen. Three possess full cubic symmetry, two are tetragonal, four are endowed with rhombic symmetry, and two are monoclinic; while triclinic, trigonal, and hexagonal crystals have each one space-lattice corresponding to their type of symmetry. In every case it is the full (holohedral) symmetry of the system which is present, no space-lattice possessing merely the lower degree of symmetry corresponding to one of the so-called hemihedral or tetartohedral classes of the system in question. Now in the solid crystal, not only are the grosser units arranged so that their representative points are repeated in space with extraordinary accuracy of position, with production of unit cells or "bricks" of absolutely identical dimensions throughout the crystal, but the shapes of the grosser units themselves are identically similar and identically similarly orientated in space. Suppose, however, that the force of crystallisation, the directive molecular force concerned in bringing the molecules together in this

substances for the special purpose of this lecture, and of Mr. Poser, of Messrs. Zeiss, who construct an admirably conveniently form of heating microscope and projection arrangement for demonstrating the formation of liquid crystals and their behaviour in polarised light, it is possible to exhibit some of the typical phenomena of these interesting objects on the screen. The substances in question are chiefly such as form two or more polymorphous forms, each stable within a limited range of temperature, and the liquid crystals are usually the second phase observed on allowing the truly liquid heated substance to cool; the liquid crystal phase is produced at a definite temperature during the cooling, and persists through a definite interval of temperature during the continued cooling. It either exhibits distinct attempts at the formation of pyramidal or prismatic crystals, more or less rounded at their edges, as in the case of ammonium oleate shown on the screen (Fig. 3), or manifests itself as doubly refractive and brilliantly polarising drops or streams, such as the drops of para-azoxyanisol, which are set rotating independently by

regular order of marshalling, is only adequate just to attain this marshalling of the grosser units into a space-lattice formation, without being able to fix the units about their own centres of gravity, a certain amount of wobbling about the latter being still permitted. We might, in such circum stances, expect that some of the properties of a crystal, dependent on the space-lattice formation on lines of definite symmetry, such as the optical property of double refraction and polarisation of light, would be developed and exhibited, while the production of exterior plane faces would be either only partial, with rounded edges and the exhibition of plasticity and viscosity, or would not be achieved at all, the objects produced being still fluid. One cause of such a condition of partial success at crystallisation might well be that the substance was composed of a large number of atoms arranged in a long chain, such as the well known "long chain compounds" of organic chemistry, which would offer considerable resistance to marshalling. The author believes that herein lies the explanation of the remarkable "liquid crystals" which Professor Lehmann has made the subject of his particular study, many of which are of just such long chain character.

By the kindness of Professor Lehmann, who has sent over specimens of some of the most characteristic of his

the addition of a little colophonium, as demonstrated in brilliant polarisation colours on the screen; it then suddenly passes with further cooling into the final solid phase, often with the production of brilliantly coloured acicular crystals, as in the case of para-azoxyphenetol exhibited on the screen, or of beautiful star-like or flower-like apparitions, radiating from innumerable centres all over the field, as in the exceedingly beautiful case of cholesteryl acetate. The view here put forth is apparently in agreement with that of Lehmann himself, as most recently expressed both in letters to the lecturer and in a memoir of July 27, 1912, to the Heidelberg Akademie der Wissenschaften, in which he says that in all probability :-" Die Rundung der Formen hänge zusammen mit der Plastizität der Stoffe und habe ihren Grund in unzureichender molekularer Richtkraft, welche wohl genügt, ein Raumgitter herzustellen, nicht aber regelmässige Treppenstufen, wie es nach Hauys Theorie zur Bildung ebener Krystallflächen notig wäre." The formation of regular stepped faces (of invisibly minute steps, "Treppenstufen ") the lecturer considers to occur only when the grosser units become fixed about their centres of gravity or representative points, with production of a truly solid crystal.

But now let us pass to the consideration of the internal

the Cambridge meeting of the British Association in 1904. "A crystal-considered as indefinitely extended-con sists of n interpenetrating regular point-systems, each of which is formed from similar atoms; each of these pointsystems is built up from n interpenetrating space lattices, each of the latter being formed from atoms occupying parallel positions. All the space-lattices of the combined system are geometrically identical or are characterised by the same elementary parallelopipedon."

(To be continued).

Now, just as the genius of Frankenheim and Bravais revealed to us the 14 kinds of space-lattices, so Sohncke made us acquainted with 65 regular systems of points, including many of the 32 classes of symmetry, but not all, which von Lang had shown crystals to be capable of possessing. Later the number was brought up to 230 by simultaneous and wonderfully concordant geometrical researches by Schönflies in Germany, von Fedorow in St. Petersburg, and Barlow in England, and among these 230 all the 32 crystal classes are represented, and no others. Hence, we come to the conclusion that the skeletal framework of crystal structure is the molecular or poly. molecular space-lattice, and the detailed ultimate structure the atomic point-system. The latter determines the class of symmetry (which of the 32 classes is exhibited) and therefore governs any hemihedrism or tetartohedrism, as the development of less than full systematic symmetry used to be called. But it is the space-lattice which governs the crystal system; that is, which determines whether the symmetry is cubic, tetragonal, rhombic, monoclinic, triclinic, trigonal, or hexagonal, and which also determines the crystal angles and the disposition of faces in accordance with the law of rational indices, the law which limits the number of possible faces to those which cut off small whole-number relative lengths from the crystal axes. Indeed, it is because only those planes which contain the points of the space-lattice are possible as crystal faces that the law of rational indices obtains. For any three points of the space-lattice determine a plane in which similar points are analogously regularly repeated, and which is a possible crystal face obeying the law of rational indices. Moreover, those facial planes which are most densely strewn with points are of the greatest crystallographic importance, being what are known as the primary faces, either parallel to the crystal axes or cutting off unit lengths therefrom, as well as being usually the planes of cleavage.

As the space-lattice units are all sameways orientated, any one atom of the molecular or polymolecular grosser unit might be equally well chosen as the representative point of the lattice, so long as a similar choice were made in every space lattice unit, and the resulting space-lattice would be the same whichever atom were so selected. Consequently, the space-lattice is afforded by the similarly (identically) situated atoms of the same chemical element throughout the crystal structure. The combined pointsystem (one of the 230 possible point-systems) may thus be considered to be built up of as many identical but interpenetrating space-lattices as there are atoms in the space-lattice grosser unit. These facts are concisely expressed in the definition of crystal structure

A STUDY OF THE

FRUIT OF CRATAEGUS MACRACANTHA. By W. BRUCE ARMSTRONG.

THE fruit was gathered in a thorn patch between Mount Vernon and Lisbon, Iowa. This species of the Crataegus grows on shrubs about 15 to 20 feet in height. It is indigenous to the territory from Quebec to Dakota, and as far south as Virginia and Missouri.

We decided to call this species Macracantha, although it did not coincide with the description in every detail (according to the classification of Lodd), resembling Rotundifolia in a few minor respects. But on account of the difficulties in identifying the species of this genus, it was thought best to name it according to the species which it most closely resembled.

The colour of the fruit is orange bordering on the red. Each apple contains two or three seeds, and the meat is woody. The fruit is about 0.7 cm. to 1 cm. in diameter, and nearly spherical. It was impossible to grind it with a mortar and pestle, and very difficult to cut. The fruit, when burned, gives off an odour resembling that of burning oak leaves, and so irritating as to produce sneezing. At one stage they burned with a distinct sodium flame. This came from the blossom end; they burned with force enough to move them about in the dish. There were about 450 grms. available. The average weight per berry was 0.916 grm. The Sugars.

About 200 grms. of the fruit were taken for the sugar extraction. They were placed in a litre flask, fitted with an inverted condenser, and treated with successive portions of alcohol for about twenty-two days. The alcoholic extraction was removed each day and a fresh portion applied. When the alcohol was distilled off a thick, almost black, syrup remained. The first extraction was neutral to litmus. But after standing eight days the syrup solution reacted slightly acid to litmus, probably due to fermentation. The colour of the first extraction was dark brown, and it smelled considerably like wet brown sugar. After standing a few hours a light brown sediment settled and the liquid above darkened.

A test with Fehling's solution at the end of twenty-two days showed that the sugars were almost extracted, and distilled water was substituted for alcohol. The berries under the alcohol treatment remained hard, but on the addition of water they became mushy. The last alcoholic extraction was a light yellow colour, but the water was darker than thick maple syrup or vinegar, and smelled like dates which had been boiled in water. The water extraction acted acid to litmus. About twelve days were required to remove all the remaining sugar by the water treatment.

The percentage of the sugars was determined by the Fehling's solution of such a strength that 10 cc. corresponded to o'05 grm. of sugar. One cc. of the extraction was taken and diluted to 50 cc. with water in a small beaker. This was heated to boiling and titrated with Fehling's solution. The end of the reaction was ascertained by placing a drop of the solution on a folded filter