SOME PHYSICAL PROPERTIES OF NACREOUS SULPHUR BY H. WHITAKER
The attention of the author was drawn to a micro-crystalline form of sulphur in the course of an ultra-microscopic investigation of the properties of sulphur clouds. It was desired t o repeat the experiments of Ehrenhaft on the resonance colours of sulphur paiticles and for this purpose the sulphur was vaporised at a low temperature in a U-tube. It was then noticed that the deposit which formed slowly above the molten sulphur underwent a change from liquid globules to a mass of tiny crystalline plates which exhibited striking colours. A microscopic examination of this deposit showed the presence of hexagonal plates, which at first were regarded as a new crystalline variety, but later proved to be the nacreous form which has been investigated by Muthmann. Nacreous sulphur was in all probability observed as far back as 1852, according to Muthmann,l who, in this connection refers to the work of Payen (1852), Ste. Claire-Deville (1852), Debray (1858)~Geinez (1884), Bruhns (1887-8), the latter obtaining the crystals in sufficient size so that their physical constants could be determined. I n 1890, Muthmann took up the detailed study of sulphur and selenium, repeated the experiments of Dr. Bruhns, and carefully measured the interfacial angles of nacreous sulphur crystals. In 1899, Salomon2 obtained these crystals by heating a little sulphur in a watch glass, covered with a microscope slide, upon which a film of sulphur droplets was allowed to condense. Salomon briefly refers to their crystallographic and optical properties and bases the identity of his crystals with those investigated by Bruhns and Muthmann, upon measurement of the interfacial angles. It seemed worth while to investigate the mode of origin, stability, and optical properties of these crystals and in the following pages a short account is given of experiments on these points.
Experimental Crystals of nacreous sulphur are conveniently obtained by growing them from a film of sulphur droplets. This can be accomplished in several ways. At first the film was condensed on the walls of a U-tube. Later, it was found more advantageous to substitute a rectangular glass cell, made up of microscopic slides, for one limb of the U-tube. The bend of the U-tube, containing a little roll sulphur, was gently heated in a bath of iron fillings, whilst a slow stream of dry air was aspirated through the cell. A fine film of sulphur droplets condensed on the walls of the cell, which was examined periodically under the microscope and photographed a t intervals. W. Muthmann: Z. Kryst. Min., 17, 336 (1890). Salomon: Z. Kryst. Min., 30, 6 0 j (1899).
* W.
H. WHITAKER
Plate I shows the appearance of the droplets after five days. Plate I1 shows the appearance of another portion of the same film after five days, under higher magnification. The solidified droplets are distinguished by a prickly appearance and by the absence of the central spot of axial light. The crystals appear first as needles, then as trapezia, and finally assume the shape of hexagonal plates, branching out from the solidified droplets. The mechanism of crystal growth seems to be as follows: sulphur vapour evaporates from the still liquid droplet8 and condenses upon the solidified droplets as crystalline nacreous sulphur. At least three processes are going on simultaneously, (I) differentiation in size among droplets, the larger droplets growing at the expense of the smaller, (2) crystallization of some of the droplets, the others
PLATE I
PLATE I1
Appearance of sulphur droplets after 5 days, X 80 diameters.
Appearance of sulphur droplets after 5 days, X 2 j o diameters.
remaining liquid for longer or shorter intervals, (3) growth a t the expense of the surrounding droplets of crystals of nacreous sulphur and of octahedral sulphur. Neighbouring droplets evaporate for the most part more quickly than those more remote, and, since it is well known that small droplets evaporate more quickly than large ones, it is to be expected that after a suitable interval of time, a crystal will be surrounded by a space free from droplets. Plate I11 shows such a crystal surrounded by a clear space. The ideal crystal of nacreous sulphur, obtained by growth from droplets, is hexagonal in shape, according to Salomon, having three pairs of opposite sides parallel, the three principal angles being 124' 45', 146' 43 ' and 88' 16', measured microscopically. A single crystal was obtained from a cluster and laid flat upon a slide. I n this operation one end of the ciystal was unavoidably fractured and only three angles could be measured. Difficulty was found in obtaining a perfect crystal but eventually one was obtained (Plate IV) by
SACREOUS SULPHUR
40I
forming a film of droplets (average diameter of a droplet =O.I mm.) on a microscope cover slip, which was ipverted and cemented to a microscope slide, leaving a thin air film between. Measurements of the six angles gave the following mean values: 88' 8', 146" 23') 125' 4', 88" 3 0 ' , 146" 36', 124' 45'. Actual length of crystal= 0.16 mm. Between crossed Nicols, the directions of extinction were apparently parallel and perpendicular respectively to a long edge of the crystal. Plate JT shows the appearance of the crystals after eight days growth. Four types of crystals were observed: , ( I ) octahedra; the plate shows one
PLATEI11 il single crystal of nacreoua sulphur, x 8 0 diameters.
PLATE IV A single crystal of nacreous sulphur, with six complete sides. Age 12 days, X 330 diameters.
large well-developed specimen, ( 2 ) nacreous plates, imperfectly developed, (3) hexagonal plates, (4) curved hair-like crystals (estimated average thickness, 0 . 0 2 mm.). The last named do not appear on the plate. An edge of a hexagonal plate, after ten days growth, measured 0.024 mm. The two smaller angles measured 90°, (min. 89" 45 ', max. 90") ; the other four, 135' each, (min. 134" 45') max. 135" 18'). The crystals were colourless and between crossed Nicols the directions of extinction were apparently parallel and perpendicular respectively to the long axis. The crystals are possibly rhombic or monoclinic, but they were not studied in further detail so as t o determine definitely the system to which they belong. The angular measurements distinguish them from the nacreous plates and also from Muthmann's hexagonal tabular sulphur1 (SI>.), the plane angles on the predominating face being 120". W. Muthmann: Zeit. Kryst. Min., 17, 343 (1890).
402
€1. WHITAKER
Plate VI shows a cluster of nacreous sulphur crystals and one octahedron, two days old, its chief feature being that itghows the possibility of simultaneous growth of both crystalline forms, growing in the same way from the droplets. In this plate, the crystals shown are on the same surface as the droplets; in Plate V, the crystals are developing on the free glass surface nearest to the droplets, the hazy background being due to the droplets being out of focus. Plate VI1 shows a cluster of nacreous sulphur crystals, possessing various colours by transmitted light. Their appearance under the microscope, due to the assemblage of different colours, is often very striking. The large parallelogram-shaped crystal, to the left of the centre of the plate, was grey on its outermost edge. Within this was a green band, and in the centre, a cone of
PLATE V
PLATE VI
Sulphur crystals after 8 days growth, X 80 diameters.
Rhombic and nacreous crystals, X 360 diameters.
colours ranging from red a t the tip of the cone to greyish violet at the base. The large crystal at the bottom left hand corner was ielicately coloured with alternate bands of pale red and pale green, shading into one another, resembling the pale red and green bands seen in soap films. I n general, however, the tint is uniform throughout a given crystal. To the right of the centre will be seen a small crystal possessing a curious net-work appearance on its surface, which will be referred to again. The extreme thinness and transparency of these crystals is evidenced by the fact that the black dots, representing the sulphur droplets, are readily photographed through the crystal. Thickness of crystals. This was determined in the case of some relatively thick crystals by isolating a single specimen on the end of a glass rod drawn out to a fine point, and rotating the rod until the crystal assumed a nearly vertical position. An edge was then focussed through the microscope and its thickness measured by an eye-piece micrometer. The thickness turned out to
NACREOUS SULPHUR
403
be of the order of .OOI mm. I n the case of the coloured crystals, attempts were made to isolate a single crystal showing an interference band in its spectrum. After many failures, a pink crystal was isolated showing a dark band in the region of 0 . 5 ~ . Its thickness was calculated to be 0 . 1 3 ~ . Attempts t o measure directly the thickness of its edge failed; its apparent thickness, using a I,”” objective and a high power eye-piece, was less than the thickness of a single line of the eye-piece micrometer scale. A rough calculation from these data indicated that its thickness WRS below 0 . 4 ~ . Colours of crystals. A crystal was mounted so that it could be rotated about a horizontal or vertical axis and the light reflected from its surface
PLATE VI1 Group of nacreous crystals showing interference colours.
PLATE# VI11 Crystal showing etch figures, X diameters.
2 j0
examined microscopically. If the colours are due to interferences, as seems to be the case, a change in tint should occur with change in the angle of incidence of light upon the crystal. To examine this question, a piece of mirror was smoked on its surface and a slide, containing some nacreous crystals, was inverted and pressed gently upon the smoked surface, leaving thereon a pumber of the crystals. The smoked glass was then mounted so that a given crystal could be brought into the centre of the field of the microscopic and rotated about a horizontal or a vertical axis. By using a smoked surface, reflections from the glass slide, supporting the crystals, were eliminated, and at the same time a dark background was provided, which made it possible to follow the colour changes with greater certainty. The source of illumination was diffused daylight or diffused artificial daylight from an Osram Daylight Lamp. To take one example, a green crystal was rotated about its long axis, with the plane containing the incident and reflected light perpendicular to the long
404
H. WHITAKER
axis. With change in the angle of incidence from roughly 15’ to 7 5 O , the colour of the crystal changed to blue, then purple and finally reddish purple. When rotated in a similar manner, about an axis perpendicular to its long axis, the colour changed to greenish blue. The greatest variation in tint occurred when the crystal was rotated about its long axis. The pleochroism of the crystals has already been noted by Salomon. The transmitted light resolved by a Nicol consists of two components, one pale pink and the other pale green. The feeble colours may be accounted for by the extreme thinness of the crystals. The incident light is for the most part transmitted, and the pleochroic colours are to some extent masked by white light. If the reflected light be examined through a Nicol, the colour is in consequence more saturated and the change in colour on rotating the Nicol is very striking. To give only one example: light was incident upon the crystal face of a sky-blue crystal at 30’ approximately. When the plane of vibration of the analysed light was parallel to the long axis of the crystal the colour was greenish-blue; when perpendicular to the long axis the colour was purple. These observations were repeated, allowing light to fall upon the crystal at different angles of incidence. I n the crystals examined, the variation in tint seemed most marked when the light was incident a t an oblique angle. Strong evidence for the theory that the colours are due to interference is afforded by the presence of a dark band in the spectrum. The following observations confirm this view. (I) Crystals 24 hours old did not show any colour. Pale colours were observed in some crystals two or three days old. Some crystals, two or three weeks old, which had evidently increased in thickness and area, showed no colour . (2) Occassionally two similarly coloured crystals overlapped. The resulting colour was often quite different from that of either. (3) Two or three large crystals were examined with the naked eye against a dark background and their colours, as seen by reflection noted. They were again examined against a white background when their pale complementary tints were observed. “Ageing” of crystals. After three or four weeks, it was observed that whilst many nacreous crystals remained stable, jn a few a remarkable change had taken place. Plates VI11 and IX illustrate these changes. The etch figures, which exhibit the characteristic angles of the large crystal, are in reality holes in the crystal itself. The crystal is in a process of disintegration. At the centre of each hole is a tiny speck. In the centre of some of the larger holes there are irregular masses, the details of which could not be resolved. All attempts made to resolve the detail of the tiny specks were unsuccessful. It is believed that they consist of new crystalline nuclei, of the stable octahedral variety, which are growing at the expens? of the nacreous plate. Since the octahedra grow in three dimensions, whereas the nacreous crystal may be considered for practical purposes as two-dimensional, a relatively large area of plate will be
S A C R E O U S SULPHUR
40 5
required to build up a tiny octahedron. This view is supported by the observations of Gernez who found that a nacreous ciystal changed into the rhombic form when touched with a rhombic crystal. Muthmann also noticed that when a rhombic crystal fell on a leaflet of nacreous sulphur which had separated from solution, a circular hole formed in the latter and the rhombic crystal continued to grow. It is of interest to compare the relative areas presented to the eye of a nacreous crystal and of a rhombic crystal, containing the same number of molecules, i. e. equal masses of the two varieties. Assuming that the nacreous plate has a thickness of 0.6p, and neglecting a possible small difference in the densities between the t n o varieties, a rough calculation shows that a rhombic crystal will cover an area not greater than I / I O , O O O (a and b axes in horizontal plane), of not greater than 1/4,5oo (b and c axes in horizontal plane) of the area presented by the nacreous crystal when lying flat. This corresponds to a minimum linear shrinkage of I in 67. In Plate VIII, the dimensions of one hole are I mm.Xq mm. Hence the area is that of n square whose side is 2 mm. But, since the magnification is 2 5 0 , the actual length will be 8p, and the corresponding linear dimension of the octahedron will be therefore ( 8 / 6 7 ) p= 0.12p, which is beyond the limit of microscopic resolution. In Plate IX, the process of disintegration has gone a stage further, so that the crystal i s now a mere skeleton. In the larger crystal, the acute angle is explained by the absence of one of the normal faces. Measurements of this angle gave PLATE IX the value 54' 46'. Calculated value 54' 54'. Two crystals showing etch figures, later stage, X Some other crystals, no photographed, show signs 1 2 0 diameters. of disintegration by the appearance of a serrated edge. The crystal in Plate VII, showing a net-work structure is no doubt breaking up in a similar way.
Summary The growth, development and disintegration of crystals of nacreous sulphur, obtained from sulphur droplets, has been studied and the various stages illustrated by microphotographs. Colour in crystals is shown to be due to interference. In conclusion, the author wishes to offer his sincere thanks to Professor R. Whytlaw-Gray for continual advice and encouragement throughout the work. Inorganzc Chemistry Department, T h e Universaty, Leeds.
