CRYSTALLOLUMINESCENCE.
I1
BY HARRY B. WEISER
Triboluminescence and Crystalloluminescence The term triboluminescence was first applied by Wiedemannl to the property which many crystalline substances possess of emitting a characteristic phosphorescent light when rubbed or crushed. The earliest observers of this phenomenon took it for granted that the light was caused by small particles of the solid becoming heated to incandescence by friction. However, it was demonstrated as early as 1792 by Saussure2 that in certain cases light could be obtained under such conditions that a pure temperature radiation was altogether unlikely. Thus, by fusing a mixture of chalk and phosphoric acid he obtained a substance which glowed when stroked with a quill. Dessaignes3 attempted to show that the light obtained by grinding glass, adularia and pumice stone in a mortar was an electrical effect. “He says, that a stronger light is obtained in a metal mortar than in a nonconducting one of porcelain and that better results are obtained in dry weather than in wet; if the powder obtained in a metal mortar is strewn on a hot plate it no longer luminesces, whereas that obtained in a porcelain mortar glows well under the same conditions. In the first case the electric fluid on which the phosphorescence in general depends willhave had an opportunity to escape entirely because of the good conductivity of the metal”4 These experiments have not been confirmed. Henrick5 made an exhaustive study of the phenomenon of triboluminescence but reached no definite conclusion as t o the cause. However, he emphasized the fact that the light Wied. Ann., 34, 446 (1888). See Kayser: “Handbuch der Spektroscopie,” 4,672 (1904). 3 Delamentherie: Jour. Phys., 68, 444; 69, 5 (1809). 4 Kayser : “Handbuch der Spektroscopie,” 4, 673 (1908). 5 Die Phosphoresccnz der Korper, 4, 425-570 (1820). 1
2
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was not caused by the heat resulting from rubbing or grinding. “Henrick reviews and then criticizes the previous explanations of triboluminescence and gives his own : The light in so far as it is not a burning or glowing is due, I , to the separation of particles caused by friction (to the cracking off of rough places on the crystals; 2, to the electricity resulting from friction; 3, to the chemical decomposition produced by this means. The first mentioned cause is the most important. It is evident that Henrick explains nothing whatsoever, since he merely says that rubbing phosphorescence comes from rubbing.”l It is interesting to note that Henrick recognized the possibility of some chemical action entering into the process even though he did not take the matter seriously. Schneider2 attempted to show that the phenomenon was due neither to electrification nor heating. By rubbing different diamonds together he found that there was apparently no connection between the tendency to luminesce brightly and the degree of electrification. He again pointed out that certain siliceous rocks often emit light under conditions that could not result in an appreciable rise in temperature. More recent investigators have endeavored to establish some connection between triboluminescence and the crystal structure. Among the first investigations carried out with this end in view was Pope’s: study of saccharin. He found that there was apparently no connection between the luminescence and crystal structure; however, the results of his work are particularly important since he shows that the luminescence is not caused by a number of things that were later suggested as explanations. “The method by which the crystals are broken seems without influence on the flashing; phosphorescence occurs when the crystals are hacked with a knife, rubbed together, crushed between the fingers or between pieces of glass, and also when they are caused to crack
2 3
Kayser : Handbuch der Spektroscopie, 4, 675 (1908). Pogg. Ann., 96, 282 (1905). Jour. Chem. SOC.,67, 985 (1895).
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Harry B . Weiser
by rapid heating either in the air or in the acetone mother liquor from which they crystallize. There seems to be no plane in the crystal parallel t o which breaking occurs without phosphorescence ; no matter how carefully a crystal is cleaved along the very perfect cleavage on a(Ioo), so that parting shall only occur parallel to the cleavage, vivid phosphorescence always occurs, and on carefully cutting the crystals in the two directions perpendicular to the cleavage, so that very little parting occurs along this plane, a brilliant flash almost invariably results. “An exhaustive examination of the properties of the crystals was made in order to allow of some cause being assigned to this peculiar phenomenon; it was at first thought that the crystals might be hemimorphic or hemihedral and that the phosphorescence might be in some way related to the polar properties inseparable from hemimorphism. Some support seemed to be given to this view by the fact that the crystals are usually very unsymmetrically developed, the forms r ( 101) and ~ ( O O I )being frequently represented by only one face each, and a hollow often taking the place of one of the faces of the basal plane; but these anomalies can only be accidental, for such a distribution of planes does not indicate any of the kinds of symmetry possible in the monosymmetric system. “The completely holohedral nature of the crystals is shown pretty conclusively by the following methods: No evidence of pyroelectricity is obtained on heating and testing by Kundt’s method; further, on cleaving a crystal parallel to a(1oo) and dusting with the electrified mixture of red lead and sulphur, no indication is obtained that the two surfaces acquire a difference of electrical potential such as might be expected if this form were perpendicular to a pyroelectric axis; no pyroelectricity is observed on heating the two cleaved surfaces. These facts seem t o indicate that the phosphorescence is in no way connected with pyroelectric polarity, for the crystals phosphoresce brilliantly when parted along the plane a(1oo) although no evidence of pyroelectricity is obtained from the two parted surfaces.”
CrystaIlolzl.miutesce~~ce.11
579
XndreocciI found that dextrorotatory and levorotatory ethyl santonin are triboluminescent but the racemic modification is not, Brugnatelli2 observed triboluminescence of a number of derivatives of santonin that was similar t o the light emitted by saccharin. He was of the opinion that there was no connection between the luminescence and the optical activity, although the phenomenon was not observed with racemic modifications of the substances studied. He thought that there might possibly exist some connection between cleavage and triboluminescence since the luminescent santonin derivatives all show good cleavage and a particularly bright light resulted when the crystals were broken perpendicular to the axis of symmetry. He found on the other hand, that many substances like phenacetin which possess a good cleavage do not luminesce when crushed, while others which have no cleavage like dichlormethyl-paratolylsulphone show the phenomenon to a marked degree. In a later investigation of the triboluminescence of santonin derivatives Andreocci3 concluded that there was some connection between the phenomenon and optical activity. His conclusions may be summarized as follows : I . Two optical isomerides which are antipodes show the same behavior as regards luminescence ; either both are triboluminescent or neither. 2 . Crystals of racemic forms do not exhibit triboluminescence even though they may be derived from triboluminescent forms. 3. Optically active isomers which are not antipodes do not behave similarly with respect t o triboluminescence. 4. Optically active double forms resulting from two non-antipodal components may be triboluminescent. Tschugaeff examined 400 organic substances and I I O inorganic substances for triboluminescence. He found that 30 percent of the former and only 5 percent of the latter were Gazz. chirn. ital., 25, 462-568 (1895). See pages 494, 513, 524. Zeit. Kryst. Min., 27, 78 (1897). 3 Gazz. chirn. ital., 29, 516 (1899). 4Ber. deutsch. chem. Ges., 34, 1820 (1891);Jour. Russ. Phys, Chem. SOC.,36, 1245 (1904).
