6i8 



NA TURE 



[April 27, 1899 



without white and without gaps, we have probably drops of 

 O'l mm. ; in smaller drops, gaps and white make their appear- 

 ance. A real white bow with yellow or orange and blue 

 margins requires drops of not more than 0'05 mm. Moon- 

 bows appear white owing to their feeble intensity. A small 

 change in the size of big drops does not much matter. These 



tinuous, though rather fainter, luminescence is observed. On 

 closely watching large transparent crystals whilst crushing them, 

 the illumination appears to consist of a glow which pervades 

 the whole crystal just as it breaks, and then immediately 

 disappears ; the emission of light consequently occupies so short 

 a time as to appear instantaneous. The luminescence is not 

 always apparent, and a crystal may sometimes be crushed in the 

 dark without becoming apprecialily luminous ; these cases, how- 

 ever, are rare amongst freshly prepared crystals. On crushing 

 twenty-three crystals, which had been found to be quite normal in 

 optical and geometrical properties, only one refused to flash, 

 although no other point of difference between it and the other 

 crystals could be established. 



The method by which the crystals are broken seems without 

 influence on the flashing ; luminescence occurs when the crystals 

 are hacked with a knife, rubbed together, crushed between 

 the fingers or between pieces of gla.ss, and also when they are 



statements are based upon calculations and laboratory experi- 

 ments ; a confirmation by actual observations is hardly possible, 

 since we cannot measure the size of the raindrops that produce 

 the rainbows whose colours we are studying. H. B. 



ON TRIBOLUMINESCENCE. 



THE name triboluminescence has been applied by E. 

 Wiedemann to an emission of light not due to rise of 

 temperature which occurs on crushing certain substances. 



It has long been known that on tearing cleavage sheets of 

 mica apart, or on crushing crystals of cane sugar, light is given 

 out ; and during the last few months attention has again been 

 drawn to this curious phenomenon by a paper read before the 

 British Association by Mr. J. Burke, on the luminosity pro- 

 duced by striking sugar crystals (Nature, vol. Iviii. p. 533), and 

 by Mr. T. Steel's letter to Nature (vol. li.\. p. 295) on the 

 same subject. A brief account of other work done on the 

 subject of triboluminescence during recent years may therefore be 

 of interest. 



Some years ago I contributed a paper to the Chemical 

 Society (7>aH.i. Chem. Soc, 1895, 985)onorlhobenzoicsulphinide 



CcHj^^Pj-j-yNH, the substance known commonly as saccharin, 



and which is now largely used as a substitute for sugar when 

 the use of the latter is considered undesirable upon medical or 

 other grounds. I showed that commercial saccharin crystallises, 

 on spontaneous evaporation of its solution in acetone, in large, 

 transparent, monosymmetric crystals having the geometrical 

 constants -.—a : h : c = 27867 : i : 17187, fl =76' 8' 30". On 

 breaking or crushing the freshly prepared crystals, they emit a 

 very vivid, bluish-while light, which, however, is only of 

 momentary duration. This fl.ishing or phosphorescence of the 

 crystals is very brilliant, and is (|uite noticeable even in a well- 

 illuminated room. The luminescence was, in fact, fir.st noticed 

 whilst the crystals were being manipulated in the full glare of an 

 incandescent gas lamp. The phosphorescence may be well 

 shown on a small scale by pulverising a crystal between two 

 microscope slides, and on a larger scale by vigorously shaking a 

 bottle of the crystals in a dark room ; on grinding a quantity of 

 the material in a glass morlar in the dark, an almost con- 



caused to crack by rapid heating, either in the air or in the 

 acetone mother liquor from which they crystallise. There 

 seems to be no particular plane in the crystal parallel to which 

 breaking occurs without luminescence ; no matter how carefully 

 a crystal is cleaved along the very perfect cleavage on the pin- 

 acoid ]ioo|, so that parting only occurs parallel to the cleavage 

 plane, and with a minimum of shock, vivid luminescence always 

 occurs ; and on carefully cutting the crystals in 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 behemimorphic or hemihedral, and that the luminescence 

 might be in some way related to the polar properties inseparable 

 from hemimorphism. No positive evidence was, however, 

 obtained in this direction. 



Crystals of orthobenzoicsulphinide, obtained by other methods 

 than the above, showed no triboluminescence. The substance on 

 heating sublimes in long glistening needles, which are morpho- 

 logically and optically identical with the crystals deposited from 

 acetone, but show no triboluminescence. 



After the publication of the paper referred to, Luigi Brugnatelli 

 (Zcils. f. A'rys/alloip-apkie, 1S97, 27. 78) gave particulars of 

 several similar cases found by him amongst derivatives of 

 santonin prepared by Amerigo .Vndreocci (.//// d. A". Aciaii. 

 Line, 1895, [5<;], 2> 28). The monosymmetric crystals of 



,0.C:0 

 elhylisodesmoiroposumtonin, C,.,H,3{OEt)^ | , emit a 



\— CI I Me 

 yellow luminescence when crushed ; the monosymmetric crystals 

 of iheenantiomorphously related dextro- and l.vvo-ethylic s.anto- 

 nite, C|..Hu(OH|CUMe.CO(JEt,and themono.symmelticcrvstals 

 of hevo'desmolroposantonous acid, C,.jH,4(OH)ClIMe.CdOH, 

 all give a yellowish-green triboluminescence. L;evoethyldes- 

 motroposantonoiis acid C,.,IIi,iUEt;CIIMe.COi )11, crystallises 

 in hemihedral anorthic crystals, which when crushed glow with 

 a beautiful emerald green light. 



Crystals of saccharin have also been examined by W. Arnold 

 {Zfi/s. /. A'ljs/., 1S96, 27i 92), who describes their tribolumin- 

 escence as " stark, " and states that that of crystallised hippuric 

 acid C0H5.CO.MI.CH.J.COOH. is " sehr stark." 



NO. 1539. VOL. 59] 



