264 



NATURE 



[December 21, 191 1 



handed, we obtain a remarkable change in thi- iigurc, 

 namely, the production of the celebrated spirals of Airy. 



Now thf'se phenomena suggest a spiral, and a comple- 

 mentnrily heliial, arrangement of the atoms composing 

 the chemical molecules of silica in the two forms of quartz 

 crystals. That the supposition is correct may be proved, as 

 first shown by Reusch, by reproducing the effects by means 

 of a spirally arranged pile of biaxial mica films. Twenty- 

 four equally thin films of ordinary muscovitc mica are laid 

 over each other so that the direction of the line joining 

 the two optic axes of the mica regularly rotates by the 

 same angle, conveniently 60°. The biaxial figure normally 

 given by muscovite mica will be found, on placing the pile 

 in th> '"'--'-Tope, to have been converted into a uniaxial 

 on< . \<>r, the figure resembles that afforded by 



qu.ni r.markable degree of precision, for if the pile 



be a right-iiandcd one the figure is similar to that afforded 

 by right-handed quartz, and gives the same effects on rotat- 

 ing the analyser ; whereas, if the pile had been arranged 

 in a left-handed manner, the effects would resemble those 

 afforded by a plate of left-handed quartz. Further, if we 

 superpose the two piles of mica plates, the left-handod and 

 the riijht-Iirindod, and place them together in the polari- 

 scop' . Ail \ V >^pirals are at once produced on the screen. 



Lf'i us now study quartz plates in a parallel beam of 

 polarised light. If we take two plates, one of right-handed 

 quartz and the other of left-handed, each of 7-5 millimetres 

 thickness, we nnd that they each give with crossed Nicols 

 the well-known rose-violet tint of passage between the first 

 and second orders of Newton's spectra. But on rotating 

 the analysing Nicol clockwise, the right-handed plate 

 changes colour first to red, then to orange, vpIIow, green, 

 and blue, while the left-handed plate becornes first blue, 

 then passes through green and yellow to orange and red. 



Fic. II. — Rotations by 3'7s mm. 

 of Quartz. 



'"~~iW 



Fig. 12. — Rotations by 7*5 mm. 

 of Quartz. 



On the other hand, the two plates of 375 millimetres thick- 

 ness appear yellow under crossed Nicols, just as were the 

 centres of the interference figures in convergent polarised 

 light, and they change colour in opposite directions of the 

 spectrum on rotating the analyser, until they become violet 

 with the tint of passage when the Nicols are parallel. 



Now all these beautiful phenomena are due to the fact 

 that when a beam of light is sent along the axis of a 

 quartz crystal, the right- or left-handed arrangement of 

 the rnolecules of silica causes the plane of vibration of the 

 polarised light received from the polarising Nicol to be 

 rotated in the same direction, the amount being directly 

 proportional to the thickness of the plate. It also varies 

 considerably with the wave-length of the light. For plates 

 0/ 375 and 7-5 millimetres thickness, respectively, the rota- 

 tions by plates i millimetre thick are indicated in Figs. 11 

 and i2f Fig. 11 shows that the plane of vibration of yellow 

 light of wave-length 0000550 is rotated just 90° by a plate 

 0/ 375 millimetres thickness, and this explains the produc- 

 tion of the violet tint of passage when we have rotated the 

 analysing Nicol 90°, that is, until it is parallel to the 

 polarising Nicol. For the extinction of this yellow light 

 leaves the complementary colour, the violet transition tint, 

 predominant. .And when we double the thickness of the 

 plate to 75 millimetres, the yellow rav, of 0000550 milli- 

 metre wave-length, is rotated just '180°, as shown in 

 Fig. 12, which, when followed (by rotation of the analvser), 

 brings the Nicols into the crossed position again, and thus 

 the violet transition tint is at once given bv such a plate 

 under crossed Nicols. 



The colour produced by thick plates of quartz in polarised 

 light is thus due to optical rotation, and it is quite as 

 brilliant as that due to double refraction shown bv thin 

 plates of quartz, such as those of rock-sections, which vary 



from a twenty -fifth to a fiftieth of a millimetre in thickiie> 

 The phenomenon of optical activity is confined to crysta'. 

 belonging to those eleven classes of symmetry which cxhib 

 right- and left-handed forms, that is, in which there is \. 

 plane of symmetry developed. 



Now besides the right- and left-handed forms, showirK 



dextro and lasvo rotation, chemists havf H' • "-^ 



cases in which the optical activity is citli 

 destroyed by intimate lamellar twjnning of ■ , 



mentary varieties, or by chemical combinatiun of th«- iv 

 sets of molecules in which the atoms are oppositely spiral! 

 arranged. In the former case of regularly r< : 

 twinning the symmetry is apparently enhanced by t 



troduction of a plane of symmetry, the c< ■*- 



showing the characteristic faces of both 

 handed forms. In the latter case chemic. 

 results in the production of a new substance, and ti 

 crystalline form is altogether different, and may »n-' 

 belong to another system. This is the case with f. ■ 

 acid, for the combination of the two sets of molecul' 

 duces racemic acid, which crystallises in the trimr 

 system with a molecule of water of crystallisation, totall 

 unlike the crystals of either ordinary dextro (rit'li'-'i <'"!. 

 tartaric acid or the complementary laevo ' 

 tartaric acid, both of which belong to the sph< 

 of the monoclinic system, and are anhydrous. 



Now the racemic form of optical inactivit 



NO. 2199, VOL. 88] 



Fio. 13. Fig. 14. 



The Two Types of Quartz Twins. 



simulated to a moit remarkable extent by the repeat- 

 twinning already referred to; but such a " p- 

 racemic '' form can generally be readily distinguish^; 

 a truly racemic form by the fact that it still exhibii- . 

 symmetry of the same system, although that of a class i 

 higher symmetry in the system, owing to the introductit 

 of a plane of symmetry, about which the twinn- 

 individuals are disposed in mirror-image fashion. Th 

 case of the mechanical enhancement of the symmetry t 

 re(>etition twinning is beautifully illustrated ' by quart, 

 es|>ecially in the exquisite form of the mineral known 

 twins, one, illustrated in Fig. 13, in which the two ir 

 dividuals are of the same right- or left-handed variety, ar 

 another, represented in Fig. 14 by an example in which al- 

 complete interpenetration has occurred, in which a rigb- 

 handed crystal is twinned with a left-handed one. 



This latter kind of quartz twin is frequently found occur 

 ring among specimens from Brazil. The twin plane is 

 face of the hexagonal prism of the second order, perpet 

 dicular to a pair of the faces of the hexagonal pri«: 

 present, which is of the first order. The little ^ and ^ 

 faces are shown on every corner, as in the case of Fig. 13 

 but symmetrically. 



Now if a plate of such a crystal be cut perpendicular! \ 

 to the axis, the polarisation phenomena, due to the opposii' 

 optical activity of the two different varieties present, will 

 vary according to their mode of internal disposition. Wh^n 

 the whole of one half of the crj-stal is of right-handed and 

 the other of left-handed quartz, and the surface of junction 

 is a plane parallel to the rxis, we have a natural biquart' 



