December 21, 191 1] 



NATURE 



26 



o 



quartz and heat it gently and evenly over a small Bunsen 

 flame or other source of heat, and then allow it to cool, 

 electrical excitation is developed, positive and negative 

 electricity being produced, respectively, at alternate corners 

 of the hexagon. Now the three digonal axes of symmetry 

 emerge at the six corners, and the little trigonal pyramid 

 and trapezohedron faces s and x are present at alternate 

 corners corresponding to one end only of each axis ; and 

 it is interesting that these corners where the little distinc- 

 tive faces are present are those which become negatively 

 electrified, while those corners where no s and x faces are 

 developed become positively electrified. This disposition of 

 the pyroelectric poles is precisely in accordance with the 

 symmetry of the trapezohedral class of trigonal symmetry. 



If we treat a pair of quartz crystals, right- and left- 

 handed respectively, with a small quantity of aqueous 

 hydrofluoric acid, a chemical solvent for silica, character- 

 istic little markings, or " etch-figures," are produced on 

 the faces. They are little depressions, of the shape of a 

 candle flame blown to one side by a draught of air, to the 

 right on a right-handed crystal and to the left on a left- 

 handed crystal, and are pointing upwards and downwards, 

 respectively, on alternate faces. Thus we have trigonal 

 rather than hexagonal symmetry again demonstrated, and 

 a screw structure, clockwise in one variety and anti-clock- 

 wise in the other, such as corresponds to the trapezohedral 

 class of the trigonal system, also most clearly indicated. 



Now it has been definitely proved that specific atoms 

 of the chemical molecule are definitely orientated in 

 the crystal, and that if such atoms, say those of potassium 

 or of sulphur in potassium sulphate, be replaced by others 

 of the same family group of chemical elements, for instance, 

 the potassium by rubidium or caesium, and the sulphur by 



selenium, specific directional 

 j^ changes in the crystal 



,-•'' '■^.. angles are observed to 



occur. In the cases just re- 

 ferred to, the replacement 

 ,v.^^ of the metal brings about 



'"•.':•, an alteration in the length 

 _*• of the vertical axis of the 

 ,.-'' rhombic crystal of the 



■ ■' sulphate or selenate, while 



replacement of the sulphur 

 by selenium causes an equa- 

 torial change. If, there- 

 fore, we accept the view of 

 Barlow, and represent the 

 atoms by their spheres of influence, wv are only logically 

 following up this important experimental fact. 



Barlow has recently propounded the view, in collabora- 

 tion with Pope, that the relative size of the sphere of 

 influence of an atom in any one compound, compared with 

 that of any other atom in the same compound, is intimately 

 connected with the chemical valency of the element, being 

 proportional to the fundamental valency. This is rarely 

 the maximum valency, although in the case of carbon it 

 would appear to be so, atomic spheres of influence of carbon 

 having apparently four times the volume of those of 

 hydrogen or chlorine present in the same compound. The 

 relative size of the atomic sphere of influence of oxygen 

 appears to conform to its usual dyad character. Silicon 

 might, perhaps, from its occurrence in the same family 

 group of elements as carbon, be expected to behave also 

 as a tetrad, but there is much more evidence that its funda- 

 mental valency is only dyadic. Now Barlow has shown 

 that if we accept the view that the fundamental valencies 

 of both viliion and oxygen are dyadic, and therefore that 

 the sphori s cjf influence of the two elements in quartz are 

 of the same size, the whole of the j)rn|)i riie^ df (ni.ut/ < :in 

 be explained on the assumption fli.ii ili.- two ^irm mm' ■-. 

 right-handed and leff-li.-mdrd, nrr ( umpusid d ■ ' 

 assemblagf"^ .is .-ii'' -Ikiwm in V\'^-i. 7 .-md S, in w i 



.a;-.. 



Fig. 6. 



-Unit Cell of Rhombohedral 

 Space-lattice. 



white spill rr^ ri 



atoms, ihirc Ii.- 



former, rorri spo 



The heliral . 



• \:\'^i-^ .if siliciM 



iw fdrrTier b'ii 



irew in Fig. 7 



'WO arrangiMivnt 



I'iin; 



I silicdii a I 

 .o (,f the 1; 



(O file foril! 



Ihe 



ol the 



Fig. 7. 



-Barlow's Conception for Right- 

 handed Quartz. 



such Structural arrangements of the chemical atoms as 

 these are really present in the two varieties of quartz is, 

 indeed, highly probable 



The most important and convincing evidence of the right- 

 and left-handed screw structure of quartz is afforded, how- 

 ever, by the optical properties of the mineral. 



We may study, first, the effect of passing the light from 

 the lantern, an image of a slit in front of which is focussed 

 on the screen by a lens, through either of two 60° prisms, 

 one cut so that the 

 light passes through 

 the crystal at minimum 

 deviation parallel tu 

 the axis, as shown in 

 Fig. 9, and another 

 with the refracting 

 edge parallel to the 

 axis, as shown in 

 Fig. 10. In the latter 

 case we see two spectra 

 on the screen, which 

 are separately ex- 

 tinguished by a large 

 Nicol prism placed in 

 the path of the rays, 

 when rotated to two 

 positions 90° apart ; 

 while in the former 

 case only a single spec- 

 trum is produced on 

 the screen, and remains 

 permanent when the 

 Nicol is placed in posi- 

 tion and rotated, just, 

 in fact, as if the quartz 

 prism were made of 

 glass. This is the 

 mode of cutting quartz 

 prisms employed for 

 use in experiments with ultra-violet light, such as the in- 

 vestigation of the ultra-violet spectrum, quartz being 

 remarkably transparent to these ultra-violet rays of short 

 wave-length, which are entirely cut off by glass. 



We may next investigate plates of quartz in polarised 

 light. Placing a plate of quartz i millimetre thick in the 

 polariscope ananged for convergent light, we perceive a 

 more or less normal uniaxial figure, although the rings are 

 large and diffuse and the dark cross present is also very 

 diffuse. If we now take a thicker plate, conveniently one 

 3-75 millimetres thick, the coloured rings are more 



Fig. 8.- 



-Barlow's Conceptiorj for Left- 

 handed Quariz. 



Fig. 9.-60° Prism Cut for 

 Light to Traverse Axis. 



Fig. 10.-60" Prism uiih Kefrac'ing 

 lvls;r I'ar;,n<-1 Axis. 



light-hand glove is to a leti-ii.iiid 

 NO. 2199, VOL. 88] 



numerous, the innermost being sm;illi 

 eross are also sharper near the margin 

 h.ivc entirely disappeared from the cent 

 the whole of the interior of 

 instead with yellow light, 

 analysing Nicol clockwise, tli 

 has been cut from a right-h 

 -..ei.-i.tv if the crystal be a ^ 



ill' ring also alterini; until 

 we superpose two such j) 

 iiuek, one of :■■ ;ht-li;in(|eil ini.ut/ .111 



lai. 



til. 



arms of the 



the 



1 1 late 



but 



. ular 



. ,11 i\ si|uaro. 

 ; niillinieires 

 >lher ,,f left- 



