1 882.] Mineralogy. 339 



the same conclusion. He shows, moreover, that the " organic 

 sulphur" in coal is not affected by the process of coking. 



Spiral Figures in Crystals. — Students in optical mineralogy 

 will be interested in an article by L. Wright, in a recent 

 number of the Philosophical M<rg,v:iin\ entitled "Some Spiral 

 Figures observable in Crystals, illustrating the relation of their 

 Optic Axes." The author places a section of the mineral to 

 be examined between a quarter-wave plate and a thick plate of 

 quartz and examines this arrangement in a polariscope with con- 

 verging rays. Beautiful spiral figures are produced, resembling 

 the well known " Airy's spirals." A uniaxial crystal, as calcite, 

 shows a system of double spirals, mutually enwrapping each 

 other (Fig. 1.). A single axis of a biaxial crystal shows a 

 simple spiral (Fig. 2), while if the section includes both axes of 

 the biaxial- crystal, as muscovite, two series of single spirals are 

 observed, which, while separated from each other, finally enwrap 

 one another (Fig. 3). 



This a beautiful demonstration of the well known fact that the 

 optic axis of a uniaxial crystal has a two-fold character. Fig. I, 

 representing a uniaxial crystal is seen to be composed of the same 

 two spirals seen in Fig. 3, a biaxial crystal. A uniaxial crystal 

 must therefore be regarded as a case in which the two axes of a 

 biaxial crystal coincide. 



Mineralogists will here perceive how slight a distinction exists 

 between a uniaxial crystal and a biaxial crystal of small optic 

 axial angle and will understand how, for example, a biotite hav- 

 ing often no appreciable biaxial character may yet be regarded as 

 monoclinic with an optic axial angle of nearly o°. 



Native Silver.— Several interesting occurrences of native 

 silver have recently been described. 



The first of these is in the province of Almeria, Spain, where it 

 has been found in iron ore. A bed of hematite of considerable 



