TRANSACTIONS OF THE SECTIONS. 9 



An Account of the Cause of the Colours in precious Opal. 

 By Sir David Brewster, F.R.S. L.^E., Hon. M.R.I.A. 



This gem is intersected in all directions with colorific planes, exhibiting the most 

 brilliant colours of all kinds. The cause of these colours has never, we believe, been 

 carefully studied. Mineralogists, indeed, have said that they are the colours of thin 

 plates of air occupying fissures or cracks in the stone ; but tliis is a mere assump- 

 tion, disproved by the fact that no such fissures have ever been found during the 

 processes of cutting, grinding and polishing, which the opal undergoes in the hands 

 of the lapidary. In submitting to a powerful microscope specimens of precious 

 opal, and comparing the phsenomena with those of hydrophanous opal. Sir David 

 Brewster found that the colorific planes or patches consist of minute pores or vacui- 

 ties arranged in parallel lines, and that various such planes are placed close to each 

 other, so as to occupy a space with three dimensions. These pores sometimes 

 exhibit a crystalline arrangement, like the lines in sapphire, calcareous spar, and 

 other bodies, and have doubtless been produced during the conversion of the quartz 

 into opal by heat under the peculiar circumstances of its formation. In some speci- 

 mens of common opal the structure is such as would be produced by kneading cry- 

 stallized quartz when in a state of paste. The difi^erent colours produced by these 

 pores arise from their difi'erent magnitudes or thicknesses, and the colours are gene- 

 rally arranged in parallel bands, and vary with the varj'ing obliquities at which they 

 are seen. 



A notice respecting the Cause of the beautiful White Rings which are seen 

 round a luminous body when looked at through certain specimens of Cal- 

 careous Spar. By Sir David Brewster, F.R,S. L. Sf E., Hon. M.R.I.A. 



By varying the inclination of the spar, the rings increase and diminish, each of 

 them in succession contracting into a luminous spot and disappearing, and then ex- 

 panding into rings as before. The two rings are produced from the two images 

 formed by double refraction, and hence the light of one ring is oppositely polarized 

 to that of the other. When the ordinary and the extraordinary ray are refracted in 

 lines parallel to the edge of the rhomb, which they are at different incidences, their 

 respective rings disappear. At oblique incidences the rings are highly coloured, 

 and when the dispersive action is small they have a bright silvery whiteness. Sir 

 David Brewster stated that they were produced by minute tubes in the mineral, of 

 which there were many thousands in an inch, and that these tubes were parallel to 

 one of the edges of the rhomb, viz. to that edge to which the refracted ray was 

 parallel when each ring became a luminous spot. 



On Crystals in the Cavities of Topaz, ichich are dissolved by Heat and 



re-crystallize on Cooling. 



By Sir David Brewster, F.R.S. L. ^- E., Hon.M.R.I.A. 



Sir David gave a brief notice of the discovery which he had made, about twenty 

 years ago, of two new fluids in the cr\'stallized cavities of topaz and other minerals. 

 One of these fluids is very volatile, and so expansible, that it expands twenty times 

 as much as water with the same increase of temperature. When the vacuities in 

 the cavity which it occupies are large, it passes into vapour, and in these different 

 states he had succeeded in determining its refractive power, by measuring the angles 

 at which total reflexion takes place at the common surface of the fluid of the topaz. 

 The other fluid is of a denser kind, and occupies the angles and narrow necks of cavities. 

 The cavities, however, in which the soluble crystals were contained are of a different 

 kind. They (viz. the cavities) were imperfectly crystallized, and thus they exist in 

 specimens of topaz which contain the cavities with the two new fluids ; they sometimes 

 contain none of the volatile and expansible fluid, which is doubtless a condensed gas. 

 The crystals which occupy them are flat and finely crystaUized rhomboids. When 

 heat is applied, they become rounded at their angles and edges, and soon disappear. 

 After the topaz has cooled, they again appear, at first like a speck, and then recry- 

 stallize gradually, sometimes in their original place, but often in other parts of the 

 cavity, their place being determined by the mode in which the cooling is applied. 



