376 Scientific Proceedings, Royal Dublin Society. 



sponge-skeletons is removed. Since diatoms and the siliceous 

 teeth of molluscs have the same refractive index as sponge-spicules, 

 we may consider it probable that the silica of organisms generally 

 exists as opal. !;' 



The method of determining refractive indexes just described 

 admits of a wide application, and with isotropic substances presents 

 no difficulties ; it may often be employed in many cases where 

 other methods are impracticable. Thus I find that the glass of 

 the Krakatoa ashes has a refractive index of about 1-51, and it 

 would be possible to obtain a closer approximation than this were 

 one furnished with a refractometer to determine the refractive 

 index of the liquid employed : as it is I am obliged, when using a 

 mixture of two liquids in order to obtain a mean, to find its refrac- 

 tive index by discovering what mineral of known refractive index 

 disappears when immersed in it. The test mineral thus serves as 

 a refractometer. I intend, however, for laboratory purposes, to 

 provide myself with a series of fluids ; each with an accurately 

 determined refractive index, and ranging from 1'3 to 2*0, with a 

 difference of 0*002 between each. One would then make coarse 

 adjustments up to the second place of decimals, and conclude by 

 obtaining results up to the third place. Of course accurate results 

 can only be obtained by the use of monochromatic light, which 

 should be similar to that of the mineral under examination. 



The method provides an easy means of discriminating between 

 similar species of minerals. Thus, leucite and analcime often 

 closely resemble each other ; and without finding the exact vanish- 

 ing fluid, it is sufiicient for their distinction to place them either in 

 oil of santal, when the leucite becomes almost invisible, while the 

 analcime stands out in sharp relief ; or in linseed-oil, when the 

 analcime is least visible, and the leucite most so. 



With anisotropic substance the case is diiferent ; but the addi- 

 tional difiiculties which these present may be met by the use of 

 polarised light, at least when the mineral possesses planes of 

 cleavage. Thus, to take the case of aragonite and calcite. We 

 view them under the microscope between crossed nicols, a plate 

 of quartz being interposed to restore the light. The fragments of 

 both minerals are, to a great extent, bounded by cleavage planes. 

 In aragonite the indexes are — minimum, 1'530; mean, 1"682 ; 

 maximum, I'ObG: iu calcite, r^ = 1-486, r" = 1-659; the value of 



