262 Prof. J. P. Cooke on the Vermiculites. 



produce the laminae of a mica plate in the relative position in 

 which we have placed them in our artificial crystals, with only 

 this difference, that the laminse would be indefinitely thin and 

 in exact position ; and the effect of such compound molecules 

 in modifying the elasticity of the crystalline structure must be, 

 in most respects at least, like that of single molecules symme- 

 trical on all sides of one line or axis ; in other words, they must 

 produce a structure similar to that of uniaxial crystals. Under 

 what further conditions the grouping of the molecules, in right- 

 or left-handed spirals, determines the phenomena of right- or 

 left-handed circular polarization, and what bearing the new facts 

 may have on the received theory of these phenomena as they 

 appear in quartz, must be left for further analysis to discuss. 



I pass next to consider the composition of the Culsagee ver- 

 miculite; and I give below at (1), (2), and (3), the results of 

 three analyses made by myself, together with the corresponding 

 oxygen-ratios. 



The pulverized mineral, after it has been exfoliated by heat, 

 is easily and perfectly decomposed by hydrochloric acid. In 

 analysis (1), after the separation of the silica, the alumina and 

 ferric oxide were separated from the magnesia by ammonia with 

 the usual precautions. In (2) and (3) the bases were converted 

 into nitrates and separated by Deville's method. In each case 

 the magnesia was weighed as pyrophosphate, and the alumina 

 and ferric oxide were weighed together. 



All three analyses were made with material rendered anhy- 

 drous by ignition until the weight was constant ; and each is re- 

 presented by three distinct determinations — namely, the weight 

 of the silica, the sum of the weight of the alumina and ferric 

 oxide, and the weight of the magnesic pyrophosphate. The 

 oxides of iron and the water were once for all determined on 

 separate portions of the dried but not exfoliated mineral, and 

 therefore appear of the same value in all the analyses. 



The determination of the water was the only difficulty which 

 the analysis of this mineral presented. It is by far the most hy- 

 groscopic silicate I have ever examined, when once dry absorbing 

 water from comparatively dry air with almost as much avidity 

 as chloride of calcium. In two experiments with different por- 

 tions of the same powder, the material was heated in an air-bath 

 at 100° C. for seventy-two hours before the weight became con- 

 stant ; and in each case the weight was compared at intervals of 

 about six hours. The total loss in the first experiment was 

 10*27 per cent., and in the second 10-19 per cent. The mineral 

 thus dried lost, when ignited, 10*84 per cent. Another portion 

 of the same powder, which had been dried over sulphuric acid 

 for more than two months, lost, when ignited, 11*09 per cent. 



