66 PROCEEDINGS OP THE AMERICAN ACADEMY 



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 vermiculite, 

 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 sej^a- 

 rated from the magnesia by ammonia, with the usual precautions. In 

 (2) and (3) the bases were converted into nitrates and separated by De- 

 ville's method. In each case the magnesia was weighed as pyrophos- 

 phate, and the alumina and ferric oxide were w^eighed together. 



All three analyses were made with material rendered anhydrous 

 by ignition until the weight was constant, and each is represented 

 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 dithculty which the 

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

 silicate I have ever examined, when once dry absorbing water from 

 comparatively dry air with almost as much avidity as chloride of cal- 

 cium. In two experiments with different portions of the same powder, 

 the material was heated in an air bath, at 100° C, for seventy-two hours 

 before the weight became constant ; 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. This close agreement 

 indicates that all the water lost in drying, either at 100° or over sulphu- 

 ric acid, is hygroscopic ; and the conclusion is confirmed very greatly by 

 the fact that the mineral in thus drying does not change its aspect in the 

 least degree, and rapidly reabsorbs the water when exposed to the air. 

 On the other hand, when the mineral is ignited it swells up to many 

 times its volume, like other members of the vermiculite family, and under- 

 goes what is evidently a profound alteration in its molecular structure.* 



* In his valuable paper on corundum, above referred to (p. 44), Dr. Genth 



