1865.] BRITISH ASSOCIATION. 331 



elementary or compound — is of the highest importance to minera- 

 logy, a branch of study by no means so much in favor even with 

 chemists as its own merits and its collateral bearings might justly 

 deserve. Yet it is in a great measure by help of this branch of 

 study that the opinions now current regarding metamorphism of 

 rocks in situ, and the formation of mineral veins, must acquire 

 that solid support and general consent which at present they do 

 not possess. Crystals, indeed, whether regarded as to their origin 

 in nature, their fabrication by art, or their action on the rays of 

 light, the waves of heat and sound, and the distribution of elec- 

 tricity, have not been neglected by the Association or its members. 

 In one of the earliest reports, Dr. Whewell calls attention to the 

 state of crystallographical theory, and to the artificial production 

 of crystals; and in another report, Professor Johnston notices 

 epigene and pseudomorphous crystallisation ; and for many years, 

 at almost every meeting, new and brilliant discoveries in the action 

 of crystals on light were made known by Brewster (z), and com- 

 pared with the undulatory theory by Herschel, MacCullagh, 

 Airy, Hamilton, Whewell, Powell, Challis, Lloyd, and Stokes. 



The unequal expansion of crystals by heat, in different direc- 

 tions, first observed by Mitscherlich, has been carefully examined 

 in the cases of sulphate and carbonate of lime, by Professor 

 AY. H. Miller (a), who has also considered their elasticity, origin- 

 ally measured in different relations to the axis by Savart. These 

 and many other interesting relations of crystals have been attend- 

 ed to ; but the Association has not yet succeeded in obtaining a 

 complete digest of the facts and theories connected with the 

 appearance of crystals in nature — in the fissures of rocks, in the 

 smaller cavities of rocks, in the solid substance or liquid contents 

 of other crystals. Such an enquiry, however, it did earnestly 

 demand, and some steps have been taken by our own chemists, 

 mineralogists, and geologists. But more abundant information on 

 this class of subjects is still needed, even after the admirable con- 

 tributions and recent discoveries of Bischof, Delesse, and Dau- 

 bree (6). 



(c) " t Sir David Brewster must be considered as in a degree the creator 

 of the science which studies the mutual dependence of optical proper- 

 ties and crystalline forms."— [Whewell, in Report on Mineralogy, Brit. 

 Association, 1832, p. 336.] 



(«) Rep. Proc. 1837, pp. 43, 44. 



(6) Bischof, Chemical Geology (published by the Cavendish Society, 

 1856.) 



