ADDRESS. lvii 



By the labours of Lavoisier and his contemporaries, Chemistry acquired a 

 fixed logic and an accurate nomenclature. Dalton and the great physicists 

 of the early part of this century gave that law of definite combination by 

 proportionate weights of the elements which is for chemistry what the law 

 of gravitation is for celestial mechanics. A great expansion of the meaning 

 of the atomic theory took place, when Mitscherlich announced his views of 

 isomorphous, isomeric, and dimorphous bodies. For thus it came gradually 

 to appear that particular forces resided in crystals in virtue of their struc- 

 ture, lay in certain directions, and exhibited definite physical effects, if the 

 chemical elements, without being the same, were combined in similar propor- 

 tions, and aggregated into similar crystals. Some years later, ozone was 

 discovered by Schonbein, and it concurred with a few other allotropic sub- 

 stances in reviving, among philosophic chemists, the inquiry as to the relative 

 situation of the particles in a compound body, and the effects of such 

 arrangements : an idea which had been expressed by Dalton in diagrams of 

 atoms, and afterwards exercised the ingenuity of Exley, Mac Vicar, and 

 others *. 



' Everything connected with this view of the modification of physical pro- 

 perties by the arrangement of the particles- — whether elementary or com- 

 pound — is of the highest importance to mineralogy, a branch of study by no 

 means so much in favour even with chemists as its own merits and its col- 

 lateral bearings might justly deserve. Yet it is in a great measure by help 

 of this branch of study that the opinions now current regarding metamor- 

 phism 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 fabrica- 

 tion by art, or their action on the rays of light, the waves of heat and sound, 

 and the distribution of electricity, 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 meet- 

 ing, new and brilliant discoveries in the action of crystals on light were 

 made known by Brewster t, and compared with the undulatory theory by 

 Herschel, MacCullagh, Airy, Hamilton, "Whewell, Powell, Challis, Lloyd, and 

 Stokes. 



The unequal expansion of crystals by heat, in different directions, first 

 observed by Mitscherlich, has been carefully examined in the cases of 

 sulphate and carbonate of lime by Professor W. H. Miller J, who has also 

 considered their elasticity, originally measured in different relations to the 

 axis by Savart. These and many other interesting relations of crystals have 



varieties. Mr. Sorby, whose latest results are unpublished, but will be communicated to 

 the Royal Society, is of opinion that the substance of meteorites has undergone changes 

 due to physical conditions in some ancient period not now to be paralleled on our planet, 

 or on the moon, but rather to be looked for only in the immediate neighbourhood of the 

 sun. Professor Haidinger has also made a special study of meteorites. 



* Dalton, Chemistry, vol. i. 1808. A clear view of the simpler applications of Dalton's 

 ideas is given by the illustrious author in Daubeny's Treatise on the Atomic Theory, 1850. 

 Exley, Nat. and Exp. Philosophy, 1829. Mac Vicar, Rep. Brit. Assoc. 1855 ; Trans. Roy. 

 Soc. Edinb., &c. 



t " Sir David Brewster must be considered as in a degree the creator of the science 

 which studies the mutual dependence of optical properties and crystalline forms." 

 (Whewell, in Report on Mineralogy, Brit. Assoc. 1832, p. 336.) 



X Rep. Proc. 1837, pp. 43, 44. 



