ANNIVERSARY ADDRESS OF THE PRESIDENT. XXV 



dimorphism and isomorphism, that the known dimorplious bodies 

 are very few, not more than about ten in 350 crystalhzed minerals, 

 and that the isomorphous substances being ascertained, the great 

 difficulty at first contemplated by the discovery of isomorphism has 

 not been felt. M. Dufrenoy, in his excellent treatise on Mineralogy, 

 well observes, that "It is not necessary to give the same composition, 

 that minerals should exactly contain the same weight of their simple 

 constituent substances ; it is sufficient that there is an exact relation 

 between the bases and the acids they contain, or between their iso- 

 morphous substances*." 



In 1824 Berzelius published, in the Transactions of the Swedish 

 Academy, a memoir '*0n the Changes in the Chemical System of 

 Mineralogy, which the property of Isomorphous Bodies to replace 

 each other in indeterminate proportions has rendered necessary." 



In the * Jahresbericht ' for 1825 there is a critique on Gmelin's 

 system of mineralogy, with some remarks on that of Mohs ; and in 

 that for 1846, Berzelius again returns to the subject of mineralogy. 

 He there explains the changes in his views, and gives a new electro- 

 chemical arrangement of the elementary bodies according to which 

 minerals may be classified. The following are the orders he then 

 adopted : — 



I. Elementary bodies. 



II. Combinations of metals with metals. 



III. Combinations of simple elements with elements that form 

 bases (Basenbildnern) — seleniets, sulphurets, &c. 



IV. Combinations of simple elements with elements that form 

 salts (Salzbildnern) — haloid salts, &c. 



V. Combinations of the more electro-positive oxides (bases) with 

 the electro-negative oxides (acids) — hydrates, silicates, carbonates, 

 &c. 



This system has been worked out by Rammelsberg. The great 

 difficulty was still the isomorphous elements. When less than 2 or 

 3 per cent., Berzelius neglected them ; when more, they were taken 

 into account, so that some minerals, such as augite, hornblende and 

 garnet, have to be repeated in different places, while others, having 

 little in common, are brought together. 



The ' Handbook of Chemistry,' in Swedish, by Berzelius, which con- 

 tains much mineralogy, has been translated into German by Wohler, 

 and his ' Treatise on the Blowpipe ' has become a well-known work 

 in different languages. He made numerous analyses of minerals, 

 besides those noticed above. Some of those of meteoric stones were 

 published in 1832, and in the ' Jahresbericht ' for 1836 he had 

 much on the same subject. 



The analysis of wavellite by Berzelius is a good example of the 

 care needed in such investigations, and is useful in showing, though 

 made so long since as 1 8 1 9 f , how desirable it would be if analyses 



* Dufi'enoy, Traite de Mineralogie, tome i. p. 19. 



t Dr. Thomson remarks (History of Chemistry, vol. ii. p. 224) that an analysis 

 by Fuchs had given him similar results in 1818, though Berzelius did not appear 

 to have been acquainted with it. 



