ISOMERIC CHANGE 



PART I.— HISTORICAL DEVELOPMENT OF THE 



THEORY 



By T. MARTIN LOWRY, D.Sc. 



Lecturer in Physical Chemistry and Instructor in Crystallography at the Central 



Technical College 



The years that immediately followed the formulation of the 

 Atomic Theory were characterised by great energy in the deter- 

 mination of equivalent or atomic weights, and by a more zealous 

 study of the analytical composition of the most various sub- 

 stances than at any previous period in the history of chemical 

 science. The analysis of mineral bodies, such as the metallic 

 oxides and chlorides, presented, relatively speaking, only slight 

 difficulties, and it was no very serious task for Berzelius to show 

 that the three differently coloured oxides of lead differed from 

 one another in the relative proportion of lead and oxygen, the 

 brown peroxide containing twice as much oxygen as the yellow 

 litharge ; or that differences between the two oxides of sulphur, 

 repeated in the sulphites and sulphates derived from them, were 

 due to a similar inequality in the proportions of combined 

 oxygen in the various compounds. Similar instances were 

 multiplied to such an extent that a widespread conviction was 

 established that in all cases differences of properties might be 

 attributed to differences of chemical composition. 1 



Polymorphism. — It was characteristic of this attitude of mind 

 that Haiiy, in his classification of minerals, laid down the law 

 that each mineral substance possessed one specific crystalline 

 form, which could be assumed by no other species, and thus 

 denied at one stroke the possibility both of isomorphism and of 

 polymorphism. The supposed identity of composition of calcite 

 and aragonite was dismissed — in strict accordance with current 



1 For the early history I am indebted to F. P. Armitage, A History of Chemistry ; 

 o Prof. Armstrong's article on " Isomerism " in Morley and Muir's Watts' Dic- 

 tionary ; and to Roscoe and Schorlemer's Treatise, to which reference may be 

 made if it is desired to trace out the original literature. 



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