ISOMERIC CHANGE 617 



ideas as to the intimate relationship between physical pro- 

 perties and chemical composition — by the suggestion that the 

 dimorphism was due to the presence of iron in the calcite and of 

 strontia in the aragonite crystals. It is true that this assump- 

 tion was soon found to be false— strontia being by no means a 

 necessary constituent of aragonite, nor iron of calcite — but the 

 possibility of dimorphism was only regarded as fully established 

 when Mitscherlisch, in 1823, discovered that similar differences 

 of crystalline form could be produced artificially, in materials 

 of identical origin and of established purity, by crystallising 

 sulphur under slightly varied conditions. 



Allotropy. — The fact that diamond gave rise to carbonic acid 

 when burnt was established qualitatively by Lavoisier, and 

 quantitatively by Tennant in 1796, whilst Davy in 18 14 proved 

 the absence of moisture from the products of combustion, and 

 therefore of hydrogen from the diamond itself. But the allotropy 

 of charcoal and diamond, and still more so that of charcoal and 

 graphite (established by Mackenzie in 1800), was complicated by 

 the invariable presence of a greater or smaller proportion of ash, 

 and had the supreme drawback that the various allotropes had 

 to be accepted as natural products and could not be converted 

 into one another, nor even separated from their impurities, by 

 artificial methods. As a matter of fact, if the slight differences 

 resulting from dimorphism be put on one side, the first clear 

 case in which inorganic materials of identical composition were 

 proved to be capable of existing in widely different forms was 

 only forthcoming (long after the problem had been solved in 

 another field of work) when Schrotter in 1845 (P°gg- Ann. 81, 

 276) demonstrated the chemical identity of red and yellow phos- 

 phorus, and Andrews, eleven years later {Phil. Trans. 1856, p. 13), 

 proved that ozone consisted of oxygen and of oxygen only. 



Isomerism of Inorganic Compounds. — It is noteworthy that 

 both these illustrations are derived from the behaviour of ele- 

 mentary substances, and that the observed differences are known 

 to be associated with changes in the size of the molecule. If 

 this source of variation be excluded, the task of finding amongst 

 inorganic materials substances in which identity both of com- 

 position and of molecular complexity is associated with marked 

 diversity of properties becomes a very formidable one, and even 

 at the present time the number of cases known is so small that 

 it is no easy matter to quote offhand any suitable example of 



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