Dr. E. J. Mills on Isomerism. 3 



considered molecular movement, whereby one arrangement was 

 substituted for another (the German Umlagerung), to be the true 

 key to isomerism. He supposed that difference of condensation 

 was sufficient to account for what Berzelius afterwards called 

 " polymers." 



The great Swedish chemist, again returning to the subject*, 

 defines isomers, with less restriction, as substances having the 

 same composition but different properties. The second of a pair 

 of analogous isomers has the prefix irapa, as in the case of the 

 tartaric acids, the stannic oxides, the phosphoric acids, cyanic 

 and cyanuric acid. When a great difference in chemical proper- 

 ties is observed, such as exists between urea and ammonic cya- 

 nate, the adjective metaphoric is to be used instead of the prefix 

 Trapa. The phenomena themselves are thus originated : — " If 

 we allow ourselves to form hypothetical speculations on such a 

 relation, it will appear that the simple atoms of which a sub- 

 stance is composed are capable of union with each other after a 

 different fashion." In the year followingf, Berzelius reviewed 

 Dumas' s essay (v. supra), and condemned speculations on the iso- 

 merism of the elements, as being devoid of data ; some addition 

 was also made to the existing nomenclature. Isomerism now 

 refers, as at first, to the same absolute and relative number of 

 atoms. Those bodies in which the absolute number is different, 

 but the relative number the same, are polymeric. Metameric 

 substances are those which can be transformed into each other 

 without loss of weight — as cyanic acid and cyanuric acid, stan- 

 nous sulphate (Sn S) and stannic sulphite (Sn S), supposing 

 the last to exist. [It will be seen that polymers might be 

 metamers in the Berzelian sense. The use of formulae to show 

 metameric difference is also noteworthy.] 



The next important allusion to isomerism occurs in Dumas's 

 Lecons sur la Philosophic Chimique%. There we first find the 

 differences in physical properties separated from the chemical 

 differences and classed under one name, polymorphism. By this 

 term was meant, " on the one hand, the permanence of chemical 

 nature ; and on the other, the modifications which form or phy- 

 sical properties undergo" (p. 303). And further, "the ulti- 

 mate source of polymorphism is the variations in arrangement 

 of the integrant molecules of a body, variations which influence 

 its physical qualities either temporarily or permanently " (p. 308). 

 His definition of isomerism includes metamerism and polymerism : 

 " We recognize under the name of isomerism that modification 

 of grouping among the elementary molecules which produces 

 bodies quite different chemically, but having the same, funda- 



* Jahresbericht, 1832, pp. 46, 47. \ Ibid. 1833, vol. ii. p. 63. 



% 1837. 



B2 



