4 Dr. E. J. Mills on Isomerism. 



mental composition " (p. 316). His views as to the origin of 

 isomerism have become somewhat more precise : " . . . . modify 

 the effects of cohesion in a substance, and the variations will 

 belong to polymorphism. Modify the effects of affinity, and 

 you will produce a case of isomerism " (p. 317) . 



Four years afterwards*, Berzelius, always unwilling to rank 

 elements with compounds, invented the word allotropy as apply- 

 ing to the apparent isomerism &c. of the former. At the same 

 time he expresses his opinion that such phenomena arise from 

 differences in the situation (Laye) of atoms. 



Lowig seems to assign to atomic volume an important share 



in the formation of isomers. " the difference between 



the properties of radicals depends on the different grouping of 

 the atoms, and on their different atomic volumes '^f. 



At a period considerably nearer to our own time Kekule gave 

 this subject a special treatment J. He retained the strict and 

 primitive sense of the word isomerism ; restricted the application 

 of the term " metameric " to bodies of the same molecular weight, 

 and whose differences could be shown by means of rational for- 

 mulae; and established a new class of "isomers in the inner 

 sense/ 3 that is, substances whose chemical differences were as yet 

 unexplained. A distinct place was assigned to physical isomers. 



Berthelot, in his Chimie Organique fondee sur la Synthese§, 

 defined isomers as bodies formed of " the same elements united 

 in the same proportion as regards weight" (p. 658). Chemical 

 isomerism is recognized by its permanence after a given sub- 

 stance has entered and left a combination. Physical isomerism 

 is only transitory. The former has a fourfold division, under the 

 following titles (pp. 659, 660) : — (1) equivalent composition, 

 the same as the "metaphoricism" of Berzelius; (2) metamerism; 

 (3) polymerism ; (4) isomerism proper , the same as Kekule' s pro- 

 posed new division. The cause of all these various relations ap- 

 pears to be ultimately that of metamerism, which seems to be 

 referred to changes in " the distance and relative arrangement 

 of the molecules" (p. 763). In the same year Naquet pub- 

 lished a pamphlet upon allotropy and isomerism || ; but it did 

 not so much treat of the facts represented by those names as of 

 the appropriate usage which those and kindred names ought to 

 obtain. He uses the word polymorphism in the sense of phy- 

 sical allotropy, allotropy referring to the varieties of one and the 

 same thing, and their mutual convertibility ; the term isomerism 

 is limited to cases where such a conversion cannot be accom- 



* Jahresbericht, 1841, vol. ii.p. 13. 



t Chem. der org. Verbindungen, p. 124 (1846). 



% Lehrbuch, vol. i. p. 187 (1859). § Vol. ii. (I860). 



|| De Vallotropie et de Visomerie (1860). 



