CHEMICAL SCIEI!^CE. 



ISOMERISM. 



Berthelot, in a memoir on a new kind of isomerism, proposes 

 the following subdivision of this subject: Isomeric bodies — that 

 is to saj-, bodies formed of the same elements united in the same 

 proi3ortion — can be separated into a certain number of classes or 

 general groups : — 



1. Equivalent composition. — Substances which appear to have a 

 purely accidental relation to each other; for instance, butyric 

 acid, Cg Hs O4, and dialdehyde (C4 II4 02)2. 



2. Metamerism. — Bodies formed by the vinion of two distinct 

 principles, so that in their formula} a kind of compensation is 

 established; for example, methylacetic ether, C2 H2(C4H4 04), 

 and ethylformic ether, C4 H4 (C2'H2 O4) . 



3. Polymerism. — Compounds arising from the union of several 

 molecules to form one ; this is shown in the case of amylene 

 (Cio Hio) and diamylene (Cio Hio)2. 



4. Isomerism, properly so called. — Thei'e are bodies that, differ- 

 ing in their properties, retain these distinctive features in their 

 passage through certain compounds, the properties of which 

 result from the internal structure of the compound molecule 

 taken as a whole, rather than the diversity of the comj^onents 

 which have produced it. This is observed in the cases of essence 

 of terebenthine and citron, the sugars, the symmetrical tartax'ic 

 acids, and the two classes of ethyl-sulphates. 



5. Physical Isomerism. — By which is meant the different states 

 of one and the same body, the diverse nature of which vanishes 

 when the substance enters into combination. To these five 

 classes, Berthelot proposes to append a new one, called keno- 

 merism, distinct from all the others, though allied to 

 metamerism. 



6. Kenomerism. — Two diffei'cnt compounds may lose, by the 

 effect of certain retigents which bring about decomposition, dif- 

 ferent groups of elements, and the remainders be identical in 

 composition ; these two derivatives, however, may yet be distinct 

 'the one from the other, both in physical and chemical properties. 

 They I'etain to some extent the structure of the compounds from 

 which they take their origin. To take exami^les : alcohol, by los- 

 ing 2 equivalents of hydrogen, is turned into aldehvde : — 



C4 He O, — H2 = C4 H4 O2. 

 Glycol, on the other hand, by giving up 2 equivalents of water, 

 is converted into glj'colic ether (oxide of ethvlene) : — 

 C4 He O4 — Ho O2 = C4 lliO.. 

 Glycolic ether and aldehyde are isomeric ; their composition is 

 16* I80 



