170 Mr. M. M. P. Muir on Isomerism. 



sphere of one another's action ; that, molecular dissociation 

 occurs, but that in the case of molecule b this is not carried 

 out completely, that instead of the setting-free of single atoms, 

 groups (or at any rate a group) of atoms are produced ; these 

 groups become combined with the atoms which formerly con- 

 stituted molecule a : of the equivalents of these atoms some 

 are satisfied by the new atomic groups ; and in this process 

 the remaining equivalents become altered in value (Ave have 

 supposed that originally the equivalents were all of equal 

 value). Now, on our supposition, the nature of the atomic 

 group or groups which occupy certain of the positions in which 

 the atoms of molecule a are capable of exercising force, will 

 influence the amount of force remaining to be exercised at the 

 unoccupied positions ; hence we have only to imagine for the 

 production of an isomer that molecule a comes within the 

 sphere of action of a third molecule c composed of the same 

 atoms as b, but capable of splitting up into different atomic 

 groups. 



In these supposed formations of isomers we have change of 

 position, change of valency, and change of energy taking 

 place simultaneously. 



9. So-called physically isomeric bodies * appear rather to 

 belong to the category of polymeric substances. In most 

 recorded cases it is very probable that the molecular weight of 

 the substance does not remain constant throughout its modi- 

 fications. If this be so, we should of course have no difficulty 

 in explaining why varying phenomena are presented by each 

 of the modifications. 



Laubenheimer (loc. cit.) seeks to explain the formation of 

 so-called physically isomeric bodies by the supposition that 

 the molecules of those modifications which have the higher 

 melting-points are made up, so to speak, of a greater number 

 of fundamental molecules (" GrundmolekuW') than the mole- 

 cules of modifications which melt at lower temperatures. 

 This supposition takes for granted that molecular compounds 

 exist, or at any rate that by the aggregation of molecules (not 

 of atoms) new molecules may be produced, — molecule being- 

 taken to mean the smallest portion of a body which holds 

 together when the body is hot. 



In a paper published elsewhere! I used the expression 

 " molecular valency " as being perhaps applicable to certain 



* Duffy, Chem. Soc. Journ. vol. v. p. 197 ; Zincke, Dent Chem. Ges. 

 Her. vol. iv. p. 576; Tollens, ibid. vol. viii. p. 1427: Laubenheimer, ibid 

 vol. ix. p. 760, &c. &c. 



f "On Recent Chemical Researches," Quart. Journ. of Science, Janu- 

 ary 1876. 



