H)(> Mr. M. M. P. Muir on Isomerism. 



will depend on the position of the parts ; hence the two theories 

 will be complementary of each other. 



(>. One of the difficulties in the way* of explaining the facts 

 of isomerism in terms of the molecular theory arises from our 

 ignorance of the conditions which influence the linking- 

 together of atoms in a molecule. Given the formula of a 

 substance and the valency of the atoms, it is possible to ar- 

 range the symbols in a certain number of ways only ; each of 

 these methods of arrangement may represent an isomeric body. 

 But w r e have often very good reasons for believing that of 

 these possible methods of arrangement tw r o or more will 

 represent substances which are actually identical, not isomeric, 

 with each other. 



Thus if we replace one hydrogen atom in marsh-gas by 

 chlorine we might arrange the symbols representing the re- 

 sultant in four ways : — 



H CI H H 



I I I I 



H— C— H H— C— H H— C— CI CI— C— H 



I I f I 



CI H H H 



Yet we know that but one substance is Invariably produced 

 in the foregoing reaction. Hence we conclude that the four 

 " affinities " or four " equivalents " of carbon are, in this case, 

 of equal value. In other instances, however, it would appear 

 as if the " affinities " of an elementary atom were not all of 

 equal value. Thus we know T that phosphorus pentachloride. 

 PC1 5 , at a moderate temperature splits up into PC1 3 and Cl 2 ; 

 hence we might suppose that two of the chlorine atoms are 

 held to the phosphorus atom less firmly than the remaining 

 three*. Have we here the existence of weaker and stronger 

 affinities in the phosphorus atom ? If such differences actually 

 occur, the existence of isomeric bodies may be traced to other 

 causes than that which is generally understood by the phrase 

 "position of the atoms/' 



7. It would appear very probable that the valency of an 

 atom may be regarded as an expression for the number of 

 directions or of positions in which the atom is capable of 

 exercising chemical force. Thus when w 7 e say that the carbon 

 atom is tetravalent we mean, on this hypothesis, that it is 

 capable of exercising chemical force in four directions, or that 



* It may be objected that we do not know that PC1 5 actually exists. 

 Wurtz's determinations of the vapour-density of this compound (Compt. 

 Itend. vol. lxxvi. p. 601), carried out in an atmosphere of the trichloride 

 whereby dissociation was prevented, show that PCI, truly represents the 

 molecular weight of the substance. 



