Mr. M. M. P. Muir on Isomerism. 169 



composed of the same atoms as those which actually were 

 produced, but differing in atomic structure, and in the amount, 

 or at any rate in the form, of the energy possessed by them. 



We have here isomerism which is chiefly due to a dif- 

 ference in the value of the equivalents of the atoms of one of 

 the reacting molecules. 



Let us now imagine that the atoms of the reacting mole- 

 cules were capable, as before, of exercising chemical force in 

 four and in two positions respectively, but that the force 

 exercised in each position is the same in amount. 



After the atomic interchanges have taken place (we may 

 suppose, for a moment, that these interchanges are accom- 

 plished almost instantaneously, and that the resultant molecules 

 are removed from the sphere of mutual action) we have new 

 molecules, still, by supposition, capable of exercising chemical 

 force in two directions ; but we may surely suppose that the 

 amount of force to be exercised in each of these directions is 

 not the same now as it was before the atoms had become asso- 

 ciated. The occupation, so to speak, of two of the positions 

 of possible action will most probably influence the force which 

 remains to be exercised at the two residual positions, even 

 supposing that originally an equal amount of force was 

 exercised at each of the four. The phenomena of magnetism 

 and of electricity make us acquainted with actions somewhat 

 similar to this. 



If we may carry the imagination one step further and sup- 

 pose that the occupation of two positions has not only in- 

 fluenced the force to be exercised at the remaining positions, 

 but has also influenced it unequally, we shall now have a 

 molecule which is capable of forming isomers by a process 

 similar to that which was carried out in the case which we 

 first attempted to follow. 



If we grant that the occupation of a certain number of the 

 positions at which an atom is capable of exercising chemical 

 force, by another atom, causes a change in the amount of 

 force remaining to be exercised at each of the residual posi- 

 tions, we may also very well grant that upon the nature of the 

 occupying atom will greatly depend the extent of this in- 

 fluence. 



By substituting a new occupying atom of equal valency 

 with the original we shall produce not only a new molecule, 

 but a new molecule which is possessed of a different amount 

 of energy from that originally formed. On this supposition 

 we can trace the formation of isomers containing different 

 atomic groups. 



Let us suppose that two molecules a and b come within the 



