Mr. M. M. P. Muir on Isomerism. 171 



cases of formation of so-called molecular compounds. The 

 use of this term is based on the assumption, which appears to 

 be in keeping with facts, that, although in a given molecule 

 " the lines of force of its atoms are mutually satisfied, there 

 may be directions in which the molecule, as such, may exert 

 a certain amount of chemical energy." 



This idea follows naturally from the view of valency and 

 chemical affinity which I have sketched in paragraph 7 of the 

 present paper, because, if the valency of an atom be an ex- 

 pression for the number of directions or positions in which 

 that atom is capable of exercising force, but if the amount of 

 force exercised be independent of the valency, it follows that, 

 although an atom exercise its force in each of the possible 

 directions, a part of that force may possibly yet remain un- 

 exercised. And may we not imagine that this unexercised 

 force may be exercised by the newly formed molecule, and 

 that in one or more directions ? 



A view somewhat similar to this was put forward by Mills 

 some years ago*. If we grant " molecular valency," we can 

 easily see that the formation of physically isomeric bodies 

 becomes possible according to the supposition that the dif- 

 ference between these bodies is to be traced to the placing 

 together of a greater or less number of " fundamental mole- 

 cules " in the molecule of each. 



But if compounds composed of the same number of the 

 same atoms do exist the only difference in whose properties is 

 a purely physical one, we may still, I think, be able to explain 

 the existence of such truly "physically isomeric" bodies, 

 either upon the supposition that the affinities of many atoms 

 differ in value, or by supposing that these affinities are originally 

 of equal value, but that the "saturation" of some of them 

 modifies those which remain. 



10. It has been supposed that those elements which are 

 capable of assuming allotropic forms may sometimes exist in 

 these forms in their compounds f. The existence of allotropic 

 forms of elementary substances is closely allied with the ex- 

 istence of isomeric forms of compound substances. The same 

 or very similar explanations are probably applicable in both 

 cases ; in the case of elements we shall have to deal with 

 atomic interchanges of similar atoms only. If we grant 

 variability of affinities, we may suppose that the nature of the 

 particular allotrope produced by the mutual action of two ele- 



* Phil. Mag. [IV.] vol. xxviii. p. 364. 



t See Weber in this Magazine [IV.] vol. xlix. p. 300 ; also Wiedemann, 

 Pogg. Ann. vol. clvii. p. 41, a translation of which appears in the August 

 Number of the Philosophical Magazine. 



