458 PRINCIPLES OF CHEMISTRY 



their,atomic weights, indicate that this variation in atomicity is subject to 

 one perfectly exact and general law, and it is only carbon and its near 

 analogues which constitute cases of permanently preserved atomicity. It ia 

 impossible to recognise as constant and fundamental properties of atoms, 

 powers which, in substance, have proved to be variable. But by abandoning 

 the idea of permanence, and of the constant saturation of affinities that is 

 to say, by acknowledging the possibility of free affinities many retain a 

 comprehension of the atomicity of the elements * under given conditions ; ' 

 and on this frail foundation they build up structures composed of chemical 

 molecules, evidently only because the conception of manifold affinities gives, 

 at once, a simple statical method of estimating the composition of the most 

 complicated molecules. 



I shall enter neither into details, nor into the various-consequences follow- 

 ing from these views, nor into the disputes which have sprung up respecting 

 them (and relating especially to the number of isomerides possible on the 

 assumption of free affinities), because the foundation or origin of theories of 

 this nature suffers from the radical defect of being in opposition to dynamics. 

 The molecule, as even Laurent expressed himself, is represented as an archi- 

 tectural structure, the style of which is determined by the fundamental 

 arrangement of a few atoms, whilst the decorative details, which are capable 

 of being varied by the same forces, are formed by the elements entering into 

 the combination. It is on this account that the term ' structural ' is so appro- 

 priate to the contemporary views of the above order, and that the ' struc- 

 turalists ' seek to justify the tetrahedric, plane, or prismatic disposition of 

 the atoms' of carbon in benzene. It is evident that the consideration relates 

 to the statical position of atoms and molecules and not to their kinetic rela- 

 tions. The atoms of the structural type are like the lifeless pieces on a chess 

 board : they are endowed but with the voices of living beings, and are not 

 those living beings themselves ; acting, indeed, according to laws, yet each 

 possessed of a store of energy which, in the present state of our knowledge, 

 must be taken into account. 



In the days of Haiiy, crystals were considered in the same statical and 

 structural light, but modern crystallographers, having become more tho- 

 roughly acquainted with their physical properties and their actual formation, 

 have abandoned the earlier views, and have made their doctrines dependent 

 on dynamics. 



The immediate object of this lecture is to show that, starting with 

 Newton's third law of motion, it is possible to preserve to chemistry all the 

 advantages arising from structural teaching, without being obliged to build 

 up. molecules in solid and motionless figures, or to ascribe to atoms definite 

 limited valencies, directions of cohesion, or affinities. The wide extent of 

 the subject obliges me to treat only a smalfportion of it, namely otsubstitU' 

 tions, without specially considering combinations and decompositions, and 

 even then limiting myself to the simplest examples, which, however, will 

 throw open prospects embracing all the natural complexity of chemical rela- 

 tions. For this reason, if it should prove, possible to form groups similar, for 

 example, to H 4 or CH 6 as the remnants of molecules CH 4 or C 2 H 7 we shall 

 not pause to consider them, because, as far as we know, they fall asunder into 



