APPENDIX I. 459 



two parts, H 2 + H 2 or CH 4 + H^as soon as they are even temporarily formed, 

 and are incapable of separate existence, and therefore can take no part in 

 the elementary act of substitution. With respect to the simplest molecules 

 which we shall select that is to say, those of whicn the parts have no sepa- 

 rate existence, and therefore cannot appear in substitutions we shall con< 

 eider them according to the periodic law, arranging them in direct dependence 

 on the atomic weight of the elements. 



Thus, for example, the molecules of the simplest hydrogen compounds- 



HF H 4 H 3 N H 4 C 



hydrofluoric acid water ammonia methane 



correspond with elements the atomic weights of which decrease consecutively 

 F = 19, = 10, N = 14, C = 12. 



Neither the arithmetical order (1, 2, 3, 4 atoms of hydrogen) nor the total 

 information we possess respecting the elements will permit us to interpolate 

 Into this typical series one more additional element ; and therefore we have 

 here, for hydrogen compounds, a natural base on which are built up thos& 

 simple chemical combinations which we take as typical. But even they arfc 

 competent to unite with each other, as we see, for instance, in the property 

 which hydrofluoric acid has of forming a hydrate that is, of combining with 

 water ; and a similar attribute of ammonia, resulting in the formation of a 

 caustic alkali, NH 3 ,H,0, or NH 4 OH. 



Having made these indispensable preliminary observations, I may now 

 ,attack.the problem itself and attempt to explain the so-called structure or 

 father construction, of molecules -r- that is to say, their constitution and trans- 

 "formations without having recourse to the teaching of ' structuralists,' but on 

 Newton's dynamical principles. 



Of Newton's three laws of motion, only the third can be applied directly 

 to chemical molecules when regarded as systems of atoms among which it 

 must be supposed that there exist common influences or forces, and resulting 

 compounded relative motions. Chemical reactions of every kind are un- 

 doubtedly accomplished by changes in these internal movements, respecting 

 the nature of which nothing is known at present, but the existence of which 

 the mass of evidence collected in modern times forces us to acknowledge as 

 forming part of the common motion of the universe, and as a fact further 

 established by the circumstance that chemical reactions are always charac- 

 terised by changes of volume .or the relations between the atoms or the 

 molecules. Newton's third law, which is applicable to every system, declares 

 that, ' action is also associated with reaction, and is equal to it.' The 

 brevity of conciseness of this axiom was, however, qualified by Newton in- 

 a more expanded statement, 'the action of bodies one upon another ate 

 always equal, and in opposite directions.' This simple fact constitutes tb.0 

 point of departure for explaining dynamic equilibrium that is to say, systems 

 of conservancy. Jt is capable of satisfying even the dualists, and of explain- 

 ing, Without additional assumptions, the preservation of those chemical types 

 .which Dumas, Laurent, and Gerhardt created unit types, and those views of 

 atomic combinations which the structuralists express by atomicity or the 



