Ixxiv REPORT — 1873. 



atoms are classified, from their resemblance to these respectively, as Monads, 

 Dyads, Triads, Tetrads, ifee. 



The combining value which we thus recognize in the atoms of these several 

 classes has led us naturally to a consideration of the order in which atoms 

 are arranged in a molecule. Thus, in the compound of oxygen with hydro- 

 gen and potassium, each of these latter atoms is directly combined with the 

 oxygen, and the atom of oxygen serves as a connecting link between them. 

 Hydrogen and potassium have never been found capable of uniting directly 

 ■vvith one another ; but when both combined with one atom of oxygen they 

 are in what may be called indirect combination with one another through 

 the medium of that oxygen. 



One of the great difficulties of chemistry some few years ago was to ex- 

 plain the constitution of isomeric compounds, those compounds whose mole- 

 cules contain atoms of like kinds and in equal numbers, but which differ 

 from one another in their properties. Thus a molecule of common ether 

 contains four atoms of carbon, ten atoms of hydrogen, and one of oxygen. 

 Butylic alcohol, a very different substance, has precisely the same composition. 

 We now know that in the former the atom of oxygen is in the middle of a 

 chain of carbon atoms, whereas in the latter it is at one end of that chain. 

 You might fancy it impossible to decide upon any thing like consistent evi- 

 dence such questions as this ; but I can assure you that the atomic theory, 

 as now used by chemist?, leads frequently to conclusions of this kind, which 

 are confirmed by independent observers, and command general assent. That 

 these conclusions are, as far as they go, true descriptions of natural phe- 

 nomena is shown by the fact that each of them serves in its turn as a step- 

 ping-stone to further discoveries. 



One other extension of our knowledge of atoms I must briefly mention, 

 one which has as yet received but little attention, yet which wiU, I venture 

 to think, be found serviceable in the study of the forces which bring about 

 chemical change. 



The original view of the constitution of molecules was statical ; and che- 

 mists only took cognizance of those changes of place among their atoms which 

 result in the disappearance of the moleci;les employed, and the appearance of 

 new molecules formed by their reaction on one another. Thus, when a 

 solution of common salt (sodic chloride) is mixed with a solution of silver 

 nitrate, it is well known that the metallic atoms in these respective com- 

 pounds change places with one another, forming silver chloride and sodic 

 nitrate ; for the silver chloride soon settles to the bottom of the solution in 

 the form of an insoluble powder, while the other product remains dissolved 

 in the liquid. But as long as the solution of salt remained undecomposed, 

 each little molecule in it was supposed to be chemically at rest. A parti- 

 cular atom of sodium which was combined with an atom of chlorine was sup- 

 posed to remain steadily fixed to it. When this inactive solution was mixed 

 with the similarly inactive solution of silver nitrate, the interchange of atoms 

 known to take place between their respective molecules was nominally ex- 

 plained by the force of predisposing affinity. It was, in fact, supposed that 

 the properties of the new compounds existed and produced effects before the 

 compounds themselves had been formed. 



I had occasion to point out a good many years ago that molecules which 

 appear to be chemically at rest are reacting on one another when in suitable 

 conditions, in the same kind of way as those which are manifestly in a state 

 of chemical change — that, for instance, the molecules of liquid sodic chloride 

 exchange sodium atoms with one another, forming new molecules of the same 



