Mr. M. M. Pattison Muir on Chemical Classification. 201 



sumptions : — (1) that the atoms constituting a group which 

 makes its escape from a molecule during the decomposition 

 of the latter existed in immediate combination with one 

 another in the molecule; (2) that if a group of atoms can 

 be caused to pass from one molecule into another, the constitu- 

 tion or structure of the group is not, as a rule, thereby altered; 

 and (o) that if an atom or group of atoms be replaced by 

 another atom or group of atoms of equal valency, the repla- 

 cing atom or group occupies the place formerly occupied by 

 the atom or group which has been replaced*. 



These assumptions appear to be often verified by facts. 

 Whether we are wholly justified in making them cannot be 

 settled until extended researches upon the chemical analogies, 

 and more especially upon the physical properties, of groups of 

 compounds have thrown light upon the connexion which 

 exists between chemical composition and the properties of 

 chemical compounds. At present the assumptions may be 

 looked upon in the light of working hypotheses. 



I have frequently used the expression constitutional formula. 

 It is necessary to define these words. 



In a former paper j I have defined a constitutional or struc- 

 tural formula as a formula which '' generalizes the reactions of 

 formation and of decomposition " exhibited by the compound 

 formulated. In another paper J I have insisted upon the ad- 

 vantages gained by the use of these formulae as expressing 

 " in the shortest possible manner the greatest amount of in- 

 formation about the actions of the substances formulated." 



Now some chemists would, I think, be content to use con- 

 stitutional formulaB, attaching to them some such meaning 

 as that Avhich I have just mentioned, while they would not be 

 at all willing to accept the general theories of valency and of 

 atom-linking ; in fact, they might refuse to accept the atomic 

 theory itself. But while I regard constitutional formulae as 

 doing all which I have claimed for them in the passages 

 quoted above, I cannot but think that they also do something 

 more. Accepting the theory of valency, imperfectly under- 

 stood though it be, accepting the theory of atom-linking, both 

 merely as hypotheses, and being fully convinced that a con- 

 nexion between structure and properties really exists, I can- 

 not regard constitutional formulae as less than attempts to 

 exhibit, so far as can be at present exhibited, that connexion. 

 A constitutional formula generalizes facts concerning reac- 

 tions of formation and of decomposition ; it tells us much 



* See Die modernen Theorien, 2nd edit. pp. 172, 173. 



t '' Isomerism," this Magazine, loc. cit. 



X " On Chemical Notation," this Magazine, July 1876. 



