266 HISTORY OF CHEMISTRY. [LECTURE XIII. 



establish similar relations. I do not think I am mistaken in 

 regarding this fundamental principle as a new form of Laurent's 

 law of substitution. 66 It is a generalising of this law, but the 

 law has acquired, at the same time, another meaning, in so far 

 that chemists now no longer desire to determine the arrangement 

 of the atoms in space but only their relations to one another. 

 Unfortunately, no general proof of the principle has been fur- 

 nished, and experiments have not even been carried out which 

 aim at this. Its accuracy, which is certainly not beyond doubt, 

 is merely assumed because the conclusions drawn from it have 

 repeatedly yielded concordant results; that is, because these 

 conclusions led to identical formulae for the same substance 

 whichever mode of formation was considered. 



Nevertheless, this concordance is not always met with. 

 There are many cases known where the constitution deduced 

 from one mode of formation does not correspond to the 

 formula which may be derived by starting from another mode 

 of formation, or from the decomposition products. 67 In such 

 cases we are obliged to assume the conversion of the substance 

 into an isomeric modification, in one of the reactions that 

 have taken place ; that is, we must hold that the principle 

 stated above does not here apply, and that the atoms remaining 

 in the molecule have changed their mutual relations during 

 one of the reactions. Cases of this kind deserve attention. 

 They are well calculated to shake our faith in the accuracy of 

 the fundamental principle, even although an attempt has been 

 made to regard them as two-fold reactions, and in agreement 

 with this principle. A special interest attaches to those 

 investigations that define the conditions under which such 

 isomeric changes the so-called migration or wandering of 

 the atoms take place within the molecule, and to those 

 actually designed to elucidate this particular matter. As 

 examples of these, Hofmann's investigation of the conversion 



66 Compare pp. 143-144. 67 Compare, for example, Carius, Annalen. 

 131, 172 ; Tollens, ibid. 137, 311 ; P'riedel and Ladenburg, ibid. 145, 190; 

 Linnemann and Siersch, ibid. 144, 137; Butlerow and Ossokin, ibid. 145, 

 257 ; Simpson, ibid. 145, 373 ; Erlenmeyer, ibid. 145, 365, etc. 



