202 HISTORY OF CHEMISTRY. [LECTURE XI. 



be altered. It was unquestionable, however, that even with only 

 a moderately consistent application of atomic considerations, 

 there was a certain connection between the different formulae. 

 A relation of this kind was attained, especially by Laurent and 

 Gerhardt, in the case of organic compounds in particular. It 

 appears to me to follow from the nucleus theory that Laurent 

 assumed the number of carbon atoms in the radical to remain 

 unchanged until carbon was separated in some form or other. 1 ' 1 

 It was Gerhardt who first clearly stated this rule 17 (which, how- 

 ever, is not always accurate, as, for example, in the formation 

 of polymeric substances), and it will readily be conceded that 

 by means of it, the molecular weights of many series of com- 

 pounds were fixed from a knowledge of these magnitudes in 

 the cases of a few substances. The so-called law of the even 

 number of atoms gave a further guide to the smallest formula, 

 and, in consequence of it, Laurent and Gerhardt found frequent 

 occasion to alter the formulae previously adopted. 



The conception of the polybasic acids exercised a very im- 

 portant influence in the fixing of formulae, since even Liebig 

 for example (who first definitely grasped this conception) was 

 induced to double the formula of tartaric acid, 18 in order to 

 make it conform with the chemical character of this substance. 

 In the case of one special class of substances the acids the 

 recognition of a criterion of their polybasicity, for which we are 

 indebted to Liebig, Laurent, and Gerhardt, was just as far- 

 reaching as the later experiments of Williamson in the cases of 

 other groups of compounds. 



The phenomena of substitution, as may readily be under- 

 stood, contributed something towards rendering the molecules 

 of different compounds comparable with one another. The 

 formula of a substance frequently required to be multiplied by 

 two or by three so that it might not be necessary to assume 

 fractions of atoms in the products arising from the substance 

 by the action of chlorine, etc. It only became necessary to 

 attend to these considerations in consequence of the unitary 



16 Compare p. 145. 17 J. pr. Chem. 27, 439. 18 Annalen, 26, 154. 



