LECTURE X.] HISTORY OF CHEMISTRY. 191 



fore obliged to halve the atomic weights of the metals, and to 

 assume K = 39, Na=23, Ca = 2o, etc. He calls that quantity 

 of a monobasic acid an equivalent which yields a neutral salt 

 by the replacement of one part of hydrogen by 39 parts of 

 potassium, whilst he assumes the equivalents of the dibasic 

 acids to be twice that quantity. 40 The formula of acetic acid 

 therefore becomes C 2 H 4 O 2 while that of oxalic acid, C 2 H 2 O 4 , 

 remains unchanged. 



That Gerhardt, in spite of these well-considered and excel- 

 lent observations, which are now for the most part adopted, 

 had not reached the point of view which we take up, is shown 

 by a part of his paper 50 where he thinks he should point out 

 that, as a consequence of his proposals, the atomic theory, the 

 theory of volumes, and the equivalent theory all coincide. 

 According to our present views this is not attainable. The 

 various conceptions were first separated from one another in 

 1846, by Laurent, 51 who thereby rendered Gerhardt's numbers 

 admissible. He showed that these values were not by any 

 means equivalent, and consequently did not deserve the name. 

 They express, as he points out, those quantities which enter 

 into reaction, and accordingly represent molecular weights. 



Although Gerhardt's endeavours in the determination of 

 equivalents tended in the direction of employing comparable 

 quantities only, this was first stated and elevated to a principle 

 by Laurent. According to the latter, it is necessary to start 

 from a " terme de comparaison " and to refer the formulae of all 

 compounds to it. Since he is quite clear as to the fact that 

 the quantities contained in equal volumes do not always pro- 

 duce the same chemical effect, he considers the question 

 whether he will compare substances in the gaseous state 

 according to the space which they occupy, or whether he will 

 compare their equivalents. He rejects the latter comparison, 

 on account of the difficulty associated with the determination 

 of equivalents of substances which are not analogous, and 

 decides in favour of the first comparison ; that is to say, he 



49 Ann. Chim. [3] 7, 129. 50 Ibid. [3] 7, 140. 51 Ibid. [3] 18, 266. 



