LECTURE VI.] HISTORY OF CHEMISTRY. 93 



Berzelius does not adopt the distinction between physical 

 and chemical atoms, introduced by Avogadro and Ampere ; 

 and he endeavours to surmount the difficulty which had led 

 Dalton to regard Gay-Lussac's law as inaccurate, by completely 

 separating from one another the elementary and the compound 

 gases. 



It is clear that the law of gaseous volumes, together with 

 the conclusions that Berzelius draws from it, is insufficient. It 

 can only be used to determine the relative number of atoms in 

 a very few compounds, and the founder of the first chemical 

 system is, therefore, obliged to seek for other generalisations of 

 more universal validity. He advances the following rules, 19 

 which are intended, however, to apply to inorganic compounds 

 only. 



I. One atom of an element combines with i, 2, 3, etc., 

 atoms of another element. 



He does not state the limit. In 1819, he thinks that 

 more than four atoms of one element seldom combine with 

 one atom of another; afterwards (1828) he drops this limita- 

 tion. 



II. Two atoms of an element combine with 3 or with 5 

 atoms of another element. 



This rule leads him to a discussion of the question as to 

 whether a compound of 2 atoms of one element with 4 or 

 with 6 of another element is identical or not with the combina- 

 tion of i atom of the first element with 2 or with 3 of the 

 second. In his Text-book (i 828) he leans to the latter opinion ; 

 by this time, isomeric compounds were known. 



The laws of combination of compound atoms of the first, 

 second, and third orders are quite similar, but certain limita- 

 tions here come into play, arising from the fact that when 

 compound atoms combine, they have either the electro- 

 negative, or else, less frequently, the electro-positive constituent 

 common to both, and the proportions in which these atoms 

 then combine are determined by the common element in such 



19 Essai etc. 29-30. 



