CHEMISTRY AND ITS DEVELOPMENT 227 



have come into existence if not for the fact that toward the 

 end of the fifties chemists had learned the true atomic 

 weights of the elements. Without a knowledge of the true 

 relative weights of atoms, it would have been impossible to 

 know their true number in molecules, and, hence, impossible 

 to know their true valencies. Atomic weights were deter- 

 mined, calculated, and re-calculated ever since Dalton first 

 established the atomic theory. Dalton himself, as stated 

 in a previous section of this article, determined atomic 

 weights on the basis of certain simple assumptions. Soon 

 afterward Berzelius devoted himself to the problem with 

 great assiduity. From the law of combining volumes, dis- 

 covered by Gay-Lussac in 1808, Berzelius inferred that 

 equal volumes of gaseous elements must contain equal 

 numbers of particles. In 1819 Mitscherlich discovered the 

 principle of isomorphism. (See Atomic Weights.) 1 Ber- 

 zelius had carried out about two thousand analyses, and 

 had thus determined the relative quantities of the elements 

 contained in a great variety of compounds. By combining 

 the principle of isomorphism with that of equal gaseous 

 volumes, he was now able to calculate the atomic weights 

 of the elements. Now, his principle of equal volumes was 

 not quite correct. To him the particles of a gaseous ele- 

 ment in the uncombined state were isolated atoms. While 

 he distinguished between the particles of compounds and 

 the atoms of elements, he failed to distinguish between the 

 free particles of elements and their atoms. That the parti- 

 cle of an element might be made up of two or more single 

 atoms, it would have been impossible for him to admit ; for, 

 according to his electro-chemical theory only unlike atoms 

 could exist in combination with one another. Avogadro's 

 memoir of 1811, in which more correct views on the subject 

 had been advanced, therefore remained unnoticed, and Ber- 

 zelius ' atomic weights were for years employed by all. Nor 

 were most of those figures wrong; for in many cases Ber- 



1 New International Encyclopaedia. 



