4 8o HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



almost the whole of the elements, apart from carbon, silicon, 

 and tungsten, which are used in the manufacture of modern 

 special-steels. Equally remarkable is the series of 

 isomorphous bivalent sulphates studied by Mitscherlich 

 (P- 372), 



MnSO 4 , FeSO 4 , CoSO 4 , NiSO 4 , CuSO 4 , ZnSO 4 , 



which run right across a series of columns labelled VII, 

 VIII, I, and II, and headed by the symbols R 2 O 7 , RO 4 

 R 2 O and RO. The horizontal relationship reaches its 

 climax in the case of the rare-earth elements which run 

 consecutively, with identical valency and almost identical 

 properties, over a range of 40 units of atomic weight. 



If injustice is not to be done to the memory of Mendeleeff, 

 it is important that these secondary relationships should be 

 recognised and emphasised. 



D. ATOMIC WEIGHTS AND ATOMIC NUMBERS. 



Front's hypothesis (1815). Almost immediately after the 

 first atomic weights had been determined, attempts were 

 made to discover numerical relationships between them. 

 The most important of these, now known as PROUT'S 

 HYPOTHESIS, was put forward anonymously in a paper " On 

 the Relation between the Specific Gravities of Bodies in 

 their Gaseous State and the Weights of their Atoms" 

 (Annals of Philosophy, 1815,6,321-330). It is generally 

 expressed by a statement that 



" The atomic weights of the elements are exact multiples oj 

 the atomic weight of hydrogen" 



The view first put forward by Prout was : 



" That all the elementary numbers, hydrogen being con- 

 sidered as i, are divisible by 4, except carbon, azote, and 

 barytium *, and these are divisible by 2 " (he. cit. p. 330). 



1 i.e. barium. 



