16 JOURNAL OF THE 
is the theory of valence or quantivalence or atomicity, and 
without it the equivalents are purely empirical, and it is most 
difficult if not impossible to clear up the confusion connected 
with their use. 
Returning now to the derivation of this idea of valence 
from the type theory, according to Wurtz’ the conception of 
valence was introduced into the science in three steps. First 
there was the discovery of polyatomic compounds. This 
term was first used by Berzelius in 1827’, he applying it to 
such elements as chlorine or fluorine where he thought sever- 
al atoms of these elements united with a single atom of an- 
other element. ‘The term was later applied by Graham, Wil- 
liamson, and others to compounds. 
The second step was the reference of this polyatomicity to 
what was called the state of saturation of the radicals con- 
tained in these compounds. ‘This was largely through the 
work of Williamson and Gerhardt. 
Thirdly, this conception of saturation was extended to the 
elements themselves. ‘This was chiefly due to the work of 
Frankland upon the organo-metallic compounds. And so 
valence has come to refer to the number of atoms with which 
a single atom of any element will combine. 
This conception has then been one of slow growth, gradu- 
ally incorporating itself into the science as the necessity arose 
of devising a suitable explanation for accumulated observa- 
tions. It was a logical outcome of and was evolved from 
knowledge acquired step by step. It was no mere speculation 
or hypothesis, such as that of Prout, evolved by the fancy 
or imagination of one man and suddenly appearing with” 
scarcely a claim to foundation upon observed fact. 
This conception enters into the chemical theory of to-day 
almost as fundamentally as the atomic theory itself. Its ap- 
plication is of every-day occurrence and of the most varied 
character, and yet chemists admit that the nature of valence 
is one of their chief puzzles and they have advanced but little 
towards its solution during the past half century. It is quite 
1 Historie des doctrines chimiques, p. 69. 
2Jsb. d. Chem., 7, 89. 
