14 Clarke, The Atomic Theory. 



if not entirely ignored. Laurent and Gerhardt attempted 

 to reform the system, but it was left for Cannizzaro, in 

 1858, to succeed. By doubling some of the currently 

 accepted atomic weights, order was introduced into the 

 prevailing chaos, and the chemical constants were brought 

 into harmony with the physical laws. The modern 

 atomic weights and our present chemical notation may 

 be dated from this time, even though the preliminary 

 anticipations of them were neither few nor inconspicuous. 

 The second great step forward was accomplished 

 through the labours of several men. Frankland and 

 Kekule were foremost among them, but Couper, Odling, 

 Williamson, Wurtz, and Hofmann all contributed their 

 share to the upbuilding of a new chemistry, of which the 

 doctrine of valency was the corner stone. A new property 

 of the chemical atom was brought to light, and structural 

 or rational formulae became possible. Each atom was 

 shown to have a fixed capacity for union with other 

 atoms, a capacity which could be given numerical ex- 

 pression ; and from this discovery important consequences 

 followed. An atom of hydrogen unites with one other 

 atom only ; the atom of oxygen may combine with two ; 

 that of nitrogen with three or five ; while carbon has 

 capacity for four. All unions of atoms to atoms within a 

 molecule are governed by conditions of this order, and the 

 limitations thus imposed determine the possibilities of 

 combination in a given class of compounds. Inorganic 

 chemistry the conception of valency has been most fruit- 

 ful, and it has shown the prophetic power which is 

 characteristic of all good theories. It explains radicles 

 and isomers ; it predicts whole classes of compounds in 

 advance of their actual discovery ; and it has guided 

 economic investigations from which great industries have 

 sprung. The former partial theories regarding chemical 



