A CENTURY OF CHEMISTRY. 105 



and the atoms that can combine with one, two, three 

 or four atoms (or equivalent atoms or radicals) are 

 said to be univalent, bivalent, trivalent, or quadri- 

 valent respectively. Thus marsh gas CH'* illustrates 

 the quadrivalent character of carbon, and water OH^ 

 the bivalent character of oxygen. 



Another development, foretold by Wollaston, but 

 practically beginning about 1858, when Pasteur 

 founded " stereochemistry " and Kekule stated his 

 theory of chemical structure, attained epoch-making 

 expression in 1875, when VanH Hoff published his 

 work entitled La Chimie dans VEspace * — an at- 

 tempt to formulate a geometrical conception of the 

 manner in which the hypothetical atoms may be sup- 

 posed to be placed in space. Along with Le Bel, he 

 formulated what is called the theory of " the asym- 

 metric carbon-atom " f and initiated what may be de- 

 scribed as a mechanical theory of valency, which has 

 been further strengthened by the work of Wislicenus 

 (1887), and other masters of the chemist^s craft. 



Summary. — The development of organic chem- 

 istry on its theoretical side affords a fine instance of 

 the gradual specialisation of an hypothesis as the 

 facts require it. The steps indicated by theories of 

 radicals, types, nuclei, and valencies are steps to- 

 wards a conception of material architecture which 

 will consist with the facts of chemical change. 



The concept of the atom was in its first form too 

 simple; the study of gases showed the necessity of 

 recognising the molecule; the development of or- 

 ganic chemistry enlarged the concept by the sug- 

 gestion of radicals and nuclei, equivalents and val- 



* J. H. Van't Hoff. Chemistry in Space, trans, and ed. by 

 J. E. Marsh, Oxford, 1891. 



t One whose four valencies are satisfied by four atoms 

 or radicals of different kinds. 



