104 PROGRESS OF SCIENCE IN THE CENTURY. 



conception of " nuclei." " The radical, as the per- 

 manent constituent in organic compounds, cor- 

 responding to the elements in inorganic chemistry, 

 gave way to the changeable nucleus, which only pre- 

 served its form ; the unchangeable principle was 

 found in the form, the structure or type, instead of 

 in the substance of the simple or composite consti- 

 tuents." 



Valency. Time and ability alike fail us to dis- 

 cuss how the endeavour after systematisation and 

 simplicity was continued by Kekule (18291896), 

 Kolbe (1818-1884), A. W. von Hofmann (1818- 

 1892),"Wurtz (1817-1884), and many others. The 

 radical theory was characteristically German, the 

 type theory, French ; and now we have to notice a 

 more distinctively British contribution, the idea of 

 the " atomicity " or " valency " of chemical substan- 

 ces, whether elements or compounds. With this idea 

 the name of Frankland (1852) ought perhaps to be 

 particularly associated. 



The conception of " valency," or the capacity of 

 saturation of the atoms, was used with great effect 

 by Kekule. Almost simultaneously, in 1858, he and 

 Couper suggested that the carbon atom should be con- 

 sidered as quadrivalent; i.e., able to unite with four 

 univalent atoms or radicals (such as can replace one 

 atom of hydrogen), but not with more. Kekule 

 found in this a key to the constitution of many car- 

 bon compounds. 



" We have chiefly," Ostwald says, " to thank 

 Kekule for carrying through this idea. In the 

 theory of valency, which is at the present time the 

 prevalent one, it is assumed that each atom pos- 

 sesses a definite limited capacity for combining with 

 other atoms. This capacity is called the valency, 



