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 KekulS (1829-1896), 

 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 Kckule". 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. Kekul6 

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

 bon compounds. 



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

 Kekul6 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, 



