CHEMISTRY SINCE TIME OF DALTON 



form molecules, it gradually dawned upon them that 

 not all elements are satisfied with the same number of 

 companions. Some elements ask only one, and refuse 

 to take more ; while others link themselves, when occa- 

 sion offers, with two, three, four, or more. Thus we 

 saw that oxygen forsook a single atom of its own kind 

 and linked itself with two atoms of hydrogen. Clearly, 

 then, the oxygen atom, like a creature with two hands, 

 is able to clutch two other atoms. But we have no 

 proof that under any circumstances it could hold more 

 than two. Its affinities seem satisfied when it has two 

 bonds. But, on the other hand, the atom of nitrogen 

 is able to hold three atoms of hydrogen, and does so in 

 the molecule of ammonium (NH 3 ) ; while the carbon 

 atom can hold four atoms of hydrogen or two atoms of 

 oxygen. 



Evidently, then, one atom is not always equivalent 

 to another atom of a different kind in combining pow- 

 ers. A recognition of this fact by Frankland about 

 1852, and its further investigation by others (notably 

 A. Kekule* and A. S. Couper), led to the introduction 

 of the word equivalent into chemical terminology in a 

 new sense, and in particular to an understanding of the 

 affinities or "valency" of different elements, which 

 proved of the most fundamental importance. Thus it 

 was shown that, of the four elements that enter most 

 prominently into organic compounds, hydrogen can 

 link itself with only a single bond to any other element 

 it has, so to speak, but a single hand with which to 

 grasp while oxygen has capacity for two bonds, ni- 

 trogen for three (possibly for five), and carbon for four. 

 The words monovalent, divalent, trivalent, tretrava- 



59 



