THE CENTURY'S PROGRESS IN CHEMISTRY 



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 powers. 

 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, nitrogen for three (possi- 

 bly for five), and carbon for four. The words mono- 

 valent, divalent, trivalent, tretravalent, etc., were coined 

 to express this most important fact, and the various ele- 

 ments came to be known as monads, diads, triads, etc. 

 Just why different elements should differ thus in valency 

 no one as yet knows ; it is an empirical fact that they 

 do. And once the nature of any element has been deter- 

 mined as regards its valency, a most important insight 

 into the possible behavior of that element has been 

 secured. Thus a consideration of the fact that hydro- 



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