300 Mr. J. F. Heyes on Valency, 



as valency or atomicity, is left open for investigation and dis- 

 cussion. It may also be useful to adopt the word adicity. 

 long ago proposed by Prof. Odling as comprehending both 

 valency and validity*. Thus it is convenient to say that the 

 adicity of oxygen is 2 or 4, that oxygon is usually divalent, 

 but in some cases quadrivalid, if not actually tetravalent, 

 leaving questions of molecular compounds not entirely closed 

 on the chemical side, especially in the case of all X .*/H 2 

 "compounds." To take another example; in the case of the 

 notorious CO molecule, if the oxygen atom is not there tetra- 

 valent the carbon atom is bivalid. The " molecular com- 

 pounds " dispute does not here arise. On the other hand, in 

 such formulas as AgKI 2 and AgT . KI sides are taken. On 

 the former side we should write Ag — 1=1 — K ; whereas 

 Ag — I =a I — K would suggest the controversy : — Is iodine 

 here trivalent or is it tervalid in either some integral sense 

 akin to atomicity or in the sense suggested by the working- 

 hypothesis of " residual affinity " ? 



There has undoubtedly been great ambiguity of expressions 

 hitherto in this matter. Thus, whilst Wurtz admitted that 

 " NH 4 C1 belongs to the type NX.,," and held " generally and 

 implicitly that the chlorine and the four atoms of hydrogen 

 are united individually to the pentavalent nitrogen," he at 

 the same time gave as two reasons why HC1 and KH 3 can 

 unite: — (i.) "the atoms uniting are in possession of a residue 

 of affinity" (ii.) "the atoms of nitrogen can admit into their 

 sphere of action a fourth atom of hydrogen and an atom of 

 chlorine " (Atomic Theory, 1880, transl. p. 247). He went 

 on to deprecate " creating and employing secondary hypo- 

 theses." " Why should we graft upon this atomic hypothesis 

 a second, a special attraction which in a completed combination 

 is exercised by one molecule upon another ?" The italics are 

 mine, and they should be compared with the views he ex- 

 presses later on and the cases of residual affinity as Prof. 

 Armstrong employs the phrase. In PC1 5 Wurtz held that 

 " two atoms of chlorine were retained more loosely than the 

 other three''''; but the retaining force was "most naturally " 

 attributed to residence in the atoms themselves ; the P atom 

 in PX 5 "can retain a residue of energy capable of fixing new 

 atoms and of developing, if we maybe allowed the expre--ion, 

 supplementary atomicities." These statements are of great 



* I note that Prof. Armstrong uses the word (p. 28) in " pentad i city 

 of phosphorus," apparently in the more open sense of validity. As Trot". 

 Odling used the word cautiously with reference to valency it may be 

 convenient to retain it in the wclusive sense of P'" and P v . 



