Validity, and Residual Affinity. 299 



chlorine in the one case, democratic chlorine or oxygen atoms 

 dance round a rare and monarchial metallic atom in the 

 other. This is illustrated by Wurtz's arguments in support 

 01 



TT TT 



of NH 4 C1 as \ /*t\/ ; and the new departure is indicated 

 H H 



by what Prof. Armstrong calls his first " apple of discord " 

 when he disputes the evidence usually taken to justify the 

 analogous domination of N" in Me 4 NI. I have been inde- 

 pendently led to similar conclusions, and would write the 

 constitutional formula as 



h 3 c x h 3 c n 



H 3 C-N=I— CH 3 or H 3 (VN * I— 0H 3 . 

 H 3 (/ H 3 (/ 



The peck-lines are simply meant to show that the point for 

 discussion is the valency or validity of the nitrogen and 

 iodine atoms. Those who accept NMe 4 I as not a molecular 

 compound usually assume (1) that N is pentavalent, (2) that 

 I is monovalent, (3) that there are four methyl groupings (or 

 other B/) directly and equally intimately associated with the 

 nitrogen atom. The formula given simply asserts: — (1) That 

 the chemical evidence favours a belief in the direct association 

 of only three of the four methyls; (2) That in whatever sense 

 the nitrogen atom is pentad not triad, in that same sense the 

 iodine atom is interdependently triad not monad. If the ni- 

 trogen is pentavalent here, the iodine atom is also trivalent. 

 But in order that the dispute may be fought out with the less 

 risk of misunderstanding, my previous paper proposed (1) that 

 the rarer or disputed valency may be referred to as validity, and 

 (2) the latin prefixes and (3) roman numerals be exclusively 

 used for this latter disputed number. Anything that tends 

 to order and method in our present notation is at least worth 

 consideration. Now nobody disputes that nitrogen is tri- 

 valent. Let us, then, in this and in other well-recognized 

 cases keep uniformly to the greek prefix and symbolize a 

 trivalent nitrogen atom as W" ', reserving the symbol N v for 

 the disputed qui?iquivalid nitrogen in such compounds as those 

 under consideration whenever there is likely to be ambiguity. 

 The first formula would thus assert that iodine was therein 

 trivalent, the second that it is tervalid ; but that whether 

 validity is a property (electrical wholly or in part), a function 

 of the atom of the same chemical intensity as that relatively 

 suggested by the numbers 1, 2, 3, already defined, and known 



