1908-9.] Life and Chemical Work of Archibald S. Couper. 203 
Founding on his mode of regarding combination, Couper writes formulae 
in which the mutual bonds of the atoms are indicated by means of con- 
necting lines, dotted in the papers in the Comptes rendus and in the 
Philosophical Magazine , continuous in that in the Annales de chimie et 
de physique — true structural formulae in our present meaning of the 
term. 
Kekule, in his derivation of the constitution of the radicals, resolved 
the formulae as completely as Couper ; but when lie had done this in words, 
he put the contracted radical formulae into typical formulae. He thus 
retained Gerhardt’s mode of writing, and developed the idea of the typical 
formulae. In the first part of his textbook, issued in the spring of 1859, he 
gave a graphic representation of the mode of union of the atoms, but un- 
doubtedly our present manner of writing structural formulae was started 
by Couper. 
There is, however, one point on which Kekule expressly establishes a 
difference between his view and that of Couper. Couper speaks, in a 
passage quoted above, of “ la puissance de combinaison la plus elevee que 
Ton connaisse pour le carbone.” He distinguishes two kinds of affinity in 
an element : — 
“ 1. L’affinite de degre, 
2. L’affinite elective.” 
What Couper wished to be understood by “l’affinite de degre” is explained 
by reference to carbon : — 
“ Prenant pour exemple le carbone, je trouve qu’il exerce son pouvoir 
de combinaison en deux degres. Ces degres sont representes par CO 2 
et CO 4 , c’est-a-dire par l’oxyde de carbone et l’acide carbonique, en 
adoptant pour les equivalents du carbone et de l’oxygene les nombres 
12 et 8.” 
Without discussing the example chosen by Couper to explain the 
“ afhnite de degre,” Kekule says : — ■“ Si M. Couper croit avoir decouvert la 
cause de cette difference de basicite dans l’existence d’une espece speciale 
d’affinite, i’affinite de degre, je suis le premier a reconnaitre que je n’ai 
aucun droit a lui contester cette priorite.” 
But Couper ’s “ affinite de degre ” is obviously nothing else than what 
was afterwards called “ variable valency,” and the passage quoted above 
from Kekule’s claim of priority is his first clear expression of his disbelief 
in variable valency. 
That Couper assumes 0 — 8, while he puts C = 12, is an inconsistency 
to which Kekule makes no reference ; I shall return to this matter 
later. 
