410 TRANSACTIONS OF SECTION B. 
of geometrical isomerism, have either been discarded or modified by the dis- 
covery of new facts, and who shall say that the theory of ionic dissociation 
stands where it did, now that ions in solution have incurred the suspicion of 
associating with the solvent, and to that extent have come into line with 
molecules, for the orthodox behaviour of which Professor Armstrong himself 
would no doubt be prepared to vouch? 
Residual Valency. 
Among the many doctrines which have suffered under the stress of long- 
sustained investigation, that of valency is a prominent example. Valency is that 
property by which an atom attracts to itself other atoms or radicals, and its 
numerical value is deduced from the structural formule of compounds in which 
that atom occurs. Claus seems to have been the first to recognise that this 
attraction between two atoms is not a constant, but depends on the nature of the 
other atoms or radicals in the molecule,‘ and it is of interest to note in connec- 
tion with what follows that he used methane and its chloro-derivatives to 
illustrate his point of view. Valency may vary, therefore, from compound 
to compound ; it is known to alter under the influence of change in temperature, 
as, for example, when carbon dioxide or phosphorus pentachloride under- 
goes thermal dissociation. But Claus’ view did not meet with ready acceptance: 
hence at the Birmingham meeting few chemists, if any, would have questioned 
the quadrivalency of carbon, despite the difficulty caused by the existence of 
carbon monoxide. Now, carbon is believed to be bivalent in the carbamines, 
fulminic acid and other compounds as well as in carbon monoxide, and its 
tervalency is coming to be accepted in the light of the latest investigations on 
triphenylmethyl and its congeners. What is true of carbon is equally true of all 
other elements, except argon and its companions. Hence the doctrine of constant 
valency for which Kekulé contended, or that of variable valency in which the 
uncombined units varied by even numbers has necessarily given place to one 
of less rigid type, although the final form has yet to be determined. 
For the purpose of this address it will be sufficient to refer only to one of 
these later theories: that in which Werner, as the outcome of his exhaustive 
study of inorganic molecular compounds, and especially of the amines, sup- 
poses that an atom may have both principal and auxiliary or residual valency.® 
There are difficulties in its application to certain problems of organic chemistry— 
for example, the structure of the benzene molecule—but the conspicuous success 
which has attended Werner’s investigation of the complicated isomerism of the 
cobalt and chromium amines is evidence of its value as a guide in stimulating 
research in the most unpromising directions. Werner’s view that valency is an 
attractive force acting from the centre of the atom. being of equal value at all 
points on the surface and independent of units of affinity, has the merit of 
meeting the objection long urged to the idea that affinity has fixed direction in 
space, but otherwise leaves untouched Van’t Hoff’s brilliant conception of 
asymmetry which plays so great a part in the chemistry of to-day. 
What licht does this conception of residual valency, dating back to 1885, if 
not earlier,” and now embodied in many theories besides Werner’s, throw: on 
some of the problems with which the organic chemist is faced? Much every 
way. The question of the distribution of valency in the molecules of carbon 
compounds is discussed probably more than any other; it arises in connection 
with the structure of unsaturated compounds, the properties of fluorescence or 
colour which many of them exhibit, and the relation between chemical consti- 
* A. Claus, Ber., 1881, 14, 435. It may be noted that Claus concludes his 
paper with the statement, ‘ .... die Annahme von Valenzen, als in den 
mehrwerthigen Atomen praexistirender ihrer Wirkungsgrésse nach bestimmter 
Anziehungseinheiten eine ebenso unbegriindete, wie unnatiirliche Hypothese ist.’ 
° A. Werner, Newere Anschauungen auf dem Gebiete der anorganischen 
Chemie (Friedr. Vieweg u. Sohn, Braunschweig, 1908); English edition. New 
Ideas on Inorganic Chemistry. HE. P. Hedley. (Longmans, 1911.) 
* A. Werner, Ber., 1911, 44, 2445, 3231. 
7 8. U. Pickering, Proc. Chem. Soc., 1885, 1, 122; H. E. Armstrong, Proc. 
Roy. Soc., 1886, 40, 285. 
