PRESIDENTIAL ADDRESS. 431 
‘he greatest advance in the discussion of the problems of valency in 
recent years is that made by Barlow and Pope, as their method of treatment 
is one which applies to solid substances—the correlation of structure with 
crystalline form which it effects promises to be of far-reaching importance. 
Apart from hydrogen, carbon is the one element of certain character, 
acting always as a tetrad—its affinities may be only incompletely satisfied 
but they are always exercised, it may be supposed, even in ethenoid and 
similar compounds ; carbon monoxide apparently is the only exception to 
this rule, its relative inactivity being one of the most puzzling enigmas of our 
science, especially as the oxide becomes one of the most active of known 
substances when only two atoms of hydrogen are added to it. Most other 
elements (non-metallic) seem to vary in valency, the valency beyond a 
certain minimum being dependent on the nature of the association. Of late 
years, attention has been drawn in particular to the quadrivalency of oxygen 
in many of its compounds. 
The quadrivalency of sulphur in substances such as trimethylsulphonium 
iodide, Me,SI, having been proved to demonstration by the production of 
optically active compounds of this type (Pope and Peachy), it can no longer 
be supposed that in such cases we are dealing with compounds in which 
the negative constituents of the parent molecules are conjoined, e.g., 
Mel: SMe,. And yet we must contemplate the existence of such compounds 
as possible—in the case of nitrogen, for example, as ammonia must be sup- 
posed to form the compound NH,:OH, in preference to the hydroxide 
NH,O8, the latter being only a very minor constituent, the former the 
major component of the aqueous solution of the gas; hydrogen chloride, on 
the other hand, appears to afford only one product with ammonia, viz., 
NH, Cl. The existence of such differences is clear proof in the case of 
the non-metallic elements other than carbon that valency is not merely a 
variable but also a reciprocal or dependent function. 
There is no reason to suppose that hydrogen ever acts otherwise than as a 
simple monad; and the behaviour of the alkalies and alkaline earths in 
salts would seem to justify the conclusion that they have no tendency to 
vary in valency, were it not for the existence of well-defined non-volatile 
hydrides of these metals which are clearly substances of some degree of 
molecular complexity. Such compounds are illustrations of the difficulties 
which surround the subject. It has long been clear that the exhibition of 
the higher valency by an element is a process of a different order from that 
manifest when it exerts only its lower proper valency measured in terms of 
positive radicles such as H or C,H,,,; radicles. What that difference is 
we are not able at present to decide—carbon (together with silicon) differs 
from almost all other elements especially in combining with hydrogen and 
analogous radicles to the extent of its maximum valency. 
The proposition I made in 1888 (Phil. Mag., Series V., 25, 21) that the 
valency lines should, in some cases, be represented as passing through the 
atom, so that each is capable of acting in two directions, is the only con- 
sistent mode of expressing varying valency which has been devised, the 
only one, moreover, by which attention is drawn to the great difference. 
In many cases probably there has been a tendency to exaggerate the 
valency value-—in the case of chlorine, for example, in assuming that it 
functions as a heptad in the perchlorates. In this and many other instances, 
it suffices to assume that the chlorine and oxygen atoms are united in a 
closed ring, the chlorine functioning as a triad. Some such explanation 
will doubtless be given of the structure of the metallic ammonias and similar 
compounds. The co-ordination values introduced by Werner serve merely 
to establish certain empirical relationships and are only useful for the pur- 
poses of classification. The perhaps more rational plan of dealing with such 
compounds suggested by Abegg has a similar value, 
