TRANSACTIONS OF SECTION B. 959 
shown that during electrolysis both water and acid, according to cireumstances— 
concentration, KH. M. F., &c.—undergo change, it will be necessary to teach that in 
a similar manner the action of metals on acids is no less complex. Our views on 
the action of metals on concentrated sulphuric acid and on solutions of nitric acid 
-of various strength must also materially depend on the interpretation of the beha- 
viour of these acids on electrolysis with varying electromotive forces. 
Having thus fully explained why I venture to think that Helmholtz’s definition 
‘that ‘electrolytes belong to the class of typical compounds, not to that of molecular 
aggregates,’ is somewhat open to question, it now becomes necessary to make some 
slight reference to the constitution of these so-called molecular aggregates. 
Although opinions differ widely as to the definition to be given of a typical or 
atomic compound, and of a molecular compound or aggregate, the majority of 
chemists appear to agree that we must recognise the existence of two distinct 
classes of compounds. Professor Williamson, in his address to this Section at the 
York meeting (1881), entered at length into the discussion of this question, and in 
very forcible terms objected to the recognition of molecular combination as some- 
thing different from atomic combinations; in this I, in the main, agree most fully 
with him. He further said that he had been led to doubt whether we have any 
grounds for assigning any limits whatever to atomic values, and he adduced a 
number of cases which, in his opinion, afforded illustration of a capability of 
elements to assume greater atomic values by combining with both negative and 
positive atoms than with atoms of one kind only; for example, he cited the com- 
‘pounds K,CuCl, and K,HgCl, as proof that copper and mercury may assume 
hexad functions ; the compound K,Agl, as an illustration that silver may act as 
a pentad; and the compounds KAsF, and K,AsF, were regarded by him as 
-evidence of the heptadicity and nonadicity of arsenic. 
I have long been of opinion that the experimental investigation of this question 
is of great importance, and I believe that it must ere long attract the attention it 
-deserves. The problem will be solved, not by discussions on the fertile theme of 
valency, but by determining the structure—the constitution—of bodies such as 
were referred to by Professor Williamson. 
My own view on the question is a very decided one. So far as the mere defi- 
‘nition of valency is concerned, I entirely agree with Lossen; and, as I have said, T 
hold with Prof. Williamson that in all compounds the constituents are held together 
‘by atomic affinities, and atomic affinities only, but I believe that the formation of 
so-called molecular compounds is mainly due to peculiarities inherent more especi- 
-ally in the negative elements—.e., the non-metals and metalloids, and not in the 
positive elements—the metals; in other words, to the fact that, as was first pointed 
-out, I believe, by Lothar Meyer, the negative elements tend to exhibit a higher 
valency towards each other than towards positive elements. The view I take, then, 
is, that in the majority of so-called molecular compounds the parent molecules are 
preserved intact in the sense in which a hydrocarbon radical, such as ethyl, is pre- 
-seryed intact in an ethyl compound, being held together by the ‘ surplus affinity ’ of 
‘the negative elements. Thus I would represent the compounds K,CuCl, and 
K,HgCl, as containing copper and mercury of the same valency as the metal in 
‘the parent chloride, and regard them as compounds of the radicals (CuCl,), 
(HgCl,) and (KCl) ; a view which may be expressed by the formule 
Cl. ClK Cl. ClK 
Cac | Clk He C1 | OK 
"The arsenic compounds referred to may be similarly represented 
F.FK 
F.FK 
We do not hesitate to attribute to the so-called double cyanides this order of 
structure, without in any way supposing that the metal changes in valency. Evi- 
dence that the ‘constituent radicals exist unchanged in molecular compounds’ is 
afforded by facts such as that ferrous and potassium chlorides, for example, form a 
F,AsF . FK F,As 
_ compound which obviously is still ferrous, being of a green colour, which would 
