470 
NALTORE 
4 ea 7 
[Sept. 17, 1885 
important bearing on the discussion of the nature of the chemical 
changes which occur during the dissolution of metals. Formerly 
it was said that when zinc acts upon dilute sulphuric acid, the 
zinc displaces the hydrogen of the water and the resulting zinc 
oxide dissolves in the acid, forming zine sulphate ; the modern 
explanation advocated by most chemists has been that the metal 
directly displaces the hydrogen of the acid: in fact, that this is 
the nature of the change whenever an acid is acted upon by a 
metal. Ifin a solution of sulphuric acid, of whatever strength, 
the acid be the actual electrolyte, I imagine that we are right in 
accepting this modern view ; but if the water be the electrolyte, 
we must, to be consistent, return to the view that the oxide— 
more probably in most cases the hydroxide—is the primary pro- 
duct. And if it can be shown that during electrolysis both water 
and acid, according to circumstances—concentration, E. 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 behaviour 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 some- 
what 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. Prof. 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 recog- 
nition of molecular combinations as something 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 compouuds K,CuCl, and K,HgCl, as proof that 
copper and mercury may assume hexad functions ; the compound 
K,AglI, 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 bodics 
such as were referred to by Prof. Williamson. 
My own view on the question is a very decided one. So far 
as the mere definition of valency is concerned, I entirely agree 
with Lossen; and, as I have said, I 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 forma- 
tion of so-called molecular compounds is mainly due to pecu- 
liarities inherent more especially in the negative elements—z.e. 
the non-metal; 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 posi- 
tive elements. The view I take, then, is, that in the majority 
of so-called molecular compounds the parent molecules are pre- 
served intact in the sense in which a hydrocarbon radical, such 
as ethyl, is preserved intact in an ethyl compound, being held 
together by the ‘‘surplus affinity” of the negative elements. 
Thus I would represent the compounds K,CuCi, 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. CIK Cl .«GUS 
fic) oiKy We crson 
The arsenic compounds referred to may be similar] ; represented 
“ F,AsF . FK RyAsea ee 
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. Evidence that the ‘‘ constituent radi- 
cals exist unchanged in molecular compounds” is afforded by 
facts such as that ferrous and potassium chlorides, for example, 
form a compound which obviously is still ferrous, being of a 
green colour, which would hardly be the case if the valency of 
| the iron were increased ; and that in like manner the compounds 
formed from stannous chloride manifest all the properties of 
stannous derivatives. 
Whatever be the nature of chemical affinity, it is difficult to 
re ist the conclusion that the ‘‘ charge” of a negative radical 
especially is rarely, if ever, given up all at once, that its affinity 
is at once exhausted. It would also appear that the amount of 
residual charge—of surplus affinity—possessed by a radical after 
combination with others depends both on its own nature and 
that of the radical or radicals with which it becomes associated. 
Differences such as are observed in the composition and stability 
of the hydrates of the salts of an acid—the sulphate:, for ex- 
ample—clearly point to this. Other illustrations are afforded 
by the manner in which chlorhydric acid yield chlorhydrates of 
some metals and chlorides of others.! 
It is noteworthy, however, that often those elements which 
from the ordinary point of view are regarded as possessed of 
feeble affinities are those which manifest the greatest tendency 
to form molecular compounds. Thus it is commonly held that, 
of the three elements, chlorine, bromine and iodine, chlorine has 
the highest and iodine the lowest affinity, and this views accords 
well with the recent observations of V. Meyer on the relative 
stability of their diatomic molecules at high temperatures; but 
nevertheless we find that the compound which HI forms with 
PH, is far more stable than that of HBr or HCl with this gas; 
and it is well known that mercuric zodide has a much greater 
affinity for other iodides than have mercuric bromide and chloride 
for the corresponding bromides and chlorides.” 
The recognition of the peculiarity in the negative elements to 
which I would attribute the formation of molecular compounds 
must, I think, exercise an important influence in stimulating 
and directing the investigation of these compounds and of com- 
pounds other than those of carbon ; in the near future the deter- 
mination of the structure of such compounds should oceupy an im- 
portant share of the chemist’s attention. It will perhaps afford a 
clue in not a few cases which are not altogether satisfactorily inter- 
preted in accordance with the popular view of valency. I may in- 
stance the formation of (?) polymericimetaphosphates, of complex 
series of silicates and tungstates, and of compounds of hydrocar- 
bons with trinitrophenol. It may even serve to explain some of 
the peculiarities of the more complex carbohydrates. 
It is one of the most clearly established of the ‘‘laws of sub- 
stitution”? in carbon compounds that negative radicals tend to 
accumulate : numerous instances are afforded by the behaviour 
of paraffnoid compounds with chlorine, bromine and oxidising 
agents, and by that of unsaturated paraffinoid compounds when 
combining with hydrogen bromide and iodine. The special 
affinity of negative elements for negative is not improbably the 
cause of this accumulation. A similar explanation may perhaps 
be given of some of the peculiarities which are manifested by 
benzenoid compounds. 
I would even venture to suggest that in electrolysing solutions 
the friction arising from the attraction of the ions for each other 
is perhaps diminished, not by the mere mechanical interposition 
of the veutval molecules of the solvent—in the manner suggested 
by Kohlrausch—but by the actual attraction exercised by these 
molecules upon the negative ion in virtue of the affinities of the 
negative radicals. 
One result of increased attention being paid vo the investigation 
1 The name chlorhydric acid is here applied to the compound HCKOH,), 
—probably x = 1—which, according to Thomsen, is present in an aqueous 
solution of hydrogen chloride. It would be an advantage if we ceased to 
speak of HF, HCI, HBr, HI, as acids, and always termed them hydrogen 
fluoride, chloride, bromide and iodide respectively. The names hydric 
chloride, bromide. &c., might with equal advantage be altogether abandoned ; 
hydrochloric acid is objectionable, as suggesting a relation to chloric acid. 
The names fluor-, chlor-, brom-, and iodhydric, as applied to the acids pre- 
sent in aqueous solutions of the hydrides, are especially appropriate as indi- 
cating that they are compounds containing the radical water—that they are 
hydrates : indeed, it would be well to restrict the use of hydric and hydro- 
to bodies of this kind, and to speak of hydrides as hydri-, not as hydro-, de- 
rivatives. It would then be possible to give comparatively simple names 
even to complex hydrates. 7 
2 Yhomsen gives the values in heat units as— 
HGCl,,2KClAq = 1380 . 
HgBr,,2KBrAqg = 1640 
HglI.,2K1Aq = 345° 
HgCy.,2KCyAq = 8830 
