394 J. P. Cooke on the Numerical Relation 
Here the solubility diminishes until we come to KCl, which is 
less soluble than KCy; then it increases to the last. Here, of 
course, the singular point is at KCl. With the corresponding 
compounds of sodium, the solubility diminishes to NaFl, which 
is the least soluble of the series, and then increases constantly to 
the end. 
These facts at least seem to show that apparent variations from 
the law of series in properties, which evidently are unknown fune- 
tions of several variables, should not be allowed to outweigh 
strong analogies, and certainly the analogies between Fluorine 
and the other haloids are very marked. Fluorine itself possesses 
properties such as we should expect to find in a member of the 
series above chlorine. The strong and active affinities of fluorine 
might be indeed predicted, after seeing the rapid increase both in 
the strength and activity of the affinities in passing from iodine 
to chlorine. In passing from bromine to chlorine, we pass from 
a liquid to a gas, permanent under any natural conditions ; and 
we should expect, therefore, in rising still higher in the series, t0 
find in fluorine a gas less easily reduced to a liquid than chlorine. 
Now although, on account of its remarkably active affinities, this 
fact cannot be demonstrated on the gas itself, it can, neverthe- 
less, be inferred with perfect certainty from its compounds. Fi- 
nally, the isomorphism of fluorine and the other haloids may be 
urged as indicating close analogy. From these considerations, I 
cannot but think that those chemists who have questioned the 
propriety of classing fluorine with the other haloids will, on Te- 
viewing the facts, and regarding the haloids in the light of a 
series, and not simply as a group of elements possessing certain 
general properties, be led to change their opinion. ; 
Cyanogen, though a compound radical, has been classed with 
the other haloids, not only from its atomic weight, but also from 
its other analogies. Its properties are in most cases those which 
we should expect from an element occupying its position in the 
series; but in others it presents remarkable variations, OWIDg 
probably to the fact that it contains a radical which is easily de 
composed. As is well known, it is perfectly isomorphous with 
chlorine. 
The propriety of classing oxygen in this series seems to be 
placed beyond doubt by the discovery of ozone, which, although 
it does not seem to possess such energy as we should expect 18 
an element higher in the series than fluorine, may, nevertheless, 
be found to fulfil all anticipations should it ever be obtained 1n 4 
perfectly unmixed condition. The isomorphism of oxygen Wl! 
chlorine, and therefore with the other haloids, seems sufficiently 
established by the determination both of Proust and Mitscherlich 
of the tetrahedral form of CuzCl. It must, however, be admitt 
that oxygen presents as strong analogies with sulphur as it does 
