ELISHA MITCHELL SCIENTIFIC SOCIETY 23 
cation of the atomic theory is most highly important as a step 
towards the clearing up of the problems springing from the 
conception of valence and from the periodic system. 
NVote.—Since certain points in this paper require treatment 
at greater length than was practicable in an address, it will 
be followed by a second paper elaborating such portions. 
THE NATURE OF VALENCE. 
[SECOND PAPER | 
As the preceding paper upon this subject was in the form of 
an address before one of the local sections of the society, the 
hypothesis as to the cause of valence there suggested was 
given in outline only and could not be enlarged upon as far as 
may have been necessary. In the present paper it is proposed 
to elaborate certain points and to test, as far as possible, the 
reasonable nature of the hypothesis. 
While the whole subject of valence has been much catPuseny 
and the use of some of the terms connected with it unfortun- 
ate, no part of it has given greater trouble than its variabil- 
ity. This is the very point, however, which affords the best 
clew to its solution and should therefore be treated at some 
length. 
The most instructive cases of varying valence are those 
where the variation is shown towards the same element, 
as in the compounds PCl, and PCI,, FeCl, and FeCl,, 
Hg,O and HgO, CO and CO,, and many other similar 
compounds. There are two possible views regarding 
these. Hither the valence varies or the valence remains 
the same aud the differences are explained by some such 
assumption as that of a state of saturation of the atom 
and of various unsaturated states. 
