ELISHA MITCHELL SCIENTIFIC SOCIETY 35 
trous and nitric, chlorous and chloric, etc. A possible dis- 
tinction might be made that the more negative the element 
the greater the number of changes of valence; the more pos- 
itive the element the less variation in valence is observed. 
This would be an approximation only. 
So great is the difference caused by this variation in the 
valence that some have even thought it best to atrange what 
have been called the lower and higher stages under different 
groups. ‘Thus Mendeléeff placed cuprous copper in Group I, 
and cupric copper in Group VIII; aurous gold in Group I and 
auric gold in Group VIII. Such an arrangement would, how- 
ever, greatly confuse the periodic system. Mercury,thallium, 
chromium. manganese, phosphorus, arsenic, sulphur, selen- 
ium, and others would have to be similarly provided for. It 
is better to retain them in the positions to which their atomic 
weights would assign them and to study them more thorough- 
ly, so that we may understand why certain elements, as cop- 
per, gold, and mercury show this peculiarity while others 
closely akin to them, as silver, zinc, and cadmium, do not. 
In studying the nature of valence from the standpoint of its 
variability, the means by which these variations can be 
brought about must have an important bearing upon the sub- 
ject. ‘There are a number of these agencies. 
Light.—It is a matter of common observation that light 
can bring about physical, and the most varied chemical, 
transformations. In some cases it is apparent that the trans- 
formation is one from a higher to a lower valence or vice ver- 
sa. ‘Thus, certain mercurous compounds can be changed to 
mercuric. 
Hg,O = HgO+Heg. 
An alcoholic solution of ferric chloride is changed by light to 
ferrous chloride. 
2FeCl,+C,H,O = 2FeCi,4+-C,H,O-+2HCI1. 
Ferric oxalate under the influence of light gives off carbon 
dioxide and becomes ferrous oxalate. 
Fe,(C,0,);=2Fe(C,O,)+2CO,. — 
An alcoholic solution of cupric chloride becomes cuprous 
