24 JOURNAL OF THE 
The terms ‘saturated’ and ‘unsaturated’ present a 
number of anomalies as commonly used. In the first 
place the term saturated is not always used for that con- 
dition of the atom in which it is united with the largest 
number of other atoms. ‘Thus, ferrous oxide (FeO) is 
called unsaturated, and ferric oxide (Fe,O,) saturated, 
though there is a larger proportion still of oxygen in fer- 
ric acid and the ferrates. The same is true of the three 
series of manganese and chromium compounds. 
Again the term saturated does not carry with it any defin- 
ite relation to the stability of the compound. Sometimes the 
compound called unsaturated, and containing the least num- 
ber of atoms is the most stable, sometimes that with the larg- 
est number. Phosphorus trichloride is more stable than the 
pentachloride, but the pentoxide is more stable than the tri- 
oxide. The most stable of the manganese compounds are the 
so-called unsaturated manganous salts; in the case of chrom- 
ium it would appear to be the chromic salts. In the case of 
carbon the saturated compounds are the most stable. It is 
manifest that these two terms cannot cover all cases of com- 
bination for a number of elements. It would seem wiser and 
simpler then to speak of the valence directly when discussing 
the elements, as bivalent carbon or quadrivalent carbon; bi- 
valent or trivalent iron, etc. In the case of carbon com- 
pounds the terms have acquired a somewhat different mean- 
ing and are too thoroughly incorporated in the literature for 
achange to be suggested. Saturated here means a com- 
pound which can take on no further atoms by addition, 
while an unsaturated compound can have such atoms add- 
ed. 
Certain cases of change of valence, as in cuprous and cup- 
ric compounds, mercurous and mercuric, ferrous and ferric, 
etc., have been looked upon as presenting some peculiar rela- 
tionships, Such cases are spoken of by some as if they oc- 
curred among positive elements only. It is not clear how 
any distinction can be drawn between these and the classes 
phosphorus and phosphoric, sulphurous and sulphuric, ni- 
