164 
tion ; but it is well known that metallic oxides exhibk 
every variety of colour, with little or no regard to 
the quantity of oxygen they contain. Thus, many 
oxides, which contain but little oxygen, are white ; 
others, which contain a great deal, are black. 
Mercury, with different portions of oxygen, forms a 
black and red oxide, but these colours are not pecu- 
liar to such combinations ; for sulphur, in union with 
mercury, produces compounds, which exhibit simi- 
lar colours. Iron, when combined with 27 per cent. 
of oxygen, is black ; with 48 per cent, it is red 5 and 
these same combinations exhibit sometimes green and 
brown colours. So many, indeed, and so diversified 
are the colours which the oxides of iron are capable 
of assuming, that all the fine varieties of colours, 
employed in Wedgwood's pottery, are said to be 
produced by the oxides of this single metal *. Hence, 
then, it appears, that, although oxygen combines with 
metals in various proportions, and the compounds 
exhibit various colours, yet no uniform colour, even 
in the same metal, accompanies the varying propor- 
tion of oxygen. On the contrary, similar colours are 
produced by very different combinations of this ele- 
ment j and different substances, by uniting with me- 
tals, exhibit similar colours where no oxygen is con- 
cerned* Consequently, no particular colouring pro- 
perty belongs exclusively to oxygen, and, therefore, 
the colours of metallic oxides must be attributed to 
the properties of the compound, and not exclusively 
to those of the oxygen they may contain. 
* Bancroft on Perm. Colours, p, 1 7. 
