Species of hard Carbonate of Lime, &c. 
335 
CUBIC OXIDE OF IRON. 
Amongst the various examples which mineralogy, when stu- 
died with attention, constantly offers to us, of several different 
species being included in the combination of the same modifying 
substance with the same base, iron may be considered as afford- 
ing one of the most striking. This metal, in its combinations 
with oxygen, varies considerably ; insomuch that it presents us 
with several different species, according as the proportion of 
oxygen in the combination is increased. Thus, by the first 
degree of oxidation in which this metal offers itself to our notice, 
is formed the very attractable oxide of iron, which crystallizes 
in regular octaedrons. By the second degree of oxidation, in 
which there is a greater proportion of oxygen, a different oxide 
is produced, which is much less attractable than the former, and 
crystallizes in the form of a slightly acute rhomboid. Lastly, in 
the third degree of oxidation, in consequence of a still greater 
proportion of oxygen, the attractable property no longer exists ; 
the power of crystallizing entirely ceases ; and we have the ores 
called haematites, and the other earthy oxides, for instance, the 
brown, the red, the yellow, and the black, (the^ appearance of 
which latter very much resembles that of a bitumen,) between 
which there certainly exists some real chemical difference, that 
will probably hereafter be discovered. 
The distinction of species here spoken of will perhaps appear 
extraordinary, to those persons who are accustomed to consider 
the combination of oxygen with iron as forming of itself a 
species in the genus, (which genus is determined by the nature 
of the metal, namely, iron,) because it may appear to them like 
dividing into various species, that which merely constitutes a 
single one. But I must observe, that in mineralogy it is not 
X x 2 
