OXI 
for whatever length of tima they are ex- 
posed to the atmosphere. This oxydation, 
and the quantity of oxygen absorbed, is 
proportional to the temperature. Tliere 
are, however, many metals which Uombine 
with a determinate proportion of oxygen at 
certain temperatures, and from this may be 
estimated the quantity of oxydation from 
the degree of heat which has been applied. 
The rapidity of the oxydation is almost al- 
ways increased by the elevation of tempe- 
rature. In this way actual combustion or 
inflammation is pr^uced. Thus filings of 
metals thrown upon a body in the state of 
ignition, give out brilliant sparks ; and 
steel, struck upon a flint, burns with a vivid 
flame in the aii-, in consequence of the 
great heat which is communicated to it by 
percussion. Metallic substances combing 
with very difterent proportions of oxygen ; 
and this quantity varies according to the 
manner in which the process has been con- 
ducted, or the temperature to which the 
metal has been exposed. 
In these different states and conditions 
of oxydation diflFerent phenomena are ex- 
hibited. Sometimes the metal becomes 
red-hot, and is inflamed ; sometimes the 
oxydation takes place without fusion, or 
does not combine with oxygen till alter it 
has been melted ; sometimes it is covered 
with a brittle crust, or with a substance in 
the form of powder. At oflier times a pel- 
licle, exhibibiting different colours, forms 
on the surface; but, in all cases, the metal 
is tarnished, loses its brilliancy and its co- 
lour, and assumes another, which announces 
the change that has thken place. Another 
difference which takes place among metals, 
is the different degrees of force with w'hich 
the oxygen adheres to the metal. The 
knowledge of this, and the different degrees 
of affinity between oxygen and metallic 
substances is of great importance in many 
operations and chemical results. During 
the fixation of oxygen in metallic sub- 
stances, it is absorbed by some in its solid 
state, and gives out a great deal of caloric. 
In others it is combined, without giving out 
the same quantify. This proportion of ca- 
loric given out corresponds to the facilily 
with wdiich oxides part with their oxygen, 
or are reduced to the metallic state. Those 
which have combined with oxygen, with 
the greater proportion of caloric, are most 
easily reduced ; hut those, on the contrary, 
in which the oxygen has been deprived of 
its caloric, are reduced to the metallic 
state by a great addition of caloric, and the 
0X0 
greatest number of oxides require the advfi- 
tion of substances whose affinity for oxygen 
is greater than that of the metal. Jletallic 
oxides are extremely difierent in different 
metals, and even in the same metal, accord- 
ing to the proportion of oxygen. They 
are, however, possessed of some common 
properties. They are all in the form of 
powder or earthy substance, or so brittle 
as to be easily reduced to this state. They 
exhibit every shade of. colour from pure 
white to brown and deep red, and they are 
heavier than the metals from which they 
have been obtained. Some oxides are re». 
vived, as it is called, or are reduced to the 
metallic state, merely by being in contact 
' with light or calorie. Some require the ad- 
dition of a combustible substance and a 
high temperature ; while others have so 
strong an affinity for oxygen, that they can- 
not be depriyed of it by the strongest heat, 
hut become fusible in the fire, and afford a 
glassy matter more or less coloured, and 
even serve as a flux to flie earths. Some 
oxides are volatile, but the greatest number 
are fixed. Some have an acrid and caustic 
taste, are more or less soluble in water, and 
even possess an acid quality ; others are in- 
soluble and insipid. 
OXOPHYLLUM, in botany, a genus 
of the Monadelphia Pentandria class and 
order. Natural order of Trihilat®. Me- 
liae, Jussieu. Essential character; one- 
styled ; calyx five-toothed ; petals five, long ; 
filaments sheathing the style, five-toothed 
at top; teeth antheriferoiis ; stigma one; 
capsule five-celled. There is but one spe- 
cies, viz. O. foetiduni ; this is a shrub about 
fen feet in height, and nearly six inches in 
diameter; the bark is green and smooth, 
the wood white, tender, and fragile ; the 
branches twiggy, garnished with alternate 
leaves, each leaf digitated, having three 
large lobes growing on a foot-stalk of five 
or six inches in length ; each lobe is divided 
by a longitudinal nerve, which is prominent 
beneath; the flowers spring from the bo- 
soms of the leaves, at the extremity of the 
twigs and branches ; their common foot- 
stalk is about a foot in height, dividing at 
its summit into several smaller ones, on 
each of which are placed alternate sessile 
flowers ; the corolla is white, eaich petal 
being an inch long, and, as it were, glued 
to each other longitudinally by their bor- 
ders, so as to form a kind of tubular tigure, 
the upper part spreading ; these petals co- 
ver a white membranaceous tube, wliich on 
its upper part divides into five short fila- 
