OPI 
be composed of tlie sulphates of lime and of 
potash, extractive matter, gluten, mucilage, 
resinous matter, and an oil, besides the nar- 
cotic matter, to which its peculiar proper- 
ties are owing. By digesting opium in W'a- 
ter, part of it is dissolved, and by evapo- 
rating the solution to the consistence of 
syrup, a gritty precipitate appears, which 
becomes more copious with the addition of 
water. This precipitate is composed of re- 
sinous and extractive matter, besides the 
peculiar narcotic matter which is crystal- 
lized. When alcohol is digested on this 
precipitate, the resinous and narcotic mat- 
ters are dissolved, and the extractive mat- 
ter remains behind. As the solution cools, 
the narcotic matter crystallizes; but the 
crystals are coloured with a portion of re- 
sin. By repeated solutions and crystalliza- 
tions it may be obtained tolerably pure. If 
alcohol be digested on the residuum, it be- 
comes of a deep red colour, the same crys- 
tals are deposited on cooling, and may be 
purified in die same way from the resinous 
matter with which they are contaminated, 
riie narcotic matter, when properly purified, 
is of a white colour ; crystallizes in right- 
angled prisms, with a rtiomboidal base ; and 
has neither taste nor smell. It is insoluble 
in cold water, and requires 400 parts of 
boiling water for its solution, from which it 
is precipitated by cooling. The solution 
does not redden the tincture of turnsole. 
It IS soluble in 24 parts of boiling alcohol, 
and requires about 100 parts when it is cold. 
When water is added to the solution in alco- 
hol, it is precipitated in the form of a wliite 
opaque matter. One of the most decided 
characters of this substance is its easy solu- 
bility in all the acids, and without the aid of 
heat. It is precipitated from these solu- 
tions by means of an alkali, in tlie form of 
white powder. Pure alkalies increase the 
power of its solubility in water, and the 
acids, when not added in excess, occasion a 
precipitate. When nitric acid is poured on 
the crystals reduced to a coarse powder, it 
communicates to them a red colour, and 
readily dissolves them. When the solution 
IS heated and evaporated, it yields crystals 
of oxalic acid in considerable quantity. 
The residuum has a very bitter taste. From 
the etfects of heat and of nitric acid on this 
substance it appears to be composed of 
oxygen, hydrogen, carbon, and azote. This 
narcotic substance is also found in the 
®'*^hy juice, and in the extracts which are 
obtained from several other plants, as from 
different species of lactuca, or lettuce ; hy- 
OPT 
oscyamus niger, or henbane. The leaves cC 
some plants also produce similar effects, as 
those of the deadly nightshade, fox-glove, 
and conium maculatum, or hemlock. See 
Poppy. 
OPOP ANAX. See Gum resin, 
OPOSSUM. See Didelphis. 
OPTICS, the science of vision, includ- 
ing Catoptrics and Dioptrics, and even Per- 
spective ; as also the whole doctrine of light 
and colours, and all the phenomena of visi- 
ble objects. See Perspective. 
Optics, in its more extensive acceptation, 
is a mixed mathematical science, which ex- 
plains tlie manner in which vision is per- 
formed in the eye ; treats of sight in gene- 
ral ; gives the reasons of the several modi- 
fications or alterations, which the rays of light 
undergo in the eye; and shows why objects 
appear sometimes greater, sometimes smalk 
er, sometimes more distinct, sometimes more 
confused, sometimes nearer, and sometimes 
more remote. In this extensive significa- 
tion it is considered by Sir Isaac Newton, 
in his Optics. Indeed optics make a con- 
siderable branch of natural philosophy; 
both as it explains the laws of nature, aV- 
cording to which vision is performed, and 
as it accounts for abundance of physical 
phenomena, otherwise inexplicable. 
The reflection of the rays of light is, in- 
deed, an occurrence too frequent and ob- 
vious to have escaped the notice even of 
the earliest observers; a river or some 
other piece of water was probably the first 
mirror ; its effect was afterward imitated by 
metallic mirrors : hence was discovered the 
equality of the angles of incidence and re- 
flection. It was known at an early period 
that an oar, or other straight piece of wood, 
partially immersed in water, no longer ap- 
peared straight, yet ages after this elapsed 
before any attempts were made to ascer- 
tain the relation between the angles of inci- 
dence and refraction. Empedocles was the 
first person on record that wrote systema- 
tically on light; and Euclid composed a 
treatise on the ancient optics and catoptrics ; 
dioptrics being less known to the ancients, 
though it was not quite unnoticed by them* 
for among the phenomena at the beginning 
of that work, Euclid remarks the effect of 
bringing an object into view, by refraction, 
in the bottom of a vessel, by pouring water 
into it, which could not be seen over the 
edge of the vessel before the water was 
poured in ; and other authors speak of the 
then known effects of glass globes, &c. 
both as buriuBg glasses, and as to bodies 
