COLON. 
nauseous taste. A pulp or mucilaginous extract, 
obtained by infusion of the fruit in boiling water, 
is the substance used in medicine; and this has 
a gelatinous consistence and a golden-yellow col- 
our, and appears to consist principally of mucus, 
resin, gallic acid, and a peculiar bitter principle 
which has obtained the name of Colocyntine. 
The pulp or extract is a very powerful drastic 
purgative; and is generally administered in com- 
bination with milder and modifying purgatives 
as a cathartic. It was formerly employed, but 
with great caution, in cases of dropsy, lethargy, 
and melancholy. Veterinary writers class it 
among substances which are poisonous to the 
horse. 
COLOMBO. See Catumsa. 
COLON. The first or uppermost of an ani- 
mal’s large intestines. It has its name, which 
signifies “a hollow,” either from its great capa- 
city, or from the circumstance of its being gen- 
erally found empty or filled only with gas on 
dissection. The colon of the horse has capacity 
for no less than about twelve gallons of.liquid or 
pulpy matter; and the colon of the cow, viewed 
jointly with the succeeding intestines, has the 
surprising length of about 33 feet. The upper 
end of the colon is supported by the common 
mesentery ; and the rest floats loosely or freely 
in convolutions. The matter of an animal’s sus- 
tenance, on arriving at the colon, ceases to yield 
any more aliment, and commences to be wholly 
feculent, thenceforth to hasten onward to expul- 
sion. 
COLOUR. A property of light, the knowledge 
of which can be gained from no description, but 
is acquired by means of the organ of sight. Col- 
| ouring substances, or paints, often improperly 
| termed colours, are made use of to impart a colour 
to other substances, either by application or ad- 
mixture. White and black are counted among 
colours in the latter sense, but not at all, or sel- 
dom, in the former, in which sense a white body 
is very properly called colourless, Black is merely 
the absence of all light. Colours, both alone and 
united, have different properties, and produce 
different effects upon the organs of sense, by 
means of their harmony or contrast, which are 
particularly important to painters, and are pro- 
perties arising from the nervous sensibility. Thus 
scarlet is a burning colour, injurious to the eyes; 
and it is probably on this account that beasts are 
so violently excited hy it. Yellow is the bright- 
est, red the warmest, deep brown and violet the 
softest among colours. 
The doctrine of colours, in a general sense, is 
the science of the origin, the mixture and effects 
of colour, as a property of light. How, for in- 
stance, is it, that light at one time is coloured, at 
another white? and by what laws are the ap- 
pearances of colours governed? The glass prism 
was the first contrivance that gave a satisfactory 
solution of these questions, and Sir Isaac Newton 
the first philosopher who explained and published 
COLOURS OF PLANTS. 
845 
the solution. Ifa ray of light is allowed to pass 
into a dark room through a small opening in a 
shutter, and is made also to pass through a smooth, 
three-sided glass prism, we find, Ist, that the ray 
of light, at its entrance into, and at its passage 
out of the glass, is turned from its direct course ; 
it is said to be refracted into a different direc- 
tion ; 2d, that the ray of light, which, falling di- 
rectly wpon a piece of paper before the prism, 
produces a round white spot, produces, when the 
paper is held behind the prism, a coloured figure, 
about five times as long as it is wide, and exhi- 
biting the colours of the rainbow, arranged in the 
same order as they are seen in that phenomenon. 
This figure or appearance is called the prismatic 
spectrum. The length of it is found to be ina di- 
rection perpendicular to the axis of the prism. 
It is red at the end which is nearest to the re- 
fracting angle of the prism, and violet at the end 
most remote from it, while orange, yellow, green, 
blue, and indigo, follow each other in the inter- 
vening space. Newton concluded from this, and 
a great variety of similar experiments, that these 
coloured rays are the simple rays of light, and | 
that white light is composed of the union of them | 
all, according to the relations which they exhibit | 
Every white ray of 
light, therefore, contains all the coloured rays | 
in the prismatic spectrum. 
united; but they are not recognised by us, since 
they produce upon the retina, where they are 
thus united, the impression we term whzte. These | 
coloured rays are reflected from all bodies accord- 
ing to similar laws, so that reflected white light 
is still white; but they are refrangible in differ- 
ent degrees; this property being least in the red 
rays, moderate in the green, and in the greatest 
degree in the violet; and they are, on this ac- 
count, separated from each other whenever they 
are refracted; since, from their different refran- 
gibility, although they are parallel, when they fall 
upon the refracting substance, they take different 
lines of direction in passing through it. They fol- 
low each other, in this respect, in the following 
order; first violet, then indigo, blue, green, yel- 
low, orange,and red. When these same coloured 
rays are rendered parallel again, and so fall upon 
the eye, they appear white, as at first. Most 
bodies possess the property of fixing or absorbing 
some of these coloured rays, which fall upon 
them, and thus only reflect or transmit rays of a 
particular colour; and upon this property, ac- 
cording to Newton, the colours of all bodies de- 
pend. Blue silk, for example, absorbs six col- 
oured rays, and reflects only the blue; and a s0- 
lution of cochineal transmits only the red, and 
absorbs all the other rays. All this is confirmed 
by the experiments with coloured disks revolving 
rapidly upon a rod, and with the coloured spec- 
trum falling upon coloured bodies. 
COLOURS OF PLANTS. We find in plants 
eight fundamental colours, which are called pure 
and unmixed colours—white, grey, black, blue, 
green, yellow, red,and brown. Lach of these ex- 
——_—. 
