122 
PHYSIOLOGICAL VIEWS 
TTie green 'principle. It is to this principle that all the green parts, 
exposed to Ught, owe their colour ; it undergoes changes in the dif- 
ferent states of the plant, in autumn becoming brown or yellow. 
Davy attributes the change of colour to the formation of an acid. 
Every one knows that a drop of sour wine, lemon juice, or any other 
acid, will change green to a brown, or yellowish colour. 
The second, class of proximate principles consists of substances 
which, like the first class, are formed of carbon, hydrogen, and 
oxygen ; but to these is added nitrogen. We here find ; 
Opium,, a narcotic principle, extracted from the poppy. It is solu- 
ble in alcohol, slightly in water. 
Hematine, the colouring principle from the Campeachy wood. 
Indigo, a colouring substance, obtained from several species of 
Indigofera, or indigo plant. 
Gluten, is extracted from the cotyledons of the seeds of legumi- 
nous plants, as peas, beans ; and from the albumen of wheat, rye, &c. 
It is obtained by separation from the starch. Flour owes much of 
its nourishing properties to gluten, which, in some respects, is analo- 
gous to animal principles, being, hke them, subject to puti^efaction. 
Jelly, is the thickened juice of succulent fruits; as currants, 
quinces, and apples ; it is soluble in hot water, though scarcely so 
in cold; when heated, it loses its jelly-like form, which is that of a 
coagulated mass, sasceptible of a tremulous motion ; by too long 
boiling, the juice loses this property, which gives to jelly its peculiar 
appearance. Many colouring principles have never been separated 
from the substances to which they are united ; as those of saffron, 
logwood, &c. 
It has already been suggested, that the red colour of fruits arises 
from the combination of an acid, with a blue colouring principle. 
Every beginner in chemistry knows that the effect of mixing an 
acid with an infusion of blue violets, or any vegetable blue, is to give 
a red tinge, varying in shade from a purple red to a brilliant scarlet, 
in proportion to the quantity of acid. It has, upon the same princi- 
ple, been supposed that the purple, red, and blue colouring of the 
petals of flowers, is owing to different proportions of acid ; this may 
explain the change of colour which appears in some flowers, which 
pass from blue to red, as the changeable hydrangea. This change 
may be attributed to increase of acid,* combining with the blue co- 
louring principle. Some red flowers become bine ; they are* in this 
case supposed to have parted with some portion of the acid, which 
was united with their colouring principle. 
Chemical composition of the Sap. 
The sap is a transparent, colourless fluid, imbibed by the vegeta- 
ble from the earth and air; or more properly, from the water exist- 
ing in them, which holds in solution oxygen, hydrogen, carbon, nitro- 
gen, earths, mineral-salts, and animal and vegetable matter. We 
might suppose, that being derived from the same source, the sap in 
all vegetables would be alike, but it is never obtained pure ; it is 
more or less mingled with the proximate principles, or proper juices, 
and thus differs in different species of vegetables ; water, however 
constitutes the principal part in all. 
Sap of the elm (Ulmus campestris) has by analysis been found to 
* Iron is supposed to be combined with the oxygen of the acid. 
What is said of the green principle 7— What new element is f9und in the second 
class of proximate principles 7 — What substances are found in this class 7— Cause of 
the red colour of fruit— Of the various hues of the petals of flowers— Sap of the elm. 
