PROXIMATE PRINCIPLES. 
117 
though scarcely" so ia cold ; when heated, it loses its jelly-like form, which is that 
of a coagulated mass susceptible of a tremulous motion ; by too long boiling the 
juice loses this property, which gives to jelly its peculiar appearance. Many color- 
ing principles have never been separated from the substances to which they are 
■nnited ; as those of saffron, logwood, &c. It has already been suggested, that the 
"ed color of fruits arises from the combination of an acid (or of oxygen, the great 
icidifying principle) with a blue coloring 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 been supposed, upon 
the same principle, that the purple, red, and blue coloring of the petals of flowera 
is owing to different proportions of acid ; this may explain the change of color 
which appears in some flowers, which pass from blue to red, as the changeable hy- 
drangea. This change may be attributed to increase of acid,* combining with the 
blue coloring principle. Some red flowers become blue ; they are in this case sup- 
posed to have parted with some portion of the acid which was united with their 
coloring principle. White is most common in roots, sweet berries, and the petals 
of spring flowers ; black in roots and seeds ; yellow in anthers and the petals of 
compound flowers ; red in the petals of summer flowers and acid fruits ; blue 
and violet in the petals ; green in the leaves and calyx. 
142. Chemical Composition of the Sap. — The sap is a transparent, colorless fluid, 
imbibed by the vegetable from the earth and air ; or more properly, from the 
water existing in them, which holds in solution oxygen, hydrogen., carbon, nitrogen, 
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 prin- 
ciples, or proper juices, and thus differs in different species of vegetables ; water 
however, constitutes the principal part in all. Sap of the ebn (TJlmus campestris) 
has by analysis been found to contain water, volatile matter, acetate of potash, 
carbonate of lime, vegetable matter, sulphate of potash. Sap of the beech (Fagus 
sylvatica) contains water, acetate of lime, with excess of acid, acetate of potash, 
gallic acid, tannin, mucous extract, and acetate of alumine. Sap of the horse-chestnut 
^^scuLus hippocastanum.) contains water, extractive mucous matter, niter, acetate 
of potash, and carbonate of lime.f 
143. Though it seem wonderful that of so few elementary substances such a great 
variety should exist in the taste, smell, color, consistence, medicinal and nutritious 
ualities of vegetable combinations ; it is equally wonderful, that with the nine 
igits and the cipher, we may make such varied combinations of numbers ; or with 
our twenty-six letters of the alphabet, form every variety of composition. Thus, 
by various combinations of a feio simple principles, are formed all vegetable o,nd 
animal productions. The presence of nitrogen was formerly considered as a test 
of animal substance, and the want of it of a vegetable substance ; but it is now as- 
certained that animal substances may exist without nitrogen, and that this principle 
»5 contained in several vegetables. The elements of the compounds being the same, 
the question naturally arises, What causes the great diversity in the properties ? 
Two causes may be assigned for this, viz.: 1st, The different proportions in xohich 
the elements are combined; 2d, The various modes of their combination. In vin- 
egar and sugar, the one substance a liquid and of a sour taste, the other solid and 
sweet, are found the same elements in different proportions and differently coin- 
hined. In gum, starch, and sugar, the elements are the same, the proportion luarly 
the same, but they are combined differently. 
144. When we know by chemical analysis the combinations which exist in in<«- 
ganized bodies, we can often form similar substances by putting the same to 
* Iron is supposed to be combined with the oxygen of tre acid, 
t These results of .the analysis of sap are extracted from Vauquelin. 
Uause of the color of fruits, petals, &c.— 142. Sap considered chemically— Sap of the elm— Of the 
beech— Of the horse-chestnut.— 143. All vegetable and animal productions composed of a few simple 
principles — Illustration — What two causes assigned for the different properties of compounds formed 
from the same elements ?— 144. Organized bodies not produced by the skill of man. 
