PROXIMATE PRINCIPLES. 117 



though scarcely so in 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 

 uniteil ; as those of saffron, logwood, &c. It has already been suggested, that the 

 red color of fruits arises from the combination of an acid (or of oxygen, the great 

 acidifying 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 flowers 

 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 hi 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 ahke, but 

 it is never obtained pure ; it is more or less mingled with the proximate priri- 

 ciples, or proper juices, and thus differs in different species of vegetables ; water, 

 however, constitutes the principal part in all. Sap of the elm (IJlmus 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 w%ater, 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 hippocastanuni) 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 

 qualities of vegetable combinations ; it is equally wonderful, that with the nine 

 digits 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 few simple principles, are formed all vegetable and 

 animal productioyis. 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 

 is 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 which 

 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, tlie other solid and 

 sweet, are found the same elements in different proportions and differently com- 

 bined. In gum, starch, and sugar, the elements are the same, the proportion nearly 

 the same, but tJiey are combined differently. 



144. When we know by chemical analysis the combinations which exist in inoi- 

 ganized bodies, we can often form similar substances by putting the same to 



• Iron is supposed to be combined with the oxygen of the acid, 

 t These results of the analysis of sap are extracted from Vauquelin. 



Uanse of the color of fruits, petals, &c.— 142. Sap considered chemically— Sap of the elm— Of the 

 br-efh— Ot the horse-chestnut.— 143. All vegetable :ind animal productions composed of a few simple 

 pririiM;)lcs- Illustraiion — What two causes assigned for the diflerent ])roperiies of compounds formed 

 from the same elements ?— 144. Organized bodies not produced by the skill of man. 



