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LECTURE XIX.

Chemical Composition of Plants. We have, according to our method of arrangement, considered the anatomy of the vegetable in connexion with its phy. siology: that is, when treating upon each particular organ, we have remarked upon its uses in the life and growth of the whole plant. We have treated of the germination of the seed, the minute vessels which constitute the vegetable fabric, with the fluids which circulate through these vessels ; we have, as it were, gathered these vessels together and considered them as constituting, in various ways, three essential parts of woody plants, the bark, wood and pith. We have inquired into the manner in which these separate parts are formed, and observed the great distinction in the growth of the stems of monocotyledonous and dicotyledonous plants

Yet, although we have seen how plants grow, it is no easy thing to explain how they live. The great principle which operates in organic life, appears not to have been laid open to the eye of man.

But by a careful observation of facts, we can learn all that it is important for us to know in order to cultivate plants successfully; their habits, food, and the causes of their diseases and death.

The physician who spends a long and laborious life in the study of the human frame, can give only the result of observations, made by himself or others. He finds a certain article efficacious in the relief of a particular disease ; but he knows not why this should be so; or if he is able to give some rea. sons, he is ultimately arrested in his speculations by a barrier, which he cannot pass. Thus he knows that soda or pearl-ash correct acidity in the stomach ; ask the reason of this, and he tells you that these are alkalies, substances which neutralize acids, and thus render them harmless ; inquire still farther, why alkalies do thus affect acids, and the physician is as igno. rant as yourselves.

Before closing our view of the vegetable structure, we shall, by the aid of chemistry, examine the elements which com

pose it.

The growth of vegetables, and the increase of their weight, show that they imbibe some external substances, which are in.

Retrospective view of subjects considered—Difficulty in investigating the life of plants-How far can a physician enlighten others ?– Examination of the elements which compose the vegetable structure.

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corporated into their own substance. This constitutes nutrition, and distinguishes living substances from dead matter. A stone does not receive nourishment, although it may increase by an external accumulation of matter. « Vegetable substances, analyzed by a chemical process, have been found to contain carbon, oxygen, hydrogen, and sometimes nitrogen, sul. phur, silex (a flint-like substance), the oxide of iron, soda, mag. nesia, and chalk.' These different substances are by the root, stems, and leaves of the plant, derived from the earth, air, and water.

Proximate Principles. Vegetation produces chemical combinations, which are distinguished by the name of proximate principles. Although the proximate principles of plants are very numerous, but few of them are well known; they are the result of the action of the vital forces of plants, and are, therefore, important subjects of investigation to those who pursue the study of physiological botany to any great extent. Carbon, oxygen, hydrogen, and nitrogen, are the constituent parts of the proximate principles of plants. These principles may be divided into two classes.

I. Those principles which are composed of carbon, hydrogen and oxygen, without any nitrogen.

II. Such as contain, besides the substances belonging to the other class, some nitrogen. There are few of this class.

The first class of proximate principles is divided into three orders.

1st. Principles which have more oxygen than sufficient to form water.

2d. Principles in which oxygen and hydrogen exist in the exact proportion to form water.

3d. Principles where hydrogen is in excess. The 1st order includes vegetable acids : as,

Acetic acid, or pure vinegar; this is generally produced by fermentation from wine, cider, and some other liquids; it is also found in a pure state in the campeachy wood, and the sap of the elm.

Malic acid may be extracted from green apples and the barberry.

Oxalic acid is found in several species of sorrel, belonging to the genera Oxalis and Rumex.

Tartaric acid is obtained from the tamarind and the cran. berry; this acid, combined with potash, forms what is commonly called cream of tartar.

Mirbel,“ “ Elemens de Botanique.”

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Proximate principles—Proximate principles divided into two classes-First class divided into three orders First order.

Citric acid is found in the lemon; it is mixed with the malic acid in the gooseberry, the cherry and the strawberry.

Quinic acid is obtained from the Peruvian bark (Cinchona).

Gallic acid is obtained from the oak, and the sumach ; it is highly astringent.

Benzoic acid is found in the LAURUS benzoin, and in the Va. nilla; this is highly aromatic; it is thought to give the agreea. ble odour to balms.

Prussic acid; this acid gives out a strong odour like bitter almonds; it is an active poison ; it is obtained from peach meats and blossoms, from bitter almonds, &c.

The 2d order includes gums, sugar, &c.

The Gums. Of these there are many kinds ; they have neither taste nor smell: dissolved in water, they form a muci. lage more or less thick. The principal gums are,

Gum Arabic, which flows from the plant Mimosa nilotica ;

Common Gums, such as issue from the peach tree, the cherry tree, and many others.

Sugar is a substance which dissolves in water, and has a sweet taste, it is obtained from the sugar cane, the sugar maple, from the stalks of indian corn, pumpkins, beets, and sweet apples. All vegetables which have a sweet taste may be made to yield sugar.

The 3d order includes oils, wax, resins, &c.

Oils. These are fluid and combustible substances, which do not unite with water. They are divided into Fixed and Volatile. The fixed oils are thick, and have little odour.

The oil of sweet almonds, and olive oil, grow thick and opaque by being exposed to the air.

The Oil of Flax Seed, called linseed oil, and some other oils, dry without losing their transparency ; it is this quality which renders linseed oil so valuable to painters.

The Volatile oils are distinguished from the fixed oils by their aromatic odours, and their tendency to fly off, from which circumstance the term volatile is derived. Some of these oils are those of the orange, lavender, rose, jasmine, peppermint, and wintergreen. These oils are sometimes greatly reduced by being mixed with alcohol, and are then called essences. The volatile oils may be found in a great variety of plants, particularly those of the Labiate family.

The Aroma, or aromatic property, consists chiefly of the odours which are exhaled from plants, containing volatile oil ; it is this oil which throws out the aromatic odour of the ginger plant, of the myrtle, rose, and other sweet scented plants.

Sooond order-Third order-What substances besides oils belong to the third order of the first class of proximate principles ?

Aromatic plants are much more common in hot, than cold countries ; most of our aromatic spices are found in the equatorial regions.

Wax is found on the surface of the fruit of the bay-berry (MYRICA cerifera). Beeswax is an animal production, made by the bees from pollen or farina of plants.

Camphor has much analogy with the volatile oils ; it is an extract from the LAURUS camphora, or camphor tree of Japan.

Resin exudes from the pine, and some other trees; it is dry, insoluble in water, but soluble in alcohol and very inflamma. ble.

The people in new countries often use, as a substitute for lamps, pine knots, which, abounding in resin, burn with a bright flame.

The difference between resin and the volatile oils, appears to consist in the action of oxygen upon the resin; for the oil in absorbing oxygen from the air, passes into the resinous state.

Resins mixed with volatile oils form balsams; thick, odor. ous, and inflammable substances of this kind are the balsam copaiva, dragon's blood, which, notwithstanding its terrific name, is but the simple extract of a plant, (DRACENA draco ;) the balsam of Tolu is the extract of the TOLUIFERA balsamum.

These resins are sometimes mixed with gums, they are then called gum-resins; of this kind are gamboge, assafoetida, guaia. cum, aloes, an extract from the Aloe perfoliata. These gumresins in flowing from vegetables are sometimes white and liquid like milk, but they usually become brown and hard by expo. sure to the air.

Indian rubber,* or as it is sometimes called, gum elastic, is the product of a South American tree, (called the SIPHONIA, elastica,) an East Indian plant, (the URCEOLA elastica,) and some other trees in the equatorial regions; by exposure to the air it hardens, becomes brown, and takes the appearance of leather ; it can neither be dissolved by water nor alcohol. The juice of the milk weed is said to be similar to that of the Siphonia elastica, and that of other plants from which the In. dian rubber is made.t

The green principle. It is to this principle that all the green parts, exposed to light, owe their colour; it undergoes changes in the different states of the plant, in autumn becoming brown or yellow. Davy attributes the change of colour to the form. ation of some acid; you know that a drop of sour wine, lemon

* Caoutchouc.

+ Mr. H. Eaton, assistant professor at the Rensselaer Institution, prepared a small quantity of the juice of the milk weed (Asclepias) in such a manner that it could not be distinguished from the imported Indian rubber, either in external appearance, or in its properties.

juice, or any other acid, will turn green to a brown or yellow colour.

The second class of proximate principles consists of substances which, like the first class, are formed of carbon, hy. drogen, and oxygen ; but to these is added nitrogen. In this class we find,

Opium, a narcotic principle, extracted from the poppy. It' is soluble in alcohol, slightly soluble in water.

Hematine ; this is the colouring principle, from the campeachy wood.

Indigo, a colouring substance, obtained from several species of Indigofera, or the indigo plant.

Gluten, is extracted from the cotyledons of the seeds of leguminous plants, as peas, beans; and from the albu nen of wheat, rye, &c. It is obtained by separating it from the starch ; flour owes much of its nourishing properties to gluten ; which in some respects is analogous to animal principles, and like them is subject to putrefaction.

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, between a liquid and a solid, suscepti. ble of a tremulous motion ; by long boiling, the juice loses the 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 ; you know that the effect of mixing any 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 also, upon the same principle, 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 colouring principle.* Some red flowers become blue; they are in this case supposed to have parted with some portion of the acid, which was united with their colouring principle.

* Iron is supposed to be combined with the oxygen of the acid.

What new element is found in the second class of proximate principles ? What substances are found in this class -Cause of the red colour of fruit Of the various hues of the petals of flowers.

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