;

fruits and roots taste sweet, they contain sugar; the branches and leaves of the grape-vine have a sour taste, they contain an acid salt; those of wormwood have a bitter taste; they contain a peculiar bitter principle. The latter possess also a powerful odour, which proceeds from a volatile oil. In the seeds of our various kinds of grain, and in the tubers of the potato, we find a substance of a mealy nature, starch in the seeds of the rape and flax plants, a greasy juice, fixed oil. From cherry and plum trees there exudes a viscous juice, soluble in water; from fir and pine trees a similar product, but insoluble in water; we call the former gum, mon air) explodes violently, the hydrogen being burnt by the help of the oxygen, and the result is water, that is hydrogen chemically combined with oxygen.

To find the ultimate elements of plants, we must decompose them chemically, so as to obtain the simple substances either free or in combination with other substances, added to them, which have the power of combining with them in known proportions. The elementary analysis may of course be applied either to the proximate constituents separately, or to the whole plant, or part of a plant at once, and this latter is the method usually adopted, as the proportion of the elements in the whole affords data from which we can calculate pretty nearly the proportions of the proximate constituents, or at all events the classes of these constituents, posessing similar practical value.

Thus, vegetable substances are combustible and leave their mineral constituents behind as ash. The ash thus left is chemically analysed for the fixed elements (salts of the metals, &c.). Another portion is dried, and the loss of water noted (the quantity of volatile ammonia is also estimated by the aid of heat), then a portion of the dried substance is mixed with a mineral compound (such as oxide of copper), which will give up its oxygen at a high temperature. The mixture is then violently heated in a glass tube, and the oxygen set free from the oxide of copper and from the substance itself, burns the hydrogen and carbon, while the nitrogen also passes off as a gas. These products are carried off by a tube through weighed quantities of chloride of calcium and caustic potash; the former absorbs the water produced, and the latter the carbonic acid, so that the increased weight of the former will indicate how much hydrogen was driven off, of the latter how much carbon. The quantity of nitrogen is estimated by a separate burning, in which the nitrogen is made to pass off alone and measured, or is driven off in combination with hydrogen as ammonia, and the quantity calculated from this. Then, knowing the amount of these three elements, by subtracting their sum from the amount of the volatile ingredients (ascertained by burning the dried substance and subtracting the weight of the ash), the remainder will give the quantity of oxygen existing in the form of organic com pounds.-A. H.

the latter resin. That which gives mechanical support to plants, forming as it were their bones and blood-vessels, bears the name of vegetable fibre, or, when it has become tough, insoluble, or indigestible, of woody fibre. In the sap of plants we meet with a substance which coagu lates by boiling, like the white of an egg or the albumen of the blood; in peas and other leguminous fruits, a substance which is extremely like cow-milk cheese; in the seed of rye, wheat, oats, and other kinds of grain, a substance of identical composition with that of the flesh of animals; the first is called vegetable albumen, the second vegetable caseine, and the third gluten. Finally, if we burn the plant, an earthy or saline powder is left behind, which neither burns up nor vola tilizes by heat, consisting of the mineral constituents of plants. By separating the various proximate constituents of vegetables still further, we come to their more remote constituents and elementary substances. If man is seized with amazement and admiration, when he thoughtfully and feelingly contemplates the infinite diversity and variety he encounters in the wonders of the vegetable world, he must be filled with equal amazement and admiration at the simplicity of the means employed by Divine Omnipotence in producing this multiformity. Four elements. only, chiefly constitute the essential foundations upon which plants, nay, still more, all creatures living upon the earth, build up the structure of their bodies. These are called oxygen, hydrogen, carbon, and nitrogen. Taken collectively, they are called the organic elements, because they must be regarded as the principal ingredients of all organic substances (the substances of plants and vegetables). They are also denominated combustible elements, because upon heating in the air they entirely burn away and disappear, that is, they are converted into gaseous compounds; or, again, called putrescible elements, because they are capable of decay, putrefaction, or mouldering away (by which process they are equally converted, although more slowly than by combustion, into gaseous compounds); finally, atmospheric elements, because they are contained in atmospheric air.

a. Oxygen, in a free condition, is an invisible kind of air or gas, without taste or odour. We find it thus in our atmospheric air, of which it constitutes one-fifth. Every one

knows that men and animals cannot live without air, that fire cannot burn, and that vegetable and animal substances cannot putrefy without air. The property we here ascribe to the air, of supporting life, combustion, and putrefaction, is due to the oxygen it contains, which alone imparts to the air the power of maintaining those chemical processes. When oxygen unites chemically with hydrogen, it produces a fluid, common water; when it unites with metals and other mineral substances, it becomes solid, and in this state forms a principal ingredient in all stones and earths.

b. Hydrogen, in like manner, when uncombined, is a kind of air or gas, without colour, taste or odour, and so light as to be used for filling air-balloons. We find it most extensively diffused in nature in a solid or fluid form, in water, snow, and ice, since, as already mentioned, it forms the second constituent of water.

c. Carbon, in a separate condition, is a solid body, of a black or gray colour, as is seen in charcoal, soot, black-lead, coke, and other substances, of which it is the principal constituent. It may also assume, however, the form of a colourless, lustrous, and transparent stone; for the most precious of our stones, the diamoud, has been demonstrated by chemical examination to be pure carbon. In like manner it changes its colour and form when it unites with oxygen, hydrogen, or nitrogen; for wood, sugar, starch, &c., are not black, and yet half of each of these substances consists of carbon, as is readily shown by subjecting them to heat, which drives off in the form of vapour their oxygen and hydrogen, leaving a carbonaceous mass behind. Upon continuing the application of heat with free access of air, not only the colour, but the solid form is altered; for then the carbon combines with the oxygen of the atmosphere to form a kind of gas, which has received the name of carbonic acid gas, and is colourless like ordinary air. The same thing happens in the putrefaction and decay of animal and vegetable matter.

d. Nitrogen constitutes the great bulk, namely four-fifths, of our ordinary atmosphere, and, as is evident from the fact just mentioned, is, when uncombined, an aërial fluid or gas, and invisible. Except in the air, it is but scantily diffused in nature. In the mineral kingdom it is entirely wanting, and in soils we meet only with it when these contain

In the

decayed or putrid vegetable and animal matter. organie kingdom, we find it in far larger quantity in the bodies of animals than in those of plants. Of the vegetable organs, the seeds are particularly rich in nitrogen. It unites with oxygen to form an acid, which has received the name of nitric acid, and forms with basic or alkaline bodies (for example, potash, soda, lime, &c.) what are called nitres (nitrates), which are more especially produced in decaying vegetable or animal substances. When united with hydrogen it forms a kind of air or gas, which is called ammonia, and possesses a very pungent odour; this gas is always evolved in the putrefaction of animal and vegetable substances.

The proximate constituents of plants may be grouped, according to the elementary substances of which they are composed, into two principal divisions, namely those vegetable substances which are composed of but three elements, -carbon, hydrogen, and oxygen; and those which consist of four elements,-carbon, hydrogen, oxygen, and nitrogen. This classification possesses not merely a theoretical, but in an eminent degree also a practical interest, since very important conclusions are deducible therefrom, in relation to the value of plants as food, and the manuring value of the refuse matter derived from them. The distinction upon which it rests will be seen at once to depend upon the presence or the absence of nitrogen; hence the first group may be called the anitrogenous vegetable substances, or such as. are destitute of nitrogen; and the second the nitrogenous vegetable substances. The latter always contain, besides nitrogen, small quantities of phosphorus and sulphur.

Of the proximate constituents of plants already known, the following may be classed as under:

e. In all plants, however, there exist certain inorganic constituents, partly dissolved in the sap, and partly deposited in the

cells. These are called the mineral constituents of vegetables, because they are derived from the mineral substances contained in the soil; or the incombustible, because they are not burnt away or volatilized by heat; or ash constituents, because upon the combustion of the plant they remain behind as ashes. They are distinguished from the organic constituents by being incapable of passing into putrefaction or decay.

It was formerly supposed that these constituents were unimportant to plants, and that their presence was accidental, according as the water met with them in the soil, and dissolved and introduced them into the plant. That this assumption is nevertheless entirely erroneous may be inferred at once from the fact, that we can powerfully promote and hasten the growth of vegetables by a sprinkling with gypsum, lime, ashes, salt, or other mineral ingredients; since anything which furthers and accelerates their growth cannot be unimportant to plants. It is now positively known that vegetables always require certain mineral substances, in addition to the organic, for their nourishment and. complete development, and that they are retarded in their growth when they do not obtain a sufficient supply of them. Accordingly, we must regard the ashy constituents in like manner as necessary elements of plants.

How greatly these inorganic constituents may differ in quantity and quality, not only in different plants, but even in the different parts of one and the same plant, and at different seasons of the year, may be learnt from the following table of the ingredients of certain vegetable ashes.