If we inquire the composition of the mineral constituents of plants, chemical analysis replies, that they consist principally of potash, soda, magnesia, oxide of iron, silica, phosphoric acid, sulphuric acid, muriatic acid (chlorine), and carbonic acid. Of these substances the chemist places the first five amongst the bases or oxides, the last five amongst the acids, and understands by the former those bodies which, if soluble, have an alkaline taste, like, for example, wood-ashes or burnt lime, and by the latter, those which in a state of solution have a sour taste. Potash and soda are also called alkalies; lime and magnesia, alkaline earths. If a base unites chemically with an acid, the peculiar properties of both disappear, and the compound, which behaves as a wholly different body with entirely new properties, is no longer alkaline or acid to the taste, but saline; in this condition it is called a salt. Thus we obtain from the caustic potash and the corrosive aquafortis (nitric acid) a mild salt, the well-known saltpetre (nitrate of potash); and from the caustic soda and the corrosive oil of vitriol (sulphuric acid), an innocuous salt, the well-known Glauber salts (sulphate of soda). Wherever bases and acids come into connexion with each other, they combine to form salts; this is the case in the mineral constituents of plants, and hence in vegetable ashes we meet with the above-named bodies, not in a free state, but combined into salts.

The chief of these combinations are:

a. Soluble in water: the alkaline salts (salts of potash and soda).

b. Soluble in diluted muriatic (hydrochloric) acid: the earthy compounds (salts of lime and of magnesia, together with salts of oxide of iron).

c. Insoluble in water and acids: the silicates.

Whether one or the other class of these salts predominates in a plant, may be ascertained, although only approximatively, by treating its ashes first with water, and then with diluted muriatic acid. 50 ounces of the ashes of potatoes contained 40 ounces of alkaline salts; on the other hand, 50 ounces of the ashes of the haulm, only 2 or 24 ounces; 50 ounces of ashes made from the young leaves of trees contained 25 ounces of alkaline salts, but when made from the old mature leaves, only 7 to 10 ounces, &c.

2. WHENCE DO PLANTS OBTAIN THEIR CONSTITUENTS?

This inquiry follows directly from the first, and leads us to consider the food of plants; for we must regard every substance which supplies the plant with one or more of the elements necessary to the building up its body, as a means of nutriment thereto. Plants can absorb their food only through the pores-so fine as to be altogether invisible to the naked eye of their root-fibres and leaves, hence everything which can usefully contribute to their nourishment must be either liquid or aëriform, since solid bodies cannot penetrate into their structure. The results which have been obtained as yet by investigation into the sources of supply of these nutrients, furnish the following answers to the above inquiry.

a. Plants obtain their oxygen and hydrogen from water, without which, indeed, it is, generally speaking, wholly impossible that they can live and thrive. In addition to this, water is indispensable to vegetation, from the fact that it supplies a medium for dissolving all those nutritive substances which cannot of themselves become fluid or aëriform, and because, moreover, its fluid constitution is the means of the formation of the solid vegetable structures; for it is from the juice made liquid by the water, that all the solid constituents of plants are produced.

b. Plants absorb carbon in the form of carbonic acid, which is a constant ingredient in our atmospheric air and springwater, and is formed in every soil that contains humus. Carbonic acid is a kind of air which is unceasingly produced in extraordinary quantities by the three chemical processes most universally diffused in nature; we mean, the respiration of men and animals, the combustion of wood, coal, &c., and the putrefaction or decay of animal and vegetable matter. It is, moreover, evolved in fermentation, and causes the effervescence and "rising" of the fermenting mass, as likewise the sparkle of beverages not thoroughly and completely fermented, such as bottled beer, champagne, &c. Lastly, it streams forth from crevices in many places where volcanic forces are active, or, as we may conjecture, were active in former times.

All the carbonic acid generated in these very different ways is taken up into the atmosphere. If it should continue there, the air must of necessity become gradually deteriorated and unfit for respiration; more especially as in all the processes of breathing, combustion, and decay, free oxygen or vital air is removed from it. But this is not the case. The oxygen does not decrease, the carbonic acid does not increase. The vegetable world discharges the function, not only of a supporter, but also of a protector of animal life. It not only provides the whole animal kingdom with nourishment, but also restores again to the air the oxygen abstracted by the former. For plants absorb carbonic acid by their roots and leaves as their most important article of food, and again exhale its oxygen by their green or herbaceous parts during the daylight. On the other hand, they firmly retain the carbon of the carbonic acid, and appropriate it to the construction of their leaves, blossoms, seeds, and the proximate constituents contained in these*.

*The following general considerations on the sources and consumption of carbon may prove interesting here. It has been calculated that the quantity of carbonic acid produced annually upon the globe, by combustion, respiration, and decomposition of various kinds (excluding therefore the products of volcanos), amounts at least to such a quantity, that in 100 years the quantity contained in the atmosphere would be increased one quarter, and that in 5000 years the atmosphere would become irrespirable by animals and man, unless some process were provided by nature to remove the carbonic acid.

All the carbon consumed in the above processes is derived from the vegetable kingdom, so that it is inconceivable that the store of organic substance upon the surface of the earth should long suffice to supply the material for the growth of the vegetable kingdom. The respiration of man and animals probably consumes annually as much carbon as is furnished by a rich harvest from a surface twice the size of all France. It is evident that vegetables derive a large proportion of their carbonic acid from the atmosphere, and still more, there is no doubt that in their ordinary process of growth they fix carbon in a solid form more quickly than they give it off in processes of decay, &c. Whatever might have been the mode of origin of the vegetation now clothing the earth's surface, no one who has considered the bearings of the question, will imagine that the mountains and plains were originally covered with a layer of humus or mould, in which the first vegetation was to flourish. There is no doubt that the extension of vegetation was gradual; that, as we see at the present day in the islands of the Pacific, or on tracts laid bare of vegetable mould by extensive engineering operations, &c., the ground first became occupied by a sparing growth of those hardy kinds of plants

Carbonic acid is generated in the soil wherever plants are produced. Fallen leaves, roots remaining in the ground, and the worms and insects which feed thereon, all become, as soon as life has left them, the prey of corruption and decay, and by this means the carbon they contain is converted into carbonic acid. We call such decaying organic matters humus, when, as very speedily happens, it has assumed a dark colour. Humus, when air and moisture can act upon it, is slowly but unceasingly decomposed still further, and therefore continually furnishes fresh supplies of carbonic acid to the roots of plants as nutriment. At the same time, also, the nitrogenous and mineral substances which it contains, become soluble and capable of being received as food by plants, and can thus in like manner be appropriated to their nourishment. The farmer is, therefore, quite correct in attributing to humus an especially beneficial influence upon the growth of plants, and consequently in labouring with all his energies to render his land rich in humus, especially as at the same time he makes the soil at once looser, warmer, and better suited to the absorption and diffusion of moisture, as well as stronger in the power of attracting the nutritive materials existing in the air: as will be more fully explained in the chapter upon Soil. The farmer, however, must not suppose that this enrichment of the land in humus can be achieved which draw little of their support from the soil, and that these, satisfied with a mere foot-hold, flourished and decayed, generation after generation, until a soil was formed for the vegetation of plants of a more luxurious and requiring nature.

The vast masses of carbon which we now consume in the form of coal, were produced by an enormous development of vegetation in an early period of the earth's existence, and it is very probable that the plants of which this vegetation was composed, were in great part of a character analogous to those which extend themselves so rapidly and so quickly increase the existing mass of humus in our bogs or peat-moors. The presence of moisture seems to be everywhere the only necessary condition of vegetation, and since in proportion as heat and moisture combined increase, the luxuriance of vegetation increases, and at the same time the quantity of vegetable mould or humus, resulting from the decay of plants, and their imperfect return into their elements, water and carbonic acid.

The numerous cases in which vegetation flourishes luxuriantly without the soil supplying carbon, prove that the source of this element must be the carbonic acid of the atmosphere; where, produced by the decay of humus present in the soil, it will of course act like any other carbonic acid, and greatly promote the luxuriance of vegetation.-ED.

only by directly introducing into the ground large quantities of such substances (for example, straw-manure) as have especially the power to produce much humus. This end can also be indirectly attained, and frequently with far greater pecuniary advantage, by a judicious rotation of crops, and likewise especially by the application of very powerful manures (guano, bone-dust, &c.), although these, in themselves, furnish but little humus. When by these a more vigorous growth of plants is produced, the roots and leaves become larger, and the soil acquires, therefore, by the fall of the latter and the decay of the former, more material for the formation of humus, than by less vigorous manuring and a poorer vegetation; perhaps more than it would have acquired by manuring with stable-dung.

c. Plants receive nitrogen chiefly through the ammonia which is produced in the putrefaction and decay of vegetable, and, more particularly, of animal substances. Plants are always surrounded by air, and the air consists mainly of nitrogen. Hence it might be concluded that they could never suffer from deficiency of this element for the development of their structure, inasmuch as they have the opportunity of absorbing it in any quantity from the atmosphere. And yet they are without it in many, perhaps the greater number of fields; and consequently these do not produce so many or such vigorous plants as they in general might, which will be more particularly explained in the following chapter. From the circumstance, therefore, that plants do not take up the nitrogen of the air as nourishment, we must infer that they cannot, that pure nitrogen is not digestible and suitable food for them. And this is the fact. The chemist undertakes to explain this indigestibility of nitrogen from its natural constitution. One of the distinctive characters of nitrogen is its disinclination to combine with other bodies; if this is to be accomplished, it must be brought about by compulsion, for which the chemist has frequently to employ very circuitous methods. This unwillingness to give up its natural freedom seems so strong, that the plant does not possess sufficient power to overcome it*.

*This statement is called in question by some recent observers. The researches of M. Ville, which appear to have been made with great care