ple and the birch tree; this, being obtained remote from any house, was evidently derived from the atmosphere.

There are facts here connected with cultivation, and showing the effect of different manures upon the quality of the products, which are extremely curious. Different wheats are found to contain very different proportions of gluten, of which nitrogen forms an important constituent. Some French wheat was found to contain 12.5 per cent. of gluten, while Bavarian contained 24 per cent. Davy obtained 19 per cent. from winter, and 24 from summer, wheat. Sicilian wheat afforded 21 per cent.; Barbary wheat, 29; Alsace, 17.3; wheat grown in the Jardin des Plantes 26.7, and winter wheat 3.33 per cent. In regard to these differences, Liebig remarks;

"An increase of animal manure gives rise not only to an increase in the number of seeds, but also to a most remarkable difference in the proportion of gluten, which they contain. Animal manure acts only by the formation of ammonia. One hundred parts of wheat, grown on a soil manured with cow dung (a manure containing the smallest quantity of nitrogen), afforded only 11.95 parts of gluten, and 64.34 parts of amylin or starch; whilst the same quantity, grown on a soil manured with human urine, yielded the maximum of gluten, namely, 35.1 per cent. Putrified urine contains nitrogen in the forms of carbonate, phosphate, and lactate of ammonia; and in no other form than that of ammoniacal salts." p. 136.

As illustrative of the value of aminonia in vegetation, Liebig refers to guano. This is the excrement of sea-birds, and found in large quantities on several islands in the South Sea. The effect of this manure is understood to be most powerful. It renders the soils, which consist of clay and sand, and contain, as is represented, no organic matter, highly fertile. This manure is composed principally of salts of ammonia, and a few earthy salts.

Liebig, if his theory be well founded, has solved the secrets of the operation of gypsum. It has been supposed, that gypsum acted upon plants as a stimulus, or like intoxicating liquids upon animals. But plants are not animals. They have no nerves, which may be tightly drawn or relaxed; and such suppositions, which serve only to betray our ignorance, are without foundation. No substance can cause the leaves of plants to appropriate an excess of car

bon from the atmosphere, when the other constituents of the plants are wanting. The influence of gypsum is to fix the ammonia which is brought into the soil, and preventing its evaporation, give it out as the plants may receive it. This effect is produced by the double decomposition of the carbonate of ammonia, and of the gypsum or sulphate of lime, by which sulphate of ammonia and carbonate of lime are formed. His notions on this subject, being the first satisfactory attempt at a solution of the mystery always connected with the application of this extraordinary substance, are curious and interesting.

"In order," he says, "to form a conception of the effect of gypsum, it may be sufficient to remark, that 100 lbs. Hessian of burned gypsum fixes as much ammonia in the soil, as 6,250 lbs. of horses' urine would yield to it, even on the supposition, that all the nitrogen of the urea and hippuric acid were absorbed by the plants without the smallest loss, in the form of carbonate of ammonia."— p. 143.

He is equally original in his explanation of several other facts.

"The advantage of manuring fields with burned clay and the fertility of ferruginous soils, which have been considered as facts so incomprehensible, may be explained in an equally simple manner. The oxides of iron and alumina are distinguished from all other metallic oxides by their power of forming solid compounds with ammonia. Minerals containing alumina or oxide of iron also possess, in an eminent degree, the remarkable property of attracting ammonia from the atmosphere and of retaining it."— p. 144.

Powdered charcoal is another element, which powerfully absorbs ammonia; and will take up ninety times its volume of ammoniacal gas, which it gives out upon being wet with water. Decayed wood resembles charcoal in this property, absorbing seventy-two times its own volume. This explains further the operation of humus, which supplies not only carbonic acid, but likewise nitrogen, to the growing plants.

A beautiful reflection with which Liebig concludes this chapter, we cannot forbear quoting.

"Carbonic acid, water, and ammonia contain the elements necessary for the support of animals and vegetables. The same substances are the ultimate products of the chemical

process of decay and putrefaction. All the innumerable products of vitality resume, after death, the original form from which they sprang. And thus death, the complete dissolution of an existing generation, -—becomes the source of life for a new one.” p. 147.

The next subject of discussion with Liebig, relates to the inorganic constituents of plants. These are potash, soda, lime, magnesia, oxide of iron, manganese, silica, and other substances. The plants cannot be perfected without them. Alkalies of one kind may often be substituted for those of another; but they are always found in equivalent proportions. These inorganic substances are admitted to the plants in combination with some acid. They exist independently of the plant, and are not the product of vital action. They are found in different soils, and are the result, of the decomposition of various rocks. Potash is an important constituent of most felspars. Some of the salts are evaporated in sea water, and in that way carried far into the interior, and after being spread upon the earth, are carried down by the rains. They are returned to the soil in decayed vegetable and animal matter, and in the excrements of animals. They are found in the ashes of plants in the form of carbonates; and by careful analysis their amounts in different cases have been accurately ascertained. The amount of alkaline substances required by plants is very minute. But that amount is requisite to the perfection of the vegetation. It is easy to conceive how small an amount is required in the soil, when it is understood that sea water contains only of its weight of carbonate of lime, and yet that is sufficient for the formation of all the banks of coral in the ocean, and the various shells of the marine animals.

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Having given this account of the constituents of plants, and the sources whence they are derived, Liebig proceeds to discuss the art of culture and the action of manures. We should be glad to quote the whole of this chapter, but we must limit ourselves to a brief sketch. Humus is not soluble in water; if it were, a great part of it would be carried off the ground by rains. Its office is, by the presence of water, to convert the surrounding oxygen into carbonic acid, which plants absorb and then return to the soil a large portion of carbonaceous matter, that they abstract from the air, so that

the humus of the soil is not diminished. The frequent ploughing of the soil, so as to promote, by admitting the oxygen of the air to the humus, the formation of carbonic acid, the application of alkaline substances, and whatever tends, as Liebig expresses it, to put the organic matters of the soil in a state of oxidation, increase the fertility of the soil. The oxygen then assists in the formation of carbonic acid to go to the nourishment of plants.

Knowing the substances which go to form the plants, the object of a wise agriculture will be to supply them, and to render them accessible to the plants. Potash, the most common and important of the inorganic constituents of plants, is more universally and abundantly diffused over the earth than any other alkaline substance. But the alkalies, by continual cultivation, may be exhausted, and the soil cease to be productive. This indicates the necessity of a fallow or rest to the soil, by which, under the operation of air and moisture, a further disintegration of the rocks may take place so as to furnish the necessary alkalies to the soil, or, without resting, they may be artificially supplied. Plants themselves in their decay return alkaline substances to the earth; and it is well ascertained that plants themselves act powerfully in the disintegration of rocks.

Some crops may be repeated on the same soil more frequently than others, because some consume more of the alkalies than others. One hundred parts of the stalks of wheat yield 15.5 parts of ashes; The same quantity of barley, 8.54 parts; and of oats, only 4.42. The ashes of these different plants are of the same description, but it is obvious that the demands which they make upon the soil must be different.

The interchange or rotation of crops and the application of manures are materially connected with this fact, and with another in the habits of plants to which we shall refer. Plants of different kinds absorb or take up different substances, from the soil; and one kind therefore may flourish, where another would fail. The same kind of plants cannot be cultivated in succession on the same soil for any length of time without declining in productiveness. Some plants, as flax for example, will not bear a repetition on the same soil oftener than once in five years. It has been supposed that plants assimilate to themselves, and consume in their growth, certain ingredients in the soil necessary to the perfection of VOL. LIII. - NO. 112.

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the plant, which should not be repeated on the same ground until this material is again supplied. But this is not all. No artificial supply of any ascertainable ingredient can control this general law of the necessity of a change in the rotation, growing out of other circumstances. Decandolle suggested, and may be said to have established, another theory, namely, that plants excrete from their roots certain substances, which are innutritious or hurtful to the same kind of plant in succession, but which may serve as the food of other plants. But there are difficulties, in respect to this subject, upon which we cannot dwell, which Liebig's theory solves with remarkable ability and equal reasonableness. The exudations or excretions of plants may be considered of two kinds. Plants, as we have before said, have no selection in their food but take up with little discrimination what is accessible to their organs of nutrition, and in a condition to be absorbed. They consequently may take up many things, which they can assimilate but in part, or not at all. These are exuded, and may serve as the food of other plants of a different character. But there is another class of excretions, or properly speaking excrements, which are purely the result of the vital action of the plants, and which, in the form of gum or otherwise, after having served the purpose designed in the nutrition of the plants, pass off by the appropriate organs into the soil. These, of course, cannot serve as the food of the same kind of plants, or of any other in their present condition; and these go to assist in forming the humus of the soil. In their unchanged condition, these excrements are pernicious to the kind of plants from which they were discharged, and, it may be, to others; but after becoming converted into humus, under the operation of air and moisture, the effects are the same as those of humus.

After all, where the crops are removed from the soil in the forms of seeds, roots, and leaves, the soil is of course deprived of many of the constituents requisite to a healthful and productive vegetation. The substances removed are then to be supplied by manure. The seed of the plant contains within itself the food which it first requires in order to the protrusion of its radicles. The humus in the soil will give out its carbonic acid, until the plant rises above the ground, and the leaves and other portions of its organism are formed, to enable it to gather, in the form of carbonic acid,