Except in the atmosphere, we find nitrogenous combina tions only in the structures of plants and animals; and this is the nitrogen which benefits plants, when, after the death of these living bodies, it has undergone an alteration by putrefaction and decay. These processes perform the same service to the nourishment of plants, that boiling, roasting, and baking render to the nourishment of man. The nitrogen is thereby carried over from the more complex quaternary compounds in which they exist in animals and plants, into a simpler combination. Withdrawing itself from two of these elements (carbon and oxygen), it remains in combination with the third (hydrogen), and then forms the most important and valuable nutrient of plants, namely ammonia. In a pure form this substance possesses a very strong, pungent odour and great volatility, for it is a kind of air or gas; hence it escapes into the atmosphere when the process of putrefaction does not take place in the ground. By combination with acids, for instance with sulphuric or muriatic acid, or with humus, which behaves as an acid, ammonia may be deprived of its volatility. Such combinations are called ammoniacal salts.

Any one wishing to become more accurately acquainted with this gas, so eminently important to the agriculturist, and which the farmer may regard as "putrid nitrogen,' has only to buy a small quantity of hartshorn, such as may be obtained from any druggist, to heat it in a plated spoon over the flame of a lamp, and to hold over it an empty tumbler. "Hartshorn" (or spirit of hartshorn) is water containing a large quantity of ammonia in solution; upon being heated, the latter escapes, and mounts into the tumbler. To the external eye the glass seems empty, but the and accuracy, go to prove that plants acquire an increased proportion of nitrogenous constituents during healthy growth in soil and air from which all ammonia or other compound of nitrogen has been carefully removed, so that they must derive their nitrogen directly from the atmospheric air. There is nothing to prevent our believing in the possibility of this. Nitrogen is certainly very little prone to enter into direct combination, but it may be made to combine with oxygen or with hydrogen by known artificial processes. And considering that the ammonia of decomposing animal and vegetable matters is chiefly derived in the first instance from the vegetable kingdom, it seems probable that plants really have the power of fixing nitrogen directly from atmospheric air.-ED.

odour reveals at once that it contains a pungent gas. This odour is entirely identical with that which is perceptible enough, and often very offensive, in stables, sheep-pens, or water-closets. There it originates from putrefying animal excrements, and exhales into the air.

But there is also a second combination of nitrogen which must be regarded as a means of nourishment to plants. This is produced when substances containing nitrogen putrefy in contact with bases (lime, potash, &c.). By the agency of the latter, the nitrogen, instead of uniting with hydrogen, combines with oxygen; and in this way nitric acid salts (nitrates) are formed, from which plants have the power of abstracting nitrogen. In this manner nitrate of lime is generated from the mortar of the walls of stables, and the nitrogen of urine upon rubbish-heaps.

We find nitrogen-for the most part, indeed, in small quantities-in soils and in water, sometimes in the humus, at another in the form of ammoniacal or nitric acid salts; but these are invariably derived from animal and vegetable substances which have putrefied or decayed in the earth. The more, therefore, such decaying matter is introduced into the ground, the richer will this become in nitrogen; and water will contain a greater quantity of it in proportion to the length of time during which it has percolated through soils rich in nitrogen, or to the quantity which has been washed into it. Dew, and rain-water (especially that falling at the beginning of rainy weather), always contain ammonia, because they bring down again to the earth the ammonia, which has become volatile during the processes of putrefaction and decay. The fertilizing effects of transient showers certainly depend in an essential degree on the greater quantity of ammonia which they contain.

Finally, it is more than probable that the organic matters decomposing in the soil are in a condition to enable it to convert a part of the nitrogen absorbed from the air directly into ammonia; which of course increases the importance of the humus contained in arable soils.

d. The inorganic or mineral substances requisite to the growth of plants are conveyed to them through the soil and the water. The fundamental mass of our various soils consists of crumbled rocks, and these for the most part, ex

cept perhaps inert, siliceous sand, or pure bog-earth, contain all the mineral substances which plants require for their support, although some of them in very inconsiderable quantity. In the solid rock these are insoluble in water; but nature provides for this, inasmuch as from year to year some portion of its mass is loosened and decomposed. This is accomplished by the process of weathering, which plays the same part in the preparation of the inorganic nutriment of plants, that putrefaction and decay perform in the elaboration of the organic. Chemical forces, sustained by air and water, warmth and cold, plants and animals, here effect at length the reduction of solid rocks into pulverulent earth, and of insoluble mineral compounds into soluble salts, which salts can then be taken up by the roots of plants.

But weathering takes place also in the interior of the earth's crust, indeed, wherever air and water can penetrate into the masses of rock. The substances thus rendered soluble are taken up by the rain-water, and constitute the salts or earthy ingredients contained in our common spring and river water. Accordingly in many places plants may receive inorganic matter from water also.

Finally, the atmosphere always contains inorganic substances, which have been conveyed into it by evaporation from the ocean, and by the force of the winds, and which are diffused with it over the whole earth. These are returned again to the earth in rain, dew, snow, &c., and consequently we cannot be surprised that we often find in plants mineral substances, for instance, common salt, &c., which we do not discover in the rocks from which the soil serving as a habitation for these vegetables has been derived*.

* The experiments instituted under Lavoisier's guidance by the Direction des Poudres et Salpêtres, have proved that during the evaporation of the saltpetre ley, the salt volatilises with the water, and causes a loss which could not before be explained. It is known also that, in sea-storms, leaves of plants in the direction of the wind are covered with crystals of salt, even at the distance of from 20 to 30 miles from the sea. But it does not require a storm to cause the volatilisation of the salt, for the air hanging over the sea always contains enough of this substance to render turbid a solution of nitrate of silver, and every breeze must carry this away. Now as thousands of tons of sea-water annually evaporate into the atmosphere, a corresponding quantity of the salts dissolved in it, viz. of common salt, chloride of potassium, magnesia, and the remaining constituents of the sea-water, will be conveyed by wind to the land.—Liebig.

Silica, Clay, Lime, Salts, Humus.

So much for the constituents of plants, and the sources they are derived from, the articles of food. The preceding table may be regarded as a retrospective view of the elements and nutrient materials of plants, and serve to render clearer what has been stated on this head.

How Nature proceeds in order to produce from the three nutrients, carbonic acid, water, and ammonia, with the aid of a few mineral substances, the innumerable proximate constituents of vegetables, is a subject upon which we still know nothing. The actual production of the organic substances effected in these by the creative power inherent in living plants, we cannot imitate by art, although we know with certainty that it uses chemical forces for the performance of its works. On the other hand, we can imitate several of those transformations of one vegetable substance into another, for instance, the conversion of starch into gum, of gum into sugar, of sugar into oxalic acid, which occur in plants during the period of their growth. In this respect, indeed, Art can accomplish more than Nature; for it can produce combinations such as alcohol, ether, pyroligneous acid, chloroform, gun-cotton, and a thousand other compounds, which we never find ready-made in living plants.

No doubt, however, now prevails that the elementary substances contained in the nutrient materials above mentioned are sufficient for the production of all the constituent elements of plants. The principal mass of every plant consists of substances devoid of nitrogen (ternary, or composed of three elements); all these may be produced from carbonic acid and water, the elements of the water combining with the carbon of the carbonic acid. If this occurs, the oxygen of the latter must necessarily be liberated, as the following summary shows:

CARBONICS Oxygen

ACID. Carbon

WATER.

Oxygen

Hydrogen

is exhaled.

Hence may be produced: Vegetable fibre, starch, gum, sugar, fat, oil, resin, etc.

It is, however, also possible that not the carbonic acid, but the water, is decomposed in plants, so that the elements of the carbonic acid may combine with the hydrogen of the water; and accordingly, that the oxygen which plants exhale during daylight is derived from the water. The chemical process would then, indeed, be different, but the result would still be precisely the same as that just stated.

If to the two articles of food above mentioned, is added the third, ammonia, all the fundamental materials are present which are required for the formation of nitrogenous (quaternary, or composed of four elements) vegetable substances, as is evident from the following table :

Nature provides, by means of rain and dew, decay and putrefaction, by physical, chemical, and volcanic forces, that the three universal articles of food, water, carbonic acid, and ammonia, shall not be wanting to the plants upon our globe; and man also, without exactly intending it,.contributes his share by the processes of respiration and combustion. The air contains an inexhaustible store of these substances, since the processes by which they are generated go on uninterruptedly over the whole earth. The air alone therefore suf