TABLE III.-Showing the Percentage of Constituents of Food of Live Stock.

Water-meadow

Turnips

Mongold

Potatoes

Secondly, as to the elementary substances of vegetables.—We have seen that vegetables contain water, carbon, and nitrogen, besides some mineral matters; our next inquiry is, from what sources are these substances derived by the plant?

It is a very common, and certainly not an unnatural, opinion that our crops derive their nourishment from the accumulation of vegetable matter in the soil, or, as it commonly called in Devonshire, in the meat-earth. Some persons also have been led by what they have seen in books about the elements of carbon and nitrogen, to form a hasty conclusion that plants can imbibe carbon directly from charcoal as a manure; others think that as there is a large amount of nitrogen in the air they can derive the nitrogen from the atmosphere, by absorbing it in its uncombined state through their leaves. Both of these theoretical opinions are considered by competent authorities to be at variance with facts. Again, there was a theory about humus which was very popular a few years ago. It was found that there were in the earth certain acids containing carbon, to which various names, such as humic, ulmic, and others were given, and it was too hastily inferred that they were absorbed by the plant and carried vegetable matter ready formed into its juices.

This theory is now abandoned by most English chemists. It is now supposed that plants derive the supplies of the two principal elements of their structure, their carbon namely and their nitrogen, chiefly from two sources. These sources are carbonic acid and ammonia, both of which can be absorbed in water, and when so absorbed they can be imbibed by plants through their roots in union with water as a liquid, and through the leaves in union with water as a vapour. I have spoken several times of the elements

of water; one of the elements of water united with carbon forms carbonic acid; the other united with nitrogen forms ammonia. I am now to show you that both carbonic acid and ammonia are accessible to plants, and are to be found in sufficient quantity, either in the soil or in the atmosphere, to supply all our agricultural requirements.

WHAT IS CARBONIC ACID ?

One of the most familiar forms in which carbonic accid occurs is in the bubbles which escape during the fermentation of beer or cider; it is sometimes called foul air; it is heavier than common air, and lies at the bottom of large vats: it is that air which is

well known to be inconsistent with animal life, and which often extinguishes a candle let down into a closed well. The air which we breathe out of our lungs contains much carbonic acid; the hot air which rises from a candle is chiefly carbonic acid; so that all the pure air in this room would in a short time be turned into carbonic acid and be unfit to breathe if fresh air were not constantly admitted. Half of the weight of common limestone is carbonic acid; it is this which is displaced by heat in the limekiln; or it may be displaced by adding a stronger acid to lime which will take its place in union with the lime, and then the carbonic acid which was in the solid stone, having nothing to detain it, makes its escape in effervescence, that is, bubbles of gas rapidly expanding.

We may say, wherever limestone is decomposed, wherever fuel is burnt, wherever animals breathe, or wherever fermentation takes place, during all decay of animals or vegetables, the supply of this gas is going on; and when I remind you that 35 millions of tons of coals are supposed to be annually raised in this country and to be dispersed throughout the air, you will change your question and ask, not how can plants be formed out of the air, but how can such inconceivable quantities of unwholsome gas be poured forth every year and yet the world be a fit place for man to live in?

The answer opens to us one of those marvellous vistas through which it has been vouchsafed to us to look into the works of the Almighty. It is quite true that, if a special provision had not been made, the atmosphere would long since have been unfit for respiration; but every green field, every forest, even the mighty submarine groves of the ocean, are perpetually refreshing the atmosphere with pure air, taking out of the atmosphere the carbon, and reconverting the carbonic acid into pure vital air.

A few words on the growth of plants will put this in a clear point of view.

Vegetables generally act in this way. Each of their seeds contains in itself an embryo plant, which, under the influence of heat and moisture, swells and pushes out a root downwards, and either one or two seed-leaves upwards; the food of these seed-leaves was ready formed in the seed; through these roots and leaves carbonic acid is imbibed for the stem; but as we all know in the case of the turnip, if the seed-leaf is destroyed before the rough leaf is formed the plant dies; as soon as the stem and its leaves grow the plant sucks up water containing carbonic acid from the earth; this

passes into the leaves, which are furnished with delicate mouths on the underside; through these little mouths, under the influence of heat, evaporation of the water takes place, and a still more peculiar influence of light causes the carbonic acid to be decomposed; the solid part, the carbon, is thus locked up or deposited in the leaf, the little mouths give out the pure air, to which we may now give its proper name, oxygen. Carbonic acid consists of carbon and oxygen; animals draw in oxygen, unite it to carbon, and breathe out carbonic acid. Plants do just the reverse, they draw in carbonic acid, decompose it, retain the carbon, and give out the oxygen; so is the balance of nature maintained.

WHAT IS AMMONIA ?

Ammonia used to be best known by the name of volatile alkali ; it is met with in the form of the liquid salvolatile, which is a solution of ammonia, just as a bottle of soda-water is often no more than common water charged with carbonic acid or fixed air; the ammonia itself is extremely volatile, and therefore it easily takes the form of a gas, and escapes unless kept closely corked up. It was originally made in Egypt from the droppings of camels, and derived its name from a district of that country many centuries ago. In Europe it used to be made from horn, whence it derived the name of hartshorn; and it is now made in commerce by the distillation of all sorts of animal refuse. Large quantities are also made in the process of gas-making. Such are the various sources from which the volatile alkali or ammonia of commerce is derived. It is combined (according to the process of manufacture) with various acids, which give the different names of carbonate of ammonia, sulphate of ammonia, and muriate of ammonia, which last is the same as salammoniac, the original Egyptian product. The form, however, of ammonia, which is at present most important to the farmer, is that which is to be found in the urine of animals. The urine is separated from the blood, and carries off all the waste of the muscular tissues which has been replaced by food. The solid excrements only carry off the undigested surplus matters which the system does not require to take up. Except, therefore, in the case of animals highly fed, little ammonia is formed in the solid matter of dung, but a great deal in the liquid. Lastly, wherever putrefaction takes place ammonia is formed, and from its volatile nature it immediately escapes into the air unless it is detained by some special arrangement for the purpose.

This ammonia, whether it is found in the form of a volatile gas, of a liquid solution, or of a solid crystalline salt, always contains that substance which is found in lean flesh, in cheese, and in gluten, and to which the name of azote or nitrogen is given. In 17 parts by weight of ammonia 14 parts are always nitrogen. From what has been said of putrefying bodies, and of the excrements of animals, it is obvious that a constant escape of ammonia must be going on, and therefore that there must be a certain quantity of it diffused through the atmosphere. For some time the amount of this substance in the air escaped all available chemical tests; but we owe its detection to the acuteness of Liebig.

THE ELEMENTS OF WATER.

Having now ascertained that plants may derive a supply of carbon from carbonic acid, and of nitrogen from ammonia, and that both of these gases are diffused through the atmosphere, it may be well to state the intimate relation which water bears to carbonic acid and ammonia.

Carbonic acid consists of carbon in union with oxygen; ammonia consists of nitrogen or azote in union with another substance called hydrogen. It is remarkable that these two bodies, oxygen and hydrogen, are the elements of water, united in the proportion of 8 lbs. of oxygen to 1 lb. of hydrogen, which, so combined, form 9 lbs. of pure water. You will not then wonder that water is capable of dissolving carbonic acid and ammonia. In fact, these gases diffused through the atmosphere are closely connected with the steam or vapor which the air contains; and when this vapor is condensed into clouds, and falls in rain, the water brings down from the heavens the substances which plants are chiefly made of, in the only state in which they are supposed to be capable of receiving nourishment into their circulation. It is also deserving of your notice that water charged with carbonic acid or with ammonia has a power of dissolving some of the ingredients of the soil which are necessary to the existence and health of plants, thereby bringing them, as Dr. Daubeny has observed, from a dormant into an active state.

THE MINERAL MATTER OF PLANTS.

This brings us to the last part of plants, the mineral matter which they contain in addition to the elements already spoken of. The four elements of which we have spoken, namely, carbon, oxy