LECTURES

ON AGRICULTURAL CHEMISTRY.

BY PROFESSOR SAMUEL W. JOHNSON, OF YALE COLLEGE, CONNECTICUT.

LECTURE I.

THE COMPOSITION AND STRUCTURE OF THE PLANT.

The objects of agriculture are the production of certain plants and certain animals which are employed to feed and clothe the human race. The first object in all cases is the production of plants.

Nature has made the most extensive provision for the spontaneous growth of an immense variety of vegetation; but, except in rare cases, man is obliged to employ art to provide himself with the kinds and quantities of vegetable produce which his necessities or luxuries demand. In this defect, or rather neglect of nature, agriculture has its origin.

The art of agriculture consists in certain practices and operations which have grown out of an observation and imitation of the best efforts of nature, or have been hit upon accidentally.

We distinguish here between agri-culture, or the culture (improvement) of the field, and farming, which may be anything but the imitation of nature, which often is the grossest violation of her plain precepts.

The science of agriculture is the rational theory and exposition of the successful art.

Nothing is more evident than that agricultural art impedes its own growth by holding aloof from science. In many respects the Egyptians, the Romans, and the Chinese, had, centuries ago, as perfect an agricultural practice as we now possess; but this fact so demonstrates the extreme slowness with which an empirical art progresses, that incalculable advantage must be anticipated from yoking it with the rapidlydeveloping sciences. In fact, the history of the last fifty years has proved the benefits of this union; and no farmer who by the help of science has mastered but one of the old difficulties of his art that for all time have been tormenting the thoughtful with doubt and misleading every one into a wasteful expenditure of labor or material,would willingly return to the days of pure empiricism. On the other hand, those who attempt to unfold the laws of production from considerations founded merely in the pure sciences, without regard to, or knowledge of, the truths of practice, are sure to go astray and bring discredit on their efforts. Agriculture, i. e. field culture, not husbandry or farm management in the widest sense, is a natural science, and is based principally upon physics, chemistry, and physiology.

By physics (natural philosophy) is meant the science of matter considered in relation to those forces which act among masses, or among

particles, (atoms,) in such a manner as not to alter their essential characters.

The forces of cohesion, gravitation, heat, light, electricity, and magnetism, are physical forces. A thousand fragments of iron, for example, may be made to cohere together or gravitate to the earth, may be changed in temperature, illuminated, electrified or magnetized, without any permanent change in that assemblage of properties which constitutes this metal.

Chemistry is the science of chemical force or affinity, which causes two or more bodies to unite with the production of a compound possessing essentially new characters. Thus a hard lump of quicklime when brought in contact with water greedily absorbs it, with the production of great heat, and falls to powder. In slacking, it has combined chemically with water.

Physiology is the science of the processes of life, which require, in addition to the chemical and most of the physical forces, the co-operation and superintendence of the vital principle.

The first inquiries in the natural science of agriculture are: What is the plant? Out of what materials, and under what conditions is it formed?

The plant is the result of an organism, the germ, which under certain influences begins an independent life, and grows by constructively adding to itself or assimilating surrounding matter.

The simplest plant is a single cell, a microscopic vesicle of globular shape, which, after expanding to a certain size, usually produces another similar cell division either by lateral growth or by its own.

In the chemist's laboratory it is constantly happening that, in the clearest solutions of salts, like the sulphates of soda and magnesia, a flocculent mould, sometimes red, sometimes green, most often white, is formed, which, under the microscope, is seen to be a vegetation consisting of single cells. The yeast plant (fig. 1) is nothing more

Fig. 1.

than a collection of such cells now existing singly, now connected in one line or variously branched.

The cell is the type of all vegetation. The most complex plant, a stalk of cane or an oak, is nothing more than an aggregation of myriads of such cells, very variously modi'fied indeed in shape and function,

but still all referable to this simple typical form.

In the same manner that the yeast plant enlarges by budding or splitting into new cells, so do all other plants increase in mass; and thus growth is simply the formation of new cells.

So far as the studies of the vegetable physiologist enable us to judge, all vegetable cells consist, at least in the early stages of their existence, of an external, thin, but continuous (imperforate) membrane, the cell-wall, consisting of a substance called cellulose, and an interior lining membrane of slimy or half liquid character, variously called the protoplasm, the formative layer, or the primodial utricle, (fig. 2.)

At one or several places, the formative layer is thickened to the so-called nucleus, (a fig. 2) the point from which growth and transformations proceed. Within the cell thus constructed exists a liquid, the cell-contents, from which, in course a. of time, solid cell contents of various character are found to develope.

Fig. 2.

B

In a chemical sense, not less than in a structural, the single globular cell is the type of all vegetation.

The outer wall of the cell is formed of that material which is itself the most abundant product of vegetable life, and which represents an important group of bodies, that are familiar to all, as large ingredients of our daily food.

The table which here follows gives the names and the chemical formulæ of what we may term the CELLULOSE GROUP or the VEGETAL

Cellulose is the body already alluded to as constituting the material of the outer coating of the cells. It often accumulates in some parts of

the plant by the thickening of the cell walls, thus forming the greater share of the wood (fig. 4) of trees and shrubs. Linen,hemp,(Bfig.3) and cotton (A fig. 3)are nearly or quite pure cellulose. It exists largely in the stones or shells of fruits and nuts. The so-called vegetable ivory is chiefly a very compact form of cellulose. In general, this proximate organic element is the frame-work of the plant, and the material that gives toughness and solidity to its parts.

Cellulose is characterized by its great indifference to most ordinary solvents. Water, alcohol, &c., do not dissolve it, and the stronger reagents of the chemist rarely take it up without occasioning essential changes in its constitution.* With strong nitric acid it yields nitro-cel- A

Fig. 3.

According to Pelouze, cellulose is dissolved by strong again in part (part is converted into sugar) on dilution.

B

Fig. 4.

C

hydrochloric acid, and separates Schweitzer has recently made the

lulose or gun cotton. By the continued action of oxydizing agents itis converted into that series of brown bodies known under the

Fig. 5.

A

D

name of Humus, or finally into oxalic and carbonic acid.

Next to cellulose, starch (fig. 5) is the most abundant vegetable body. It usually occurs as microscopic grains, which for many species of plants possess a characteristic form and size, being sometimes angular as in maize,but most often oval or spherical as in the other grains, the potato, &c.

Starch is insoluble in and unaffected by cold water; in hot water it swells up and forms a translucent jelly, and in this state is employed for stiffening linen.

Starch is always enclosed in the cells of the plant as seen in the ac

Fig. 6.

companying figure 6, and is exceedingly abundant, existing not only in the grains and esculent roots, but also in the trunks of trees, especially the sago-palm, and throughout nearly the whole tissue of the higher orders of plants.

Inulin closely resembles starch in many points, appearing to replace that body in the roots of the artichoke, elecampane, dahlia, dandelion, and other composite plants. It occurs in the form of small round transparent grains, which dissolve easily in boiling water, and mostly separate again as the water cools.

Unlike

starch, inulin exists in a liquid form in the roots above named, and separates in grains from the clear pressed juice when this is kept some time. The juice of the dahlia tuber becomes a semi-solid white mass in this way, after reposing 12 hours from the separation of 8 per cent. of this interesting substance, (Bouchardat.)

Dextrin is a colorless transparent body, soluble in water, and it appears universally distributed in the juices of plants, though existing in but small amount compared with the previously described proximate principles. The solution of an impure and artificially prepared dextrin, called British gum, is largely employed in calico printing, as a substitute for the more expensive natural gums, and closely resembles them in its adhesive properties. It is an im

interesting observation, that a solution of oxyd of copper in ammonia dissolves cellulose to a clear liquid, from which the cellulose may again be thrown down by an acid.

portant ingredient of bread, being formed in the loaf by the process of baking, from the transformation of starch.

Gum is a generic term, and includes a number of substances, as gum tragacanth, gum Arabic, gum Senegal, cherry gum, &c., which, though unlike in some respects, agree in composition, and have the property either of dissolving or swelling up in water with the formation of an adhesive mucilage or paste. In the bread grains there is usually found a small quantity of gum soluble in water, and in meal from the seed of millet it has been observed to the amount of 10 per cent.

The sugars are so familiar that they scarcely require special notice. Cane sugar or sucrose is the intensely sweet soluble crystallizable principle found in the juice of the cane, maple, and sugar beet. It is found, besides, in many other plants.

Fruit sugar or fructose is uncrystallizable, and exists in the juice of acid fruits, in honey and in the bread grains.

Grape sugar or glucose is found solid and crystallized in dried fruits, especially in the grape. It gradually separates from honey as the latter candies.

In the young cell this group of bodies is represented by cellulose, as the cell wall, and by dextrin and the sugars existing in its fluid

contents.

The machinery of the vegetable organism, which all the while operates as perfectly in the single cell as in the complex mass of cells, has the power to transform most if not all these bodies into every other one, and we find them all in every individual of the higher orders of plants-at least in some stage of its growth. From dextrin, which is dissolved in the juice of a parent cell, is moulded the cellulose which envelopes a new cell.

From starch, and perhaps cellulose in the stem of the maple, cane sugar is formed in the changeful temperature of spring, and, as the buds swell, this sugar is reorganized again into cellulose and starch.

The analysis of the cereal grains oftentimes reveals the presence of dextrin, but no sugar or gum; while at other times the latter are found, but not the former.

It is easy to imitate many of these transformations outside of the vegetable organism. By the agency of heat, acids, and ferments, either singly or jointly, we may effect a number of remarkable changes.

Cellulose and starch are converted, first, into dextrin, and finally into grape sugar, by boiling with a dilute acid. In this way glucose is largely manufactured from potato starch, and has, in fact, been made from saw-dust. This transformation is also effected by the digestive apparatus of herbivorous animals, and in case of starch by a roasting or baking heat. So, too, in the sprouting of seed, the same changes occur, as exemplified in the preparation of malt.

By heat and acids inulin is also converted into a kind of sugar, but without the intermediate formation of dextrin. The same is true of the gums. By these agencies cane sugar is converted into fruit sugar, and this spontaneously passes into grape sugar.