As these plants (the last named especially) will grow on poor soils, it is possible by their help to reclaim the lightest sands, and bring them up to a fair degree of productiveness in the course of a few years. - Composition of Crops, and their Value as Food. There are definite and unalterable relations between the character and habits of the animal and the composition and physical qualities of its food. In rearing and sustaining domestic animals, four distinct conditions occur, viz.: growth, or general development; fattening, or increase of flesh and fat; yielding milk; and performing labor. Different species of animals possess different degrees of aptitude in turning their food into one or other of these directions. Thus, the hog fattens most readily, the cow yields most milk, and the horse performs the greatest amount of labor. All these animals might be fed alike on a certain diet, and yet manifest their characteristic tendencies in a good degree, for the functions of all animals are the same to a certain point. That food, however, which best develops fat in the hog, is not best adapted to sustain the labor of the horse.

Where the animal's functions are required to differ in their essential nature, there the food must also differ; and we cannot carry the peculiar aptitude of an animal to the highest pitch without particular attention to the quality of the food. In fact, by a careful selection of the food we can change the character of the animal; and when at the same time other physiological circumstances, climate, etc, are suitably regulated, it is possible in the course of a few generations to impress new characters on a race. In this way the various breeds of cattle, swine, etc, have originated. A thorough understanding of the reciprocal relations between food and functional development is therefore of the highest consequence to the practical agriculturist. It cannot be pretended that science in its present state furnishes very extensive or satisfactory knowledge on these points. But physiological chemistry has developed some truths which warrant the hope of progress in this direction. The study of changes in the animal body has shown that there are two chief processes concerned in the maintenance of life, viz., nutrition and respiration.

We use the word nutrition in a somewhat qualified sense, understanding by it the support of the working parts of the animal - the muscular, nervous, and cartilaginous tissues. These tissues contain nitrogen as an invariable ingredient, and for their development nitrogenous food, or food containing albumen, caseine, and fibrine, is indispensable. No work can be done on food consisting exclusively of starch, sugar, and oil, because these bodies cannot supply the nitrogen which is required for the organization of the working tissues. In the normal growth of active animals, the non-nitrogenous principles of the food are consumed in the respiratory process. These bodies are brought into contact with the oxygen inhaled by the lungs, and are burned into carbonic acid and water, which pass off in the expired breath. The heat of the animal is sustained by this combustion. In sluggish animals which ingest large quantities of non-nitrogenous food, the excess accumulates in their bodies in the form of fat. Great activity and full respiration are incompatible with this accumulation. The application of these facts is obvious. To keep a horse or an ox in working condition, we give a food rich in nitrogen, as oats; to fatten an animal, we use a food richer in starch, sugar, and oil.

Experiments have been made with a view to determine what should be the relation between the nitrogenous and non-nitrogenous elements of the food for working, fattening, and milk-giving animals, as well as for otherwise determining the statics of nutrition. In Saxony much attention has been devoted to these subjects, and experiments in feeding, conducted in that country, have shown that breeding and dairy cattle thrive best when each animal receives daily for every 100 lbs. of its live weight 2.5 to 2.8 lbs. of food (calculated in the dry state), which contains 0.25 to 0.30 lb. of nitrogenous or nutritive, and 1.25 to 1.40 lb. of non-nitrogenous or respiratory, fat-forming material. The stomachs of cattle are adapted for a food containing a large quantity of woody fibre, which is mostly indigestible, and seems to perform a merely mechanical function in exciting the digestive apparatus. In the trials just alluded to, the best proportion of woody fibre was found to be one fifth of the whole dry matter. Years ago attempts were made to construct from chemical analyses tables of nutritive equivalents, for exhibiting the comparative value of different sorts of food. The first essays of this kind were very crude.

Later results more nearly accord with experience, being founded on more complete analyses, and with a better knowledge of the wants of the animal; but there are many circumstances whose effect on the nourishing capacity of the different kinds of food has not yet been thoroughly studied. It has been proved that the use of nitrogenous manures increases the relative as well as absolute quantity of blood-forming substances in the grain. The digestibility and consequent nutritive effect of the grasses is greatest when they are cut just after attaining full flower, or, at any rate, before the seeds have hardened, as at this period they contain the maximum of soluble matters. Afterward the quantity of woody fibre increases. The cereals yield more and better flour when cut while the berry is still in the milk, and for a similar reason. The use of cooked food for cattle depends upon the fact that the cooking of food by boiling or steaming is equivalent to the preliminary processes of digestion; as in both cases cellulose, starch, dextrine, and the gums are progressively converted into grape sugar. - Toward the end of the last century the vague and ancient notions that air, water, oil, and salt formed the nutrition of plants, began to be modified with some truer ideas.

In 1761 Wallerius, a Swede, in his treatise Fundamenta Agriculturae, Chemica, recognized to some extent the connection between the composition of the ash of plants and that of the soil. Bergman, the great Swedish chemist, Palissy, and Reaumur also sought to study the chemical conditions of vegetable growth. In 1802 Sir Humphry Davy was invited to lecture before the English board of agriculture, and thereafter made numerous important observations. He recognized the fertilizing effects of ammonia, and analyzed numerous manures, including guano. About the same time Sen-nebier and De Saussure laid the foundations of vegetable physiology, demonstrated the assimilation of carbonic acid and water from the air, and indicated atmospheric ammonia as the probable source of nitrogen to the plant. De Saussure also fully recognized the nature, importance, and source of the ingredients of the ash, and studied the life of the plant in all its phases. In 1832 Sprengel made numerous analyses of the ash of plants and of soils, employing more perfect methods than had been previously known. It was reserved for the splendid genius of Liebig to unite the fragments of truth into an organic whole.

The force of his rhetoric, not less than of his logic, excited intense interest in the chemistry of agriculture; and being the most popular teacher that this science has ever employed, he has contributed vastly to the enlistment of laborers in this important field. While Liebig discussed only "the applications of chemistry to agriculture and physiology," his celebrated work under that title having been written at the request of the British association for the advancement of science, Boussingault, a Frenchman of genius and wealth, occupied himself with the special study of the practical operations of agriculture, and in 1842 issued his Economic rurale, a mine of valuable observations and experimental results. From that time on, the number of those devoted to the study of agriculture has rapidly increased.