The development of scarcely any plant can be imagined without the assistance of nitrogen or of azotized materials. Now, under certain conditions known to all botanists, this azote must come from rain water, either in the form of atmospheric air, or under that of ammonia. Chemists have, I think, proved that it originates in the ammonia contained in the atmosphere, and not in the azote as it naturally exists in the air. The problem is put and solved in this way by Liebig, “Let us consider a farm suitably conducted, and of an extent sufficient to maintain itself, ammonia exists there in a sufficient abundance in rain water and snow; in the water of most fountains; it exists in the air in abundance, and is being constantly renewed by the decomposition of animal and vegetable bodies, and is restored to the soil by the rain, and then absorbed by the roots of plants, and produces, according to the organs, albumen, gluten, quinine, morphine, cyanogene, and a great number of other crystallized combinations.” The most decisive proof of the part played by ammonia in the nourishment of plants is furnished us by the use of manure in the cultivation of cereals and green forage. According to the distinguished chemist so often quoted in this essay, animal manure (_fumier_) acts solely by reason of its production of ammonia. The history of the Peruvian guano, a substance so highly ammoniacal, proves all these assertions; this celebrated manure, which fertilizes a soil (the Peruvian) of the most remarkable sterility, consisting mainly of white sand and argil, is composed chiefly of urates, urate of ammonia, oxalate of ammonia, phosphate of ammonia, carbonate of ammonia, and some other salts. Thus did the ancient Peruvian, like the Chinese, stumble on the solution of problems involving the fate of millions by simple experience alone, wholly unaided by science, which steps in afterwards and gives the _rationale_ of the process; teaches us that all wheats do not equally abound in gluten; that rice is poor in azote; potatoes equally so. Practical agriculturists still find difficulty in applying with success the processes recommended by the chemist; but these, no doubt, will gradually be overcome. “Since we find azote[31] in all the lichens which grow on basaltic rocks; that the fields produce more azote than is brought to them in the shape of aliment; that we meet with azote in all soils (_terrains_), even in minerals which happen never to come in contact with organic matters; that in the atmosphere, in rain-water, and in that of fountains or springs, in every description of soil we meet with this azote under the form of ammonia, as a product of the slow combustion or of the putrefaction of anterior generations; that the production of azotized principles greatly increases in plants with the quantity of ammonia presented to them in animal manure,--we may in all safety conclude that _it is the ammonia of the atmosphere which furnishes the azote to plants_.