Few of our native trees are more useful for fuel, and the manufacture of potash; and as the means of affording a great and almost inexhaustible supply of sugar, it becomes an object of great importance, even to the farmer, who is desirous of transmitting a valuable inheritance to his children. It is my intention to plant this tree in the place of a line of the Athenian poplars, which I have been obliged to cut down after eight years luxuriant growth, from their injurious effect on the adjoining fields, by the extension of their roots to sixty and seventy feet, throwing up a little forest of suckers. _Treatise on Agriculture._ SECT. III. Theory of Vegetation. 3d. Of _air_, and its agency in vegetation: A seed deprived of air will not germinate; and a plant placed under an exhausted receiver, will soon perish. Even in a close and badly ventilated garden, vegetables indicate their situation; they are sickly in appearance, and vapid in taste.--These facts sufficiently shew the general utility of air to vegetation: but this _air_ is not now the simple and elementary body, that the ancient chymist described it to be. Priestly first,[1] and Lavoisier after him, analyzed it, and found, that when pure, it consisted of about 70 parts of azote, 27 of oxygen, and 2 of carbonic acid. In its ordinary (or impure) state, it is loaded with foreign and light bodies; such as mineral, animal and vegetable vapours, the seeds of plants, and the eggs of insects, &c. Is it to this _aggregate_, that vegetation owes the services rendered to it by air? And if not, to _how many_, and to _which_, of its regular constituents, are we to ascribe them? This inquiry will form the subject of the present article. [1] See Priestly's Experiments and Observations on different kinds of Air, begun in 1767. All vegetables in a state of decomposition, give _azote_; and some of them (cabbages, radishes, &c.) give it in great quantity. This abundance, combined with the fact, that vegetation is always vigorous in the neighbourhood of dead animal matter, led to the opinion, that azote contributed largely to the growth of plants: but experiments, more exactly made and often repeated, disprove this opinion, and shew that in any quantity it is unnecessary, and that in a certain proportion it is fatal to vegetation. In _hydrogen gas_, plants are found to be variously affected, according to their local situation; if inhabitants of mountains, they soon perish--if of plains, they shew a constant debility--but if of marshy grounds, their growth is not impeded.