The functions of this humus are no doubt remarkable, and in respect of it some agricultural theories have been formed, resting on no solid basis. What seems to be tolerably well ascertained is, that in a soil permeable to air, the oxygen of the atmosphere continues to act on the humus, giving origin to carbonic acid, and thus furnishing an atmosphere for the roots of plants growing in that soil. The springing of the roots themselves seems to depend on the presence of this atmosphere; hence the labour and pains to pulverize the soil, and to give access by such processes to the atmospheric air. At this period of their growth the roots perform all the offices of their leaves which are ultimately to appear; and soon the plant has two sets of nourishing organs, the roots and the leaves. In hot summers plants derive their carbonic acid wholly from the air. Thus gradually is formed that humus or ulmine to which agriculturists attach so much importance; that vegetable mould supposed to be the richest of all soils. But where it forms, a kind of putrefaction continually goes on; the soil is influenced deeply as a residence for man. No valetudinarian takes up his abode in the centre of a rich vegetation in hopes of recovering his health and strength, his elastic step, and freedom from lassitude and weariness; he, on the contrary, seeks other regions, where vegetation is scant, humus is not forming, and the soil is never turned over by human industry. When vegetation is purely natural, that is when man does not interfere, the growth of plants does not in the least exhaust the soil. Look at the meadow and the virgin forest! Now chemistry explains this, or nearly so. But so soon as man _interferes_, he must be prepared to undertake the whole labour; if he acts on the earth, the air, and the waters, they will react on him, and sometimes with fearful effect. Beyond the processes she exhibits, and which he may read as best he can, she reveals nothing; all her secrets must be extracted from her by science, by philosophy, by the slow procedure of experiment and observation. A traveller from a distant land prepares to cross deserts of which he has had no previous _experience_; shortly he discovers an oasis, which to him seems a paradise, and he proposes resting for the night within its treacherous circle; but the wild Arab, the native guide, knows better, and explains to him briefly that the desert alone is healthy, and to rest a night within that seeming paradise is death. It is the Homeric tale of the syrens reduced to a reality; gorgeous decorated plants, sweet-smelling flowers, perfumes of Arabia, invite you to enter that island destined, should you unhappily accept the invitation, to prove the resting-place of all your labours.