It is not to that which lives, but to that which has lived and is now dead, that we must look for the sources of those terrible fevers which destroy humanity in so many fine countries. Nor is it necessary that marshes be present, nor recently inundated lands. Egypt, annually inundated, is healthy at all times, but it is always cultivated; the desert also, which is never cultivated, and incapable of any cultivation, is also healthy. The Arabian desert which skirts the cultivated spots, converting them into so many oases, is perfectly healthy; on its soil the traveller may sleep securely; but let him cross the boundary of the water drain or stream forming the oasis, and sleep within the limits of that vegetation so delightful to look at, and violent fever is sure to overtake him on the morrow, so powerfully in this instance does nature react on man, when altering the soil, he prepares with his own hand the flowery path which leads him to the grave. § 2. _On the Origin and Action of Humus_.--To Liebig we unquestionably owe the first philosophical investigation into the history of _humus_. Innumerable difficulties and prejudices beset the inquiry. It was he who first showed that all vegetables and all their component parts, so soon as they cease to live, become liable to two forms of decomposition,--to putrefaction and to rottenness, that is to fermentation, and to that slow combustion to which Liebig gave the name of _eremacausis_, a Greek term, expressing by its original meaning the fact of slow combustion, to which the illustrious German likened that process which we commonly express by the term of _pourriture_, or rottenness. By this last-named process the combustible parts of bodies in decomposition combine with the oxygen of the air. The decomposition of the rotting of the woody fibre is attended with this peculiarity--when in contact with the air, it converts the oxygen into an equal volume of carbonic acid; so soon as the supply of the oxygen ceases the rottenness stops. Now remove this carbonic acid, and add a fresh supply of oxygen, and the rotting commences, and carbonic acid reappears. The presence of water is essential to this change; the substances called antiseptic arrest it at once. Now the woody fibre in this condition of slow combustion or rottenness is precisely what we call _humus_ or _ulmine_.