In a treatise on the “Dignity and Utility of Salt,” by Jean de Marcounille Percheron, Paris, 1584, this mineral is likened in value to the four elements recognized by the ancients,—earth, air, fire, and water; and indeed, on account of its importance for the maintenance of health in the animal economy, salt has been termed a “fifth element.” So highly did the Thracians of old prize this commodity that they bartered slaves in exchange for it, whence originated the phrase _Sale emptum mancipium_. The Egyptian geographer, Cosmas, stated that a salt currency was in use in Africa in the sixth century;[325] and Marco Polo wrote that salt was a common medium of exchange among certain Asiatic peoples in the thirteenth century. In Tibet, for example, pieces of salt shaped in a mould, and weighing about half a pound each, served as small change; eighty such pieces were equal in value to a _saggio_ of fine gold, corresponding to the Roman _solidus_, worth about three dollars. Salt was, moreover, used as money at this time in Yun-Nan and other provinces of southwestern China.[326] Felix Dubois, in his “Timbuctoo the Mysterious,” p. 123, comments on the rarity of salt in the interior of the Soudan, and says that it is the most valuable commodity of that region, the _true gold_ of the Soudanese. The bulk of the salt supply of Timbuctoo comes from the salt mines of Taudeny, which are situated in the great Sahara desert, some three hundred miles away to the north. Here the salt is found in abundance beneath a scanty layer of sand, and is dug up in lumps and fashioned into blocks. Small pieces of this rock-salt are useful to the traveler as money, and are readily accepted as such by the Soudanese merchants. The camels of southern Mongolia require a certain amount of salt in order to remain in good condition. Instinctively, therefore, they browse upon the saline efflorescence which is found on the grassy plains or steppes of Asia. Baron Humboldt, in his “Aspects of Nature” (Berlin, 1808), wrote that these plains were covered with juicy, evergreen soda plants; and that many of them glistened from afar with flakes of exuded salt, which much resembled newly fallen snow. When camels do not find this efflorescence, they sometimes show their craving for its saline flavor by taking white stones in their mouths, supposing them to be lumps of salt.[327] Owing to the universality of its use, salt has been termed the “cosmopolitan condiment.” The craving for this substance is not confined to man, but is shared by the lower animals, and its hygienic value for horses and cows is well known. Wild animals travel long distances over deserts and prairies, or through swamps and jungles, to reach “salt-licks.”