The common onion is a plant too well known to need any description of its botanical character. It is chiefly cultivated for culinary uses: its root affords a large proportion of alimentary matter, particularly when boiled, as it shows by some sweetness and a large proportion of mucilage, when its acrimony is exhaled. In its fresh state it is acrid and stimulating. In bilious dispositions it produces flatulence, thirst, and headach; but in cold and phlegmatic temperaments is warming, attenuant, and promotes both expectoration and urine. It is powerfully antiseptic, and, if applied to tumours, promotes suppuration.
The root is the most active part; but it loses much of its virtue by drying. Distilled with water, all its flavour and acrimony arise. The active matter is much more volatile than that of garlic, but in other respects they agree, though the onion is much weaker, less acrid, and more mucilaginous.
Onions have a greater effect than any other alkalescent plant in dissolving gravelly concretions. The expressed juice has been serviceable in deafness (see Allium). Neumann remarks, that the characteristic principle of this root is its essential oil, though it cannot be collected in a separate state. See Lewis's Mat. Med. or Neumann's Chem. Works. Cullen's Mat. Med.