Originally, exactly as much as a volume of water equal to the cube of the hundredth part of the metre, at the temperature of melting ice. I don't find that terribly helpful. Not many did. That's why, in 1889, the mass of the kilogram was redefined as being equal to the IPK.
Boiling water is also an effective poison ivy killer. If the area where you will be killing poison ivy has no other plants you wish to keep, pour boiling water over the poison ivy plant. Boiling water will kill any part of a plant that it comes in contact with, so be careful using this around desirable plants.
Skin contact can cause redness, swelling, blisters, severe skin destruction, swelling of the eye (cornea), or loss of sight. To prevent poison ivy from causing skin irritation, wash exposed area with water within 5 to 10 minutes of contact. Use soap and water first, then ether or alcohol.