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.
Vinegar - Just sprinkle liberally on rash or blisters for instant relief from itch. caused by poison ivy. Although the smell may not be to everybody's liking, vinegar seems to help draw out the poison from poison ivy while it also helps healing. White distilled vinegar or apple cider vinegar seem to work best. Banana peel - Touted by many as a homemade miracle cure, the inside of a banana peel rubbed on poison ivy rashes seems to bring instant, cooling relief. By some accounts, banana skins may even do the trick when all other poison ivy treatments fail.