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.
If you think you have touched poison ivy, rinse your skin with cool water right away. Then, wash it with soap and water. Rinse your skin well. Do not use hot water because it may cause the oil to spread on your skin. You may also put rubbing alcohol or a solution of 1/2 alcohol and 1/2 water on your skin.