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.
Combine 1 gallon white vinegar, 1 cup salt and 2 TBSP blue dawn dish soap. Once all the ingredients have been mixed well pour mixture into a spray bottle. Spray poison ivy. Be careful not to get it on your other plants because this mixture will kill anything you spray it on!