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.
Organic Poison Ivy Control. The most difficult thing about how to get rid of poison ivy is that the whole plant must be eliminated. If any of the root survives, the poison ivy plant will come back. Killing poison ivy organically means that you will need to pull the plant out of the ground, roots and all.