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.
Each leaf on a poison ivy plant has three smaller leaflets. Contact with any part of the poison ivy plant can cause red, swollen skin, blisters and severe itching, sometimes within hours after exposure. A poison ivy rash usually resolves on its own within a few weeks. In the meantime, soothe irritated skin with an over-the-counter topical treatment, such as calamine lotion. Oatmeal baths and cool compresses also might be helpful. Consult your doctor if you have a severe poison ivy rash or if the rash involves your face or genital area.