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What is the formula for invisible ink?

Invisible ink is made from 4 drops onion juice, 4 drops lemon juice, and 17 grains of sugar. Mix the ingredients in a small container, such as a jar lid, until the 17 grains of sugar are dissolved. Dip a toothpick into the mixture and write on paper, but do not press hard. Allow the ink to dry and the message will disappear. To make the message reappear, hold the paper over a bright light bulb. The ink will appear brown.

Source: Price, Lowi. Concoctions, p. 29.

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