About Alcoholism - Alcoholism Information, Research and Treatment
Proof of the Pudding
Proofing spirits dates from the early days of distilling when the strength of liquor was measured by pouring some on a small pile of gunpowder and lighting it.
A bright blue flame meant that the liquor was too strong. A steady blue flame, which was found to come from spirits containing about 50% alcohol, indicated that the potion was about right.
It was described as "proved 100% perfect," an expression that eventually became 100 proof.-- New York Sunday News