What is proof?

Webster's New Twentieth Century Dictionary, Unabridged, 2cd ed., s.v. "proof" and "proof spirit." Proof, 8. The relative strength of an alcoholic liquor with reference to the arbitrary standard for proof spirit, taken as 100 proof. Proof spirit, An alcoholic liquor, or a mixture of alcohol and water, containing 50 per cent of its volume of alcohol having a specific gravity of .7939 at 60° F. Before the hydrometer became a common instrument of the distiller there existed a method of proofing known as "gunpowder proof". It was a simple procedure and took advantage of readily available "tools". Bourbon and gunpowder were mixed in equal proportions in a small fireproof vessel and ignited. If the flame burned yellow the liquor was too strong, if it burned blue the proof was true. A yellow proofed liquor was mellowed with spring water until it burned blue. The proof of a blue flame spirit was about 100, or 50 per cent