Proof Of “Proof”

Found the following on Alaska Distillery’s Facebook Timeline:

Back in the old days, a trapper would go to a trading post to quench his thirst for booze. However, the savvy woodsman would require “proof” that the booze he was buying wasn’t watered down. So he would pour a shot on a measure of black powder. If the mixture fizzled out, it was watered down, if it lit, then it was proof. That’s where we get the term proof that is on every bottle of alcoholic beverages.