[pandaman]

It's an often told story but I have not seen actual sources. Vodka had been made to "polugar" ("полугар",literally "half-burn") measure long before Mendeleev has been born. "Polugar" was a spirit, which could be burned down to half of the volume (when heated almost to boiling, of course) and it's about 38% ABV. More plausible theory is that when the better methods of measuring alcohol contents became available the standard had been set up to the "round" 40%.