<<So doesn't it seem likely that 'turn on' the light comes from gas lamps which predated electric ones, and is about turning on the supply of gas (rather than the supply of beer as Capfka first suggested)?>>

With gas, I think it would be "light the light," and "turn *up* the light." But, maybe, "turn on the gas" went to "turn on the light." All guesses. No authoriities.