In Mark Twain's narrative "I Sell a Dog," what expensive habit does the narrator's friend William Swinton have?

William Swinton likes to drink liquor from a jug.

William Swinton was a former Civil War correspondent and is the roommate of the narrator. They liked to drink and get drunk together.

That is why the narrator was out and about looking for ways to find money to buy liquor for their consumption.