From the novel "To Kill a Mockingbird" by Harper Lee.
6-year-old Scout does not want to go to school anymore:
“You never went to school and you do all right, so I’ll just stay home too. You can teach me like Granddaddy taught you ‘n’ Uncle Jack.”
“No I can’t,” said Atticus. “I have to make a living. Besides, they’d put me in jail if I kept you at home – dose of magnesia for you tonight and school tomorrow.”
“I’m feeling all right, really.”
“Thought so. Now what’s the matter?”

What is meant here with "dose of magnesia"? Is that like a drug that tastes so disgusting that Scout in a second feels better?

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