In 1806, the people of Leeds were sure that the rapture was upon them. Chickens had actually started laying eggs with apocalyptic messages on them. That's when a group of intrepid skeptics revealed that Leeds was in the thrall of chemistry, not the end times.
It's not entirely the fault of the citizens of Leeds that they believed that the end of the world was coming in 1806. It wasn't even their fault that they believed it because eggs, laid in the area, said, "Crist is Coming." One doesn't check spelling during the rapture. One just prostrates oneself.
Still, they might have been a tad more suspicious than they were, given that those apocalyptic messages were coming from the hens of Mary Bateman. Their suspicion should have mounted when Mary Bateman, whose business selling charms to ward off evil spells had slackened lately, cheerfully announced that she was selling blessings before the rapture. Perhaps they were just relieved that the blessings, which came in the form of pieces of paper with the initials JC written on them, were spelled right.
It took a man from out of town to realize what was happening. Bateman was taking pre-laid eggs and painting them with concentrated vinegar. Vinegar is acetic acid, and egg shell is made of calcium carbonate. When put together for long periods of time, the vinegar dissolves the calcium carbonate into calcium acetate - which washes away when the vinegar is washed away. Leftover is carbonic acid which breaks down into water and carbon dioxide.
2 CH3COOH + CaCO3 = H2O + CO2 + Ca(CH3COO)2
You see this chemistry experiment a great deal in elementary school. Leave an egg in vinegar for a few days, and the shell will dissolve. The delicate inner membrane stays intact, and keeps the yolk and egg white together. Bateman reapplied the vinegar message until the message was partially burned into the shell, and jammed the egg back up into the chicken. A skeptic found out her plot by the ingenious method of getting up early and hiding near her house. Funnily enough, after toying with people's religious devotion for the better part of a month, Bateman was most resented for cruelty to chickens.
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They hanged her three years later. Not for the hoax - there wasn't a statute on the books about faking the rapture. After her business fizzled out, Bateman started selling powders to local people to lift curses. Some people believe her powders were really abortifacients, and point to the fact that the person who sickened and died while taking her powders was a married woman. That explanation is complicated by the fact that the woman's husband was also taking Bateman's powders, and continued doing so for some time after his wife died. When he got worse, in much the same way that his wife did, he called the local police, who discovered that the powders were poison. It's not certain why Bateman would poison the man, though, as he was a regular customer. Perhaps she was trying to keep him just sick enough to keep needing her medicine.
Bateman was hanged in 1809, and earned the name, The Yorkshire Witch. Her chickens earned the kinder name of "The Prophet Hens of Leeds."
[Via Memoirs of Extraordinary Popular Delusions, Naked Egg Experiment, The Second Coming.]