Thread by @chronicleflask

Here's a fabulous bit of #chemistry I've just learned about:

French physician Séverin Icard devised a test to confirm death: write "I am really dead" on a piece of paper using lead(II) acetate (Pb(CH₃COO)₂). It's soluble & invisible when dry...

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The paper is then placed over the nose & mouth of the "corpse". One of the gases produced during decomposition is sulfur dioxide (SO₂), which reacts with lead(II) acetate to form lead sulfide (PbS) which is black. So the writing becomes visible...
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The corpse thus has a piece of a paper with "I am really dead" on their face.

(I do not know if Terry Pratchett knew about this & Granny's "I Ate'nt Dead" sign was a reference. Wouldn't surprise me. Perhaps @terryandrob knows...)

BUT...
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UNFORTUNATELY... not all corpses produced enough SO₂ near the nose/mouth to produce a positive result. It only worked for 1 in 6.

Worse, SO₂ is also produced by living humans, especially if they have a dental disease or tonsillitis, which could trigger the test...
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And thus you could find yourself declared dead for the "crime" of having bad breath. Oops.

Still, you have to give Séverin Icard points for drama, right??

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