At the bottom of the killing jar was a piece of cotton saturated with carbon tetrachloride.

NPR: Excerpt: 'At Large and At Small'

Past attempts, however, have relied on dissolving the tetrachloride (or sometimes the dioxide) into the molten calcium chloride.

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The test found that it contained nearly 4, 000 micrograms per litre of carbon tetrachloride - nearly 1, 000 times the World Health Organisation's safe limit.

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The problem with chloroform, as with potassium cyanide and carbon tetrachloride, is that these poisons freeze the butterfly's muscles into an extreme version of rigor mortis, and the wings cannot be spread.

NPR: Excerpt: 'At Large and At Small'

During the fifties, when my brother and I started chasing butterflies, potassium cyanide was still in use as well, but because it is a deadly poison, Professor Klots recommended liquid carbon tetrachloride, which is "not very poisonous unless inhaled deeply, " and which we persuaded our parents was as innocuous as smelling salts.

NPR: Excerpt: 'At Large and At Small'