Hydrogen sulfide
Hydrogen sulfide, colorless, poisonous, flammable gas (H2S) with an odor of rotting eggs. When combined with water it forms hydrosulfuric acid. The gas can be found around cesspools and in mines as a by-product of the decomposition of substances containing sulfur. The gas is also produced in laboratories for use as an analytical reagent.
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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Humber, River to Indus Valley civilization