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Sour gas plant. Industrial plant for treating sour gas. Also seen in the foreground is a freight train serving the plant. Sour gas is natural gas which is rich in the acidic compound, hydrogen sulphide (formula H2.S). At this plant, the hydrogen sulphide impurity is broken down into inert sulphur which can be marketed as a by- product. Once purified the natural gas is known as 'sweet' gas. Photographed in Alberta, Canada. (KEYSTONE/SCIENCE PHOTO LIBRARY/DAVID NUNUK)