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John Scott Haldane CH FRS (2 May 1860 – 14/15 March 1936) was a Scottish physiologist famous for intrepid self-experimenting which led to many important discoveries about the human body and the nature of gases. He also used his son J. B. S. Haldane as a guinea pig, even when he was quite young. Haldane locked himself in sealed chambers breathing potentially lethal cocktails of gases while recording their effect on his mind and body. Haldane visited the scenes of many mining disasters and investigated their causes. When the Germans used poison gas in World War I Haldane went to the front at the request of British secretary of state, Lord Kitchener and attempted to identify the gases being used. One outcome of this was his invention of the first gas mask. His son, J. B. S. Haldane became equally famous, both by extending his father's interest in diving and as a key figure in the development of the modern evolutionary synthesis.