Amedeo Avogadro was the Italian scientist who helped found physical chemistry and is best known for the hypothesis which bears his name. Avogadro was trained as a lawyer and became interested in mathematics and physics after private tuition in 1800. In 1809 Joseph-Louis Gay-Lussac demonstrated that all gases expand by the same amount if subjected to an equal rise in temperature. Seeking to explain this, Avogadro published (1811) his now famous hypothesis, stating that equal volumes of all gases contain equal numbers of particles at the same temperature and pressure. He also introduced the term molecules, reasoning that the gas particles need not be single atoms, but could be combinations of atoms. Unfortunately his work was ignored by the most eminent chemists of the time, partly due to his geographical and intellectual isolation (his experimental work was not noted for its accuracy and he often relied upon speculation and analogy). His hypothesis did not become universally accepted until the 1880s. Avogadro received little recognition for his contribution to science, although he is now well remembered for the constant named after him which gives the number of molecules per mole of a substance
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