How it works : Avogadro,Amedeo

AVOGADRO, Amedeo (1776-1856) Although he was trained as a lawyer, Avogadro's first love was science. Only after his death a was the importance of his work on gases realised. Count Amedeo Avogadro was born in Turin and spent most of his life there. There was a legal tradition in his family, and in spite of his interest in practical science he studied law and qualified in 1796, being in part-time practice for a long time afterwards. His serious study of physics began in 1800, and he became Professor of Physics at Vercelli in 1809, where he produced his famous hypothesis on the volumes of perfect gases. Between 1820 and 1850 he occupied the Chair of Physics at the University of Turin, and conducted research into the electrical properties of substances, as well as making investigations into thermal expansion and specific heat. Of Avogadro's published works, the most important was his four-volume physics text book, Fisica dei corpi ponderabili, which appeared between 1837 and 1841. Avogadro's fame rests on his proposal that equal volumes of all gases, under the same conditions of temperature and pressure, contain the same number of MOLECULES. This hypothesis was published in 1811 in the French Journal de Physique, soon after Gay-Lussac's discovery that the volumes of combining gases bear a simple ratio to each other and to the volume of the new compound. Avogadro's hypothesis, in conjunction with Gay-Lussac's law, should have allowed the molecular formulae and atomic weights of gases to he determined experimentally, but his paper attracted little attention because it was supported by so little experimental evidence. Avogadro also proposed that the simple gases, such as HYDROGEN and OXYGEN, might exist as molecules containing two atoms instead of one, a suggestion that seemed to conflict with the widely-accepted 'indivisible' atomic theory of John DALTON. Avogadro continued to publish papers on the subject, as well as on other chemical research that was much less original than his neglected hypothesis. The vindication of Avogadro's hypothesis came after his death, when in 1860 Stanislao Cannizzaro presented a system of atomic weights determined using Avogadro's work. Since then his hypothesis has been accepted without question, and his ideas on the molecular nature of the simple gases have also proved to be correct. Avogadro's hypothesis has given rise to the concept of 'gramme-molecular weight' (a mass of a substance equal to its molecular weight expressed in grammes), and to Avogadro's Number, which is the number of molecules contained h~ the gramme-molecular weight of a substance. Avogadro's Number, usually denoted N, was not accurately determined until 1941 when R T Birge evaluated it to be 6.02486 ´ 1023. Because of the volume of the molecules themselves, Avogadro's hypothesis is not strictly obeyed by real gases, but the difference is very slight except under conditions of high pressure. AVOGADRO'S LAW (see gas laws)

Reproduced from HOW IT WORKS p210