Avogadro

Amedeo Avogadro can be considered as something of a late-starter by the standards of most physicists. He followed his father into a legal career, after completing a doctorate in ecclesiastical law. Whilst practising as a lawyer, aged 24, he studied physics and mathematics privately and eventually rose to become a professor of physics. Avogadro married, had seven children and his biographer reports he “found more satisfaction and enjoyment in the serene atmosphere of his family... than in the pursuit of social and professional success.” For example, he organised a family newspaper that reported the major events in his relatives’ lives.

Avogadro built on the work of Gay-Lussac to develop his hypothesis that equal volumes of gas contained the same number of molecules. However, he was unaware of the value of the constant that came to be named after him and it took almost 50 years before the idea was widely accepted. This was for several reasons: primarily, Avogadro was isolated from the mainstream community of scientists, but also his work did not clearly define the concept of a molecule and the number of particles in a gas could not be directly measured. Ampère independently arrived at the same conclusion and dedicated a version of his book on electrodynamics to Avogadro, but misspelt his name.

Many different methods have been suggested for determining the value of Avogadro’s constant. Boltwood and Rutherford used the quantity of helium emitted by a sample of pure radium. Planck was able to determine a value of the Boltzmann constant from his derived radiation law and, as the gas constant had been experimentally measured, could then estimate Avogadro’s constant. In the early 20th century, the development of X-ray crystallography led to the measurement of the volume occupied by a single atom, allowing the value of the constant to be precisely determined.