Dedicated to Prof. Edgar Heilbronner on the occasion of his 80th birthday An account is given of the historical development of Avogadros hypothesis, and of the principal methods of determining Avogadros constant which have been used over the past 200 years. These include the kinetic theory of gases, Brownian motion, measurement of the electron charge, black-body radiation, alpha particle emission, and X-ray measurements.
early in the 17th century, and was supported later in the century by Isaac Newton and Robert Boyle. Boyle developed his mechanical philosophy around the concept that matter consisted of particles in motion.
The beginning of modern chemistry is commonly attributed to the publication in 1789 of The Elements of Chemistry by Lavoisier [1]. In this book he stressed the importance of quantitative measurements, and emphasised the principle of the conservation of matter in chemical reactions. He also revived the idea of chemical elements which were substances that could not be broken down to anything simpler by chemical means; he listed 23 such elements.
Lavoisiers, book led quickly to the development of several empirical laws in chemistry, one of the first being Richters law of equivalent proportions introduced in 1791 [2] 2 ). Richter discovered that if A and B combined with relative weights w a and w b , and A and C combined with relative weights w a and w c , then B and C would combine with relative weights w b and w c . Proust [3], in 1797, found that these relative weights were independent of how the compounds were made. Although this was disputed for many years by Berthollet, it was this evidence that finally led to the distinction between compounds and mixtures.
Atomism grew further in importance with the developments of chemistry early in the 19th century, particularly from Daltons conclusion that atoms of different chemical substances were not identical. Dalton assumed, like Newton, that atoms of the same substance repelled one another (to explain gas pressures), but he further assumed that atoms of different species did not repel one another. From this he arrived at his law of partial pressures; the total pressure of a gas is the sum of the pressures from the gases individually 3 ). This is a true result obtained from an incorrect argument.
Dalton held that atoms of different elements differed in size, weight, and number per unit volume, and he concluded that, when two elements combined to form compounds, they did so in different simple proportions of their numbers of atoms. He published his law of multiple proportions in 1804. An important step in his analysis was that, when only one combination of two elements is known, it was assumed to be a binary compound unless there was some evidence to the contrary, and from this Dalton drew up the first table of the relative weights of atoms (taking hydrogen as unity) in 1803.
In France at the beginning of the 19th century, a young chemist called Gay-Lussac was working with Berthollet on the physical properties of gases. With Humboldt [6] he did a number of experiments to examine the composition of air and how this varied