Science and Society

185 1856), Count of Quaregna and Professor of Philosophy at the University of Vercelli in Italy. Avogadro introduced a bold hypothesis - that a standard volume of any gas whatever, at room temperature and atmospheric pressure, contains a number of particles which is the same for every gas.

(Avogadro himself did not have any idea how many gas particles there are in a litre of gas; but we now know that at room temperature and atmospheric pressure, 22.4 litres contain 602,600,000,000,000,000,000,000 particles. This is the same as the number of atoms in a gram of hydrogen. To get some imaginative idea of the size of “Avogardo’s number”, we can think of the fact that the number of atoms in a drop of water is roughly the same as the number of drops of water in all the oceans in the world!)

Avogadro believed that the particles of a gas need not be single atoms, even if the gas contains only a single element. In thes way, he could explain the mysterious proportions of volume observed by Gay-lussac. for example, in the reaction where hydrogen and chlorine combine to form hydrochloric acid, Avogadro assumed that every molecule of hydrogen gas consist of two atoms joined together, and similarly, every molecule of chlorine gas consist of two atoms. Then, in the reaction in which hydrochloric acid is formed, the total number of molecules is not changed by the reaction, which fits with Gaylussac’s observation that the volume occupied by the gasses is unchanged.

Although Avogadro completely solved the problem of reconciling Dalton’s atomic ideas with Gay-lussac’s volume ratios, there was a period of 50 years during which most chemists ignored the atomic theories of Dalton and Avogadro. However, it hardly mattered that the majority of chemists where unconvinced, since the greatest chemist of the period, J¨ons Jakob Berzelius (1779-1849), was an ardent disciple of Dalton’s atomism. His belief more that made up for the other chemists’ disbelief!

After studying medicine at the University of Uppsala in Sweden, Berzelius became a chemist; and over a period of ten years, between 1807 and 1817, he analysed more than two thousand different chemical reactions. He showed that all these reactions follow Proust’s law of definite proportions by weight.

He also continued Dalton’s work on relative atomic weights; and in 1828 he published the first reasonably accurate table of these weights.

Unfortunately, although Berzelius was a follower of Dalton, he did not appreciate the value of Avogadro’s ideas; and therefore confusion about the distinction between atoms and molecules remained to plague chemistry until 1860. In that year, the first international scientific congress in history was held at Karlsruhe, Baden, to try to clear up the confusion about atomic