ABSTRACT
Eighteenth century chemists recognized a remarkable fact of chemistry, that elements react together in fixed proportions. We can mix oxygen and hydrogen in any proportion we care to, but in getting them to react to form water, they will do so only in the proportion of seven to one by weight, or one to two by volume. Other elements are found combined with one another in more than just one proportion, for instance carbon and hydrogen as methane and ethane, and oxygen and nitrogen as nitrous oxide and nitric oxide. Nonetheless, the proportions were always fixed and varying intermediate proportions were not to be found. In 1808, John Dalton resuscitated the atomic hypothesis in chemistry to explain this phenomenon. Dalton proposed that associated with each chemical element is a kind of atom. When chemical elements react together to form chemical compounds, what is happening is that the atoms of the elements are combining together to form clusters of atoms (molecules). If compounds are composed of identical clusters of this sort, then one would expect the elements to react together in a certain proportion, the same proportion indeed as the total masses of the different kinds of atom in the molecule. Nature might in some cases allow different kinds of molecules made up of the same elements and this would explain the case of the oxides of nitrogen.
