  There is a further component that needs to be added to the content of early atomic chemistry, although it did not originate with Dalton, who in fact did not fully embrace it. Gay Lussac discovered experimentally that when gases combine chemically they do so in volumes that bear an integral ratio to each other and to the volume of the resulting compound if gaseous, provided that all volumes are estimated at the same temperature and pressure. For instance, one volume of oxygen combines with two volumes of hydrogen to form two volumes of steam. If one accepts atomism, this implies that there are some whole-number ratios between the numbers per unit volume of atoms of various gaseous elements at the same temperature. Following suggestions made by Avogadro and Ampere early in the second decade of the nineteenth century, many chemists assumed that equal volumes of gases contain equal numbers of ‘atoms’, with the important implication that relative weights of atoms could be established by comparing vapour densities. As Dalton clearly saw, this can only be maintained at the expense of admitting that ‘atoms’ can be split. The measured volumes involved in the formation of water, for example, entail that, if equal volumes contain equal numbers of atoms then a water ‘atom’ must contain half of an oxygen ‘atom’. The resolution of these problems required a clear distinction between atoms of a chemical substance and molecules of a gas, the grounds for which became available only later in the century. This problem aside, the empirical fact that gases combine in volumes that are in simple ratios to each other became a central component of chemistry, although it should be noted that at the time Gay Lussac proposed his law, only a small number of gases were known to chemists. The situation was to change with the development of organic chemistry in the next few decades.