In 1799, Joseph Louis Proust was the first to recognize that chemical compounds may have a definite, constant composition. Proust formally stated a law of constant proportions. This law raised the possibility that elements might have certain characteristic tendencies to combine with each other. One gram of hydrogen, for example, was known always to combine with eight grams of oxygen in the formation of water.

After John Dalton's statement of the atomic theory, this observation could be expressed in terms of atoms. That is, each type of atom appeared to have some specific tendency to combine with other atoms. The first clear statement of this idea did not appear until about 1852, however. One reason for the delay in formulating this concept was that the techniques of chemical analysis were still too crude to yield unequivocal data about the composition of compounds. Also, the relationship among atoms, molecules, and chemical...