Chemistry of the Eighteenth and Early Nineteenth Centuries

Chemistry of the Eighteenth and Early Nineteenth Centuries

The chemistry of the eighteenth century had put the science on the road to development by the discovery of many of the gases, some inkling of the permanence of matter, and the true theory in general of combustion and respiration. The great step made in the first third of the nineteenth century was the scientific development of the atomic theory.

The theory first suggested itself to Dalton about 1804, while experimenting on marsh gas and olefiant gas. He noted that marsh gas seemed to contain exactly twice the weight of hydrogen combined in olefiant gas: also that carbonic acid gas seemed to contain just twice as much oxygen as carbonic oxide gas. To explain these facts he went back to the old idea of the atom—that matter is composed of indivisible atoms with definite weights, the ratio of which to the weight of an atom of hydrogen could be expressed by definite numbers. The weight of the smallest particle of a compound, then, would be equal to the sum of the weights of the atoms composing it. Dalton’s new theory was given to the world in Thomson’s System of Chemistry, 1807.

The next step in the theory was made by Gay-Lussac in 1808. This was that a definite volume of oxygen combines with just twice its bulk of hydrogen, and in fact, that there always exists a simple relation between the volumes of gases that combine with each other and that the volume of the compound bears, also, a simple relation to the amount of gas of which the less is used. Thus, three volumes of hydrogen combining with one of nitrogen make two of ammonia; or one of chlorine with one of hydrogen makes two of hydrochloric acid gas. Therefore, Dalton’s law of definite proportions holds good in regard to volumes as well as weights.

The third step in the argument was made by Avogadro in 1811. Noting that variations in temperature and pressure affect the volume of all gases the same, and taking into consideration the laws of Dalton and Gay-Lussac, Avogadro argued that gases must always be so constituted that the number of molecules in any given volume of any gas, whether simple or compound, must be exactly the same for the same temperature and pressure. This law was not at first accepted, but has won its right to be relied upon after long years of rigorous testing. It will be noted that it makes an important distinction between the atom and the molecule: thus the molecule of a simple gas might contain more than one atom of the same kind. This also makes the density of equal volumes of gases represent the relative weights of their atoms.

Still another law relating to atomic weights was discovered by Dulong and Petit in 1819. This is that an element’s specific heat multiplied by its atomic weight is a constant result—about 6.25.

These great laws have probably been the most influential of any in chemistry. They have led to a knowledge of the atomic weights of the elements and of the constitution of compounds which would have been considered the most impossible dream by the chemists, great though they were, of the seventeenth century.

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Chicago: Chemistry of the Eighteenth and Early Nineteenth Centuries in The Library of Original Sources, ed. Oliver J. Thatcher (Milwaukee, WI: University Research Extension Co., 1907), 360–361. Original Sources, accessed March 29, 2023,

MLA: . Chemistry of the Eighteenth and Early Nineteenth Centuries, in The Library of Original Sources, edited by Oliver J. Thatcher, Vol. 8, Milwaukee, WI, University Research Extension Co., 1907, pp. 360–361. Original Sources. 29 Mar. 2023.

Harvard: , Chemistry of the Eighteenth and Early Nineteenth Centuries. cited in 1907, The Library of Original Sources, ed. , University Research Extension Co., Milwaukee, WI, pp.360–361. Original Sources, retrieved 29 March 2023, from