The Cambridge History of English and American Literature in 18 Volumes (1907–21).
Volume XIV. The Victorian Age, Part Two.
§ 20. The Atomic Theory and Dalton
In 1808, a small book appeared, entitled A new system of Chemical Philosophy, Part 1, by John Dalton. The influence of that book on the development of chemistry, and of physics also, has been very great.
Dalton delivered a lecture in Manchester, in 1803, wherein he said, “An enquiry into the relative weights of the ultimate particles of bodies is a subject, so far as I know, entirely new; I have lately been prosecuting this enquiry with remarkable success.” Many of Dalton’s predecessors, both chemists and physicists, had used, in a vague and general manner, the Greek conception of the atomic structure of matter. Dalton showed how the relative weights of atoms can be determined. By doing that, he brought down the atomic theory to the solid earth, and made it a bold, suggestive, stimulating guide ready for the use of chemists and physicists.
Dalton was not a great experimenter; he generally used the results of other chemists’ experiments. He was a scientific thinker, characterised by boldness and caution. Dalton assumed, as Lucretius had done long before him, that matter has a grained structive; that all the ultimate particles of each particular homogeneous substance are identical, and differ in properties, one of which is their weight, from the particles of all other definite substances; he also assumed that the mechanism of chemical changes, that is, changes wherein homogeneous substances are produced different from those present when the changes began, is the coalescence of atoms of different kinds to form new sorts of atoms.
In order to find the relative weights of atoms, Dalton argued as follows: Analyses and syntheses of water show that eight grains of oxygen unite with one grain of hydrogen to form water. If this change is the union of atoms of oxygen with atoms of hydrogen, to form atoms of water, and if all the atoms of each one of these three homogeneous substances are identical in weight and other properties, it follows that an atom of oxygen is eight times heavier than an atom of hydrogen. If we take the atomic weight of hydrogen as unity—because hydrogen is lighter than any other known substance—then the atomic weight of oxygen is eight, and the atomic weight of water is nine.
In arriving at the conclusion that the atomic weight of oxygen is eight, if the atomic weight of hydrogen is one, Dalton made the assumption that a single atom of oxygen unites with one atom of hydrogen to form an atom of water. He made this assumption because it was simpler than any other. Had he chosen to suppose that two atoms of hydrogen unite with one atom of oxygen, he must have assigned to oxygen the atomic weight sixteen, and to water the atomic weight eighteen.
To make Dalton’s method perfectly general, and quite conclusive in its results, it was necessary to find means for fixing the relative weights of atoms formed by the union of other, simpler, atoms; it was also necessary to find means of determining the number of atoms of each kind which unite to form a more complex atom. A general method for solving these two problems was given to chemistry in 1811–12 by an Italian physical chemist named Avogadro, who brought into science the notion of a second order of minute particles, supplementing the conception of atom by that of molecule.
It is not possible in this brief sketch to indicate the many new fields of investigation which were opened, and made fruitful, by the Daltonian atomic theory. From the many workers who used this theory as a means for pressing forward along new lines of enquiry, two may be selected, since their work is typical of much that was done in chemistry during the first half of the nineteenth century.