The founder of that theory, who flourished about 500 B.C., said that every substance is a collocation of a vast number of minute particles, which are unchangeable, indestructible, and impenetrable, and are therefore properly called _atoms_; that the differences which are observed between the qualities of things are due to differences in the numbers, sizes, shapes, positions, and movements of atoms, and that the process which occurs when one substance is apparently destroyed and another is produced in its place, is nothing more than a rearrangement of atoms.
The supposition that changes in the properties of substances are connected with changes in the numbers, movements, and arrangements of different kinds of minute particles, was used in a general way by many naturalists of the 17th and 18th centuries; but Dalton was the first to show that the data obtained by the analyses of compounds make it possible to determine the relative weights of the atoms of the elements.
Dalton used the word _atom_ to denote the smallest particle of an element, or a compound, which exhibits the properties characteristic of that element or compound.

He supposed that the atoms of an element are never divided in any of the reactions of that element, but the atoms of a compound are often separated into the atoms of the elements whereof the compound is composed.

Apparently without knowing that the supposition had been made more than two thousand years before his time, Dalton was led by his study of the composition and properties of the atmosphere to assume that the atoms of different substances, whether elements or compounds, are of different sizes and have different weights.

He assumed that when two elements unite to form only one compound, the atom of that compound has the simplest possible composition, is formed by the union of a single atom of each element.

Dalton knew only one compound of hydrogen and nitrogen, namely, ammonia.