Early developments in atomic theory arose from research into heat and combustion, and were made possible by challenges to the rival theories of phlogiston and the caloric. Against the atomists such as Pierre Gassendi, Robert Boyle, and Isaac Newton, some theorists argued for the existence of a subtle fluid called the caloric, which gave heat to bodies. It was up to Rumford and and James Prescott Joule to demonstrate the difficulties of the caloric theory.
When the caloric and phlogiston theories were challenged, some scientists turned their attention more seriously to the atomic theory. The most important early contribution came from Dalton, who, by studying the amounts of various elements in chemical reactions, discovered their proportions could be described as ratios of simple integers. This led him to conclude that matter was not infinitely divisible, but discrete. He assigned the atomic weight 1 to hydrogen, the lightest known element, and described the weights of the other known elements as multiples of hydrogen's mass. Dalton's reasoning appears in A New System of Chemistry:
In all chemical investigations, it has justly been considered an important object to ascertain the relative weights of the simples which constitute a compound. But unfortunately the enquiry has terminated here; whereas from the relative weights in the mass, the relative weights of the ultimate particles or atoms of the bodies might have been inferred, from which their number and weight in various other compounds would appear, in order to assist and to guide future investigations, and to correct their results. Now it is one great object of this work, to shew the importance and advantage of ascertaining the relative weights of the ultimate particles, both of simple and compound bodies, the number of simple elementary particles which constitute one compound particle, and the number of less compound particles which enter into the formation of one more compound particle.It was left to Lavoisier to develop Dalton's insights into a complete system.
If there are two bodies, A and B, which are disposed to combine, the following is the order in which the combinations may take place, beginning with the most simple: namely,
The following general rules may be adopted as guides in all our investigations respecting chemical synthesis.
- 1 atom of A + 1 atom of B = 1 atom of C, binary.
- 1 atom of A + 2 atoms of B = 1 atom of D, ternary.
- 2 atoms of A + 1 atom of B = 1 atom of E, ternary.
- 1 atom of A + 3 atoms of B = 1 atom of F, quarternary.
- 3 atoms of A + 1 atom of B = 1 atom of G, quarternary.
- &c. &c.
From the application of these rules, to the chemical facts already well ascertained, we deduce the following conclusions; 1st. That water is a binary compound of hydrogen and oxygen, and the relative weights of the two elementary atoms are as 1:7, nearly; 2d. That ammonia is a binary compound of hydrogen and azote, and the relative weights of the two atoms are as 1:5, nearly; 3d. That nitrous gas is a binary compound of azote and oxygen, the atoms of which weigh 5 and 7 respectively; that nitric acid is a binary or ternary compound according as it is derived, and consists of one atom of azote and two of oxygen, together weighing 19; that nitrous oxide is a compound similar to nitric acid, and consists of one atom of oxygen and two of azote, weighing 17; that nitrous acid is a binary compound of nitric acid and nitrous gas, weighing 31; that oxynitric acid is a binary compound of nitric acid with oxygen, weighing 26; 4th. That carbonic oxide is a binary compound, consisting of one atom of charcoal, and one of oxygen, together weighing nearly 12; that carbonic acid is a ternary compound, (but sometimes binary) consisting of one atom of charcoal, and two of oxygen, weighing 19; &c. &c. In all these cases the weights are expressed in atoms of hydrogen, each of which is denoted by unity.
- 1st. When only one combination of two bodies can be obtained, it must be presumed to be a binary one, unless some other cause appear to the contrary.
- 2d. When two combinations are observed, they must be presumed to be a binary and a ternary.
- 3d. When three combinations are observed, they must be presumed to be a binary, and the other two ternary.
- 4th. When four combinations are observed, we should expect one binary, two ternary, and one quarternary, &c.
- 5th. A binary compound should always be specifically heavier than the mere mixture of its two ingredients.
- 6th. A ternary compound should be specifically heavier than the mixture of a binary and a simple, which would, if combined, constitute it; &c.
- 7th. The above rules and observations equally apply, when two bodies, such as C and D, D and E, &c. are combined.