A New System of Chemical Philosophy
Matter is atoms — each element one fixed weight, combining only in whole-number ratios.
Burn carbon two different ways and the oxygen that joins it comes in amounts with a suspiciously neat ratio — exactly one to two. Dalton saw the reason: matter comes in countable pieces.
The big idea
Dalton's claim is that everything is built from atoms — tiny, unbreakable particles. Each element (oxygen, carbon, gold) is made of just one kind of atom, all alike; different elements have atoms of different weights. When elements combine into a compound, their atoms join in fixed, whole-number groups: one to one, one to two, two to three — never one and a half.
That last point is the testable one. If atoms are real and only join in whole numbers, then when two elements make several different compounds, the amounts that combine should jump in tidy steps. They do — and that regularity is hard evidence that matter is grainy, not smooth.
How it came about
John Dalton was a Quaker schoolteacher in Manchester, largely self-taught, who kept a daily weather diary for fifty-seven years. He was colour-blind and the first to describe the condition — still called “Daltonism”. Studying how gases mix and dissolve, he was driven to imagine them as crowds of particles of different weights, and from there to a general theory of matter.
He published a first table of atomic weights in 1803 and laid out the full system in this book of 1808. A provincial outsider with no university post, he gave chemistry the quantitative backbone that the great laboratories of Europe would build on for the next century.
Why it mattered
Dalton turned chemistry from a craft of qualities and affinities into a quantitative science. Atomic weights, chemical formulas, balanced equations — the entire grammar of modern chemistry — start here. Half a century later Mendeleev would line the elements up by atomic weight to find the periodic table; everything downstream of that rests on Dalton's decision to weigh the atom.
A way to picture it
Think of money in coins. You can pay one penny or two pennies, but never one-and-a-half pennies — the coin will not split. Atoms are like that: a compound takes whole atoms, never fractions. So when you weigh what combines, the amounts come out in clean ratios — one coin, two coins, three coins — which is exactly the fingerprint Dalton found in the oxides of nitrogen and carbon.
Where it sits
The idea of atoms is ancient — Greek philosophers guessed at it 2,400 years ago — but it stayed pure speculation until Dalton made it weigh. His book stands on Lavoisier's law that mass is conserved and Proust's that compounds have fixed proportions, and it sets up Mendeleev's periodic table. In this Library it is the seed of a long thread: Boltzmann (1877) would later explain heat and gases as the statistics of these atoms, and Rutherford (1911) would split Dalton's “indivisible” atom open to find a nucleus inside.
What cannot be made or destroyed
Chemical analysis and synthesis go no farther than to the separation of particles one from another, and to their reunion. No new creation or destruction of matter is within the reach of chemical agency. We might as well attempt to introduce a new planet into the solar system, or to annihilate one already in existence, as to create or destroy a particle of hydrogen.
Therefore we may conclude that the ultimate particles of all homogeneous bodies are perfectly alike in weight, figure, &c. In other words, every particle of water is like every other particle of water; every particle of hydrogen is like every other particle of hydrogen, &c.
How atoms combine
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.
Weighing the invisible
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.