Dalton · Atomic theory
Why do substances always combine in the same ratios?
That time, that place
A teacher raised among Quakers. He could not distinguish colors himself, and was the first to study and describe color blindness. He kept a daily weather record for fifty-seven years without missing a day.
Why this question
Chemists had established one fact: hydrogen and oxygen combine to make water in a fixed ratio by weight. Add more of one and the excess simply remains.
Why always the same ratio? If matter were a continuous substance, any proportion should be possible.
What was found
It follows if matter is made of indivisible grains that combine in fixed numbers — one to one, one to two. Numbers cannot be halved, so the ratio is fixed.
The atom Democritus had reached by argument 2,200 years earlier became something you could calculate with.
The decisive move was weight. Dalton held that each kind of atom has its own weight, and built a table of relative weights with hydrogen set at one. You could not see an atom, but now you could measure one.
Many of his early values were wrong, because he assumed water was one hydrogen to one oxygen.
Matter was continuous, so any proportion should combine
The unvarying weight ratios in every compound
Physical confirmation that individual atoms exist came only in the early twentieth century