John Dalton, 6 September 1766 – 1844.
As a man John Dalton may have been a little on the boring side with the rather odd hobby of keeping daily weather records from 1787 when he was 21, till his death in 1844. (He entered more than 200,000 observations!)But it was in fact while trying to explain the behaviour of air as a mechanical system of gases in a mixture where each gas behaved independently of the others that he managed to formulate a theory about atoms that we still study in school.
This theory states that all matter is made of basic particles called atoms that can neither be destroyed nor divided, that all elements have atoms with properties unique to that element and that atoms of elements combine to form new compounds which are essentially a rearrangement of the original atoms.
He was also colour blind from birth as was his brother and this led him to study this defect and establish it as familial, long before Mendel played with his pea plants and worked out the complexities of legume sex. Red-green colour blindness is still called Daltonism.
It is amazing to think that he was born in a family so poor that he had to begin working as a teacher at the age of twelve and yet he retained the sense of wonder so vital to scientific discovery.
(The diagram above is one of the tests used to detect Daltonism. There is a number in there. Colour blindness might have been quite helpful in prehistoric times as it makes it easier to spot camouflaged animals)