One of the most important pioneers in the development of what is now known as Modern Atomic Theory, John Dalton was best known for his experiments on the formation of mixed gases, including steam, at various temperatures. These were carried out during the earlier portion of his career, and were published in a series of papers and lectures that detail the way that gases can be absorbed by water or other liquids. This capability of absorption led to Dalton's law, which discussed the law of partial pressures and how this operates in the world of chemistry and physical sciences.
The results of these experiments into water, gas, pressure, and how they all interact with one another led John Dalton to publish a table of relative atomic weights. This included six of the elements, including hydrogen, nitrogen, oxygen, sulfur, phosphorus and carbon. He broke down various gasses and liquids to help isolate these different elements, and then weighed them individually through a series of chemical experiments.
Another area that John Dalton distinguished himself in was the study of color blindness. This has also been termed Daltonism, in recognition of his contributions to the field and because he also suffered from this condition. During his time as a member of the Manchester Literary and Philosophical Society, which he was elected to in 1794, he published his first paper on the issue, theorizing that color blindness was caused by a discoloration located within the eyeball. Although that theory has since been disproven, it was one of the first scientific attempts to describe this condition and since then has been built upon by modern scientists. Other areas of study that he engaged in during this time include the color of the sky, steam, the origin of natural springs, and dew.
However, it's the work that John Dalton completed on the nature of mixed gases and atomic theory that is what he is most well known for. That includes the thermal expansion of gases, how vapors operate at different temperatures and may expand or change pressure, and what applications these results could have. This broke down the nature of gasses into different parts. Dalton's findings were published in Manchester in a series of four critically acclaimed essays, which were then used as the basis for future atomic study into the nature of gasses and their various applications under pressure, which has become modern atomic theory.