Although he dabbled in meteorology and studied color-blindness, the contribution for which Dalton is most famous, without question, is his atomic theory and the list of atomic weights for the known elements that he created.
Dalton was born on approximately September 5, 1766 in Eaglesfield, England. Since his Quaker parents did not register his birth officially, no one knows his exact birth date.
Strongly influenced by his Quaker faith, Dalton grew up in modest surroundings and continued to live simply throughout his life. He attended the local school at Eaglesfield until the age of 11. A year later, he returned as a teacher at the same school, confronted with the task of instructing a number of students his own age or older. Then, as throughout his life, Dalton continued studying a number of topics on his own.
At the age of 15, Dalton became an assistant at the Quaker school in Kendal. About four years later, he was appointed principal of the school, a post he held for eight more years. While at Kendal, Dalton also offered a series of public lectures on natural philosophy. The series was unsuccessful, however, as Dalton was shy and a less than fascinating speaker.
In 1793, Dalton accepted an assignment as tutor in mathematics and natural philosophy at the New College in Manchester. Before long, however, he decided that the post took far more time than he cared to give and, in 1799, he resigned his post at New College. For the rest of his life, he supported himself as a private tutor in mathematics and natural philosophy.
Dalton's earliest scientific interests were in the field of meteorology. For 57 years he kept a daily record of the temperature, barometric pressure, rainfall, dew point, and other weather conditions. In his lifetime, he accumulated more than 200,000 individual observations.
Another topic of interest was color-blindness , a condition that afflicted both Dalton and his brother. His first scientific paper, read to the Literary and Philosophical Society of Manchester on October 31, 1794, dealt with this topic. As a consequence of his interests, the condition of color- blindness was given the name (and is still sometimes referred to as) Daltonism.
Without question, the contribution for which Dalton is most famous is his atomic theory. Dalton's interest in weather caused him to think about the nature and composition of air. He eventually concluded--as had a few scholars before him--that air consists of tiny, individual particles. But Dalton went beyond the musings of his predecessors and hypothesized that all forms of matter--solid, liquid, and gas--consisted of these tiny particles.
He first developed a formal theory about these particles between 1803 and 1805. The ideas he presented were by no means new ones. The Greek philosopher Democritus of Abdera had proposed such a theory of matter in the fourth century B.C. He had called the tiny particles atomos, Greek for "indivisible."
Democritus' ideas were quite different from those of Dalton. He used the term atomos to describe an abstract, theoretical concept, not the concrete, marble-like particles Dalton had in mind. The Greek's ideas were not widely accepted by his contemporaries and, over the centuries, they became less fashionable than competing theories of matter proposed by Aristotle.
Still, Democritus' ideas never really died out. They filtered in and out of the writings of natural philosophers and scientists for 22 centuries until Dalton expressed them in modern form.
The atoms that Dalton had in mind (for he retained the ancient Greek term) were tiny, indivisible particles that constituted all chemical elements. If you could imagine looking at a piece of gold, for example, with the very highest magnification imaginable, what you would see is a collection of gold atoms. Similarly, an examination of a piece of copper under maximum magnification would reveal a collection of copper atoms.
The atoms of any one element, Dalton said, were all exactly alike. A single gold atom would look like any other gold atom anywhere in the world. But atoms of different elements were different from each other. A gold atom looked different from a copper atom.
Dalton's theory also dealt with the composition of compounds. The smallest particle of any compound, he said, was a compound atom. Thus, he taught that water was composed of compound water atoms. Two decades later, Amedeo Avogadro was to clarify the difference between atoms and compound atoms, which he proposed calling molecules.
Dalton is called the father of modern atomic theory partly because of his clear statement of that theory and partly because of his emphasis on atomic weights. No proponent of the concept of atoms had previously made clear the fact that atoms must have weights that can be determined experimentally. Dalton did. He said that finding the weights of atoms was a relatively straight-forward task that any chemist could accomplish.
For example, he said, suppose that we assume that the elements oxygen and hydrogen are both composed of atoms. When oxygen gas combines with hydrogen gas to form water, then, what happens on the simplest level is that one atom of oxygen combines with one atom of hydrogen to form one compound atom of water, or: H + O--- HO (Dalton had no way of knowing the actual combining ratio of hydrogen and oxygen in water [2:1], so he assumed the simplest possible ratio [1:1]).
Next, Dalton said, it is easy to determine in the laboratory what the weight ratio of hydrogen to oxygen is. A fixed weight of hydrogen will consume eight times its weight in oxygen to form water. It followed that each atom of oxygen must weigh eight times as much as each atom of hydrogen.
Dalton's discussion of atomic weights gave chemists a concrete plan for exploring at least this aspect of the atomic theory. Within a few years, Dalton had prepared a list of atomic weights for the known elements.
Dalton's theory was quickly accepted by the vast majority of chemists. One reason for its rapid success was that it explained certain experimental results that had recently been announced, most notably Joseph Proust's law of constant composition which stated that a chemical compound always contains the same constituents in the same proportions.
In his later years, Dalton was flooded with honors and awards. He died quietly at his home in Manchester on July 27, 1844.
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