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 power of water for heat; and in 1801 appeared his Constitution of Mixed Gases, wherein he proved "the total pressure of a mixture of two gases on the walls of the containing vessel is equal to the sum of the pressures of each gas; in other words, that if one gas is removed the pressure now exerted by the remaining gas is exactly the same as was exerted by that gas in the original mixture."

It may be mentioned that among Dalton's pupils was the celebrated James Prescott Joule (of the "mechanical equivalent of heat" and the "conservation of energy" fame). Both tutor and pupil are in the first rank of scientific investigators, and hence Manchester has the perhaps unique distinction of having been the home of two of the greatest natural philosophers who ever lived.

John Dalton, the Quaker philosopher, was the founder of the atomic theory, and this great generalization is one of the foundation-stones of modern chemistry.

Leucippus appears to have been the first to grasp the idea that matter is composed of ultimate particles or atoms. Democritus of Abdera (born 460 B.C.) developed the atomic theory of Leucippus, and stated that atoms were impenetrable and indivisible. Epicurus (born 340 B.C.) gave mobility to the atoms, and otherwise greatly improved the atomic theory of the Greek philosophers. According to these sages "the world is composed of an innumerable quantity of atoms, mobile,