John Dalton

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John Dalton (1766-1844) was an English scientist. He taught mathematics and physical sciences at New College, Manchester. Dalton revived the atomic theory of matter, which he applied to a table of atomic weights and used in developing his law of partial pressures (Dalton's law). He was color-blind and studied that affliction, also known as Daltonism.

Another important achievement of Dalton's was his law of multiple proportions: If two elements form more than one compound, the weights in different compounds of an element are ratios of integral numbers. For instance, consider the elements nitrogen and oxygen. The element oxygen occurs in the compounds NO and NO₂. The ratio of oxygen weights (1:2) in these compounds contains the integral numbers 1 and 2. (Note that in modern chemistry the concept "weight", used by Dalton, is replaced by "number of atoms". Now we say that the ratio of numbers of O-atoms in different NO_x compounds is a rational number. Or, by extension of Dalton's law of multiple proportions, the subscripts m, n, k, ... in a compound A_mB_nC_k⋅⋅⋅ are integral numbers.) This law led Dalton to take the mass of the lightest element, hydrogen, as unit of atomic mass.

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