Isotopes and Atomic Weights

Abstract

IN the atomic theory put forward by John Dalton in 1801 the second postulate was: “Atoms of the same element are similar to one another and equal in weight.” For more than a century this was regarded by chemists and physicists alike as an article of scientific faith. The only item among the immense quantities of knowledge acquired during that productive period which offered the faintest suggestion against its validity was the inexplicable mixture of order and disorder among the elementary atomic weights. The general state of opinion at the end of last century may be gathered from the two following quotations from Sir William Ramsay's address to the British Association at Toronto in 1897:—