  The status of atomism underwent a transformation when John Dalton formulated his version of chemical atomism early in the nineteenth century. His atomic theory had implications for the way chemicals combine by weight and, for the first time, it would seem, a direct path was uncovered that took scientists from experimental measurement to a property of atoms, namely, their relative weight. An assessment of the fruitfulness and epistemological status of Dalton's atomism can easily be distorted if we are uncritically influenced by the recognition that Dalton's basic assumptions are in fact correct from a modern point of view. This section will involve a summary of the basic features of Dalton's chemistry as he published it in 1808 together with the way in which its content can be usefully expressed using chemical formulae introduced by Berzelius five years later. The following sections will explore, first the issue of the epistemological status of this early version and then the nature and status of subsequent elaborations of chemical atomism during the first half century of its life. These latter issues very much involve developments in organic chemistry, issues that have been highlighted by historians of chemistry only in the last few decades.