Before revision: 18–19 In the early 19th century, the scientist John Dalton noticed that chemical substances seemed to combine with each other by a basic unit of weight, and he decided to use the word atom to refer to these unitsas he thought they were indivisible in essence .Pullman (1998).
After revision: 18–19 In the early 19th century, the scientist John Dalton found evidence that matter really is composed of discrete units, and so applied the word atom to those units .Pullman (1998).