  As Dalton stressed, once the chemical atomic theory is accepted, the promise is opened up of determining the relative weights of atoms by measuring the relative weights of elements in compounds. If an atom of element A combines with an atom of element B to form a compound atom of compound AB, then the relative weights of A and B in the compound as measured in the laboratory will be equal to the relative weights of atoms of A and B. However, there is a serious under-determination of relative atomic weights by measurements of combining weights in the laboratory. If the compound atom in our example were A2B rather than AB then the relative atomic weight of B would be twice what it would be if the formula were AB. Dalton himself attempted to resolve this problem with a simplicity assumption. Formulae were always to take the simplest form compatible with the empirical data. If there was only one compound of A and B known then it was assumed to be AB, whilst if there were two then a more complicated compound, A2B or AB2 became necessary. As is illustrated by the latter example, as well as the problem of the truth of the simplicity assumption there was the problem of its ambiguity. Chemical atomists were to struggle for several decades with various solutions to the problem of arriving at definitive formulae and relative atomic weights, as we shall see.