Atomic Physics: 8th Edition

They suggest themselves almost inevitably when we try to interpret the simple regularities which are at once disclosed when the masses of the substances transformed in chemical reactions are determined quantitatively with the balance. It is established in the first place, that in a reaction the total weight remains unchanged. Secondly, it is found that substances combine only in fixed simple proportions by weight, so that a definite weight of one substance can only enter into reaction with definite weights of a

The most important result, that the mass of an atom is almost wholly concentrated in a very small nucleus