manifestations of forces between the atoms. One feels, however, that these great men regarded the idea of atoms as too vague and speculative to be called upon, except as a last resort: and though Voltaire at the end of the eighteenth century could summarize the state of opinion by saying: 'bodies the most hard are looked upon as full of holes like sieves, and in fact this is what they are. Atoms are accepted indivisible and unchangeable,' it was not until 1801, the date of Dalton's Atomic Theory, that the conception of the atom played any considerable part in scientific discovery. Dalton's theory was based on the proportions by weight of the different elements in various chemical compounds; he showed that these proportions are exactly those which would exist if each element consisted of a great number of particles, all the particles of any one element being exactly alike, but each element having its own particular kind of particle. He determined the relative weights of the atoms of a number of chemical elements, and he supposed that compound bodies were formed by the union of one or more particles of one element with one or more particles of other elements.
This view gave such a clear-cut and tangible representation of chemical combination, that it was very largely, though not universally, adopted, and caused the conception of the atom to be familiar to every chemist.
Dalton traced the atoms of the different elements in all their migrations from one compound to another by means of their weight; this was a quality they could neither change nor disguise; until quite recently, however, this was about the only quality of the atom of which this could be said. Indeed, with many qualities the way the individuality of the atom is disguised is exceedingly remarkable, and sceptics had perhaps some excuse when
