metrical progression, the loss per second being
proportional to the mass of substance still left at the moment, and
independent of its state of concentration or dilution. This type of
reaction is well known in chemistry to mark a mono-molecular
change, where each molecule is dissociated or altered in structure
independently. If two or more molecules were concerned simultaneously,
the rate of reaction would depend on the nearness of the molecules
to each other, that is, to the concentration of the material. (4) The
amount of energy liberated by the change of a given mass of material far
transcends the amount set free by any known ordinary chemical action.
The activity of radium decays so slowly that it would not sink to half
its initial value in less than some two thousand years, and yet one
gramme of radium emits about 100 calories of heat during each hour of
its existence.

The energy of radio-activity is due to chemical change, but clearly
to no chemical change hitherto familiar to science. It is an atomic
property, characteristic of a given element, and the atoms undergo the
change individually, not by means of interaction among each other. The
conclusion is irresistible that we are dealing with a fundamental
change in the structure of the individual atoms, which, one by one, are
dissociating into simpler parts. We are watching the disintegration
of the "atoms" of the chemist, hitherto believed indestructible and
eternal, and measuring the liberation of some of the long-suspected
store of internal atomic energy. We have stumbled on the transmutation
dreamed by the alchemist, and discovered the process of a veritable
evolution of matter.

The transmutation theory of radio-activity was formulated by Rutherford
(Rutherford, "Radio-activity" (2nd edition), Cambridge, 1905, page 307.)
and Soddy in 1903. By its light, all recent work on the subject has been
guided; it has stood the supreme test of a hypothesis, and shown power
to suggest new investigations and to co-ordinate and explain them, when
carried out. We have summarised the evidence which led to the conception
of the theory; we have now to consider the progress which has been made
in tracing the successive disintegration of radio-active atoms.

Soon after the statement of the transmutation theory, a striking
verification of one of its consequences appeared. The measurement of
the magnetic and electric deflection of the alpha-rays suggested
to Rutherford the ide