Curve _A_ shows the variation with time of the heat emission of the radium and curve _B_ of the emanation. The sum total of the rate of heat emission of the radium and the emanation together, was at any time found to be equal to that of the original radium. The maximum heating effect of the tube containing the emanation from 30 milligrams of radium bromide was 1·26 gram-calories per hour. The emanation together with the secondary products which arise from it, obtained from one gram of radium, would thus give out 42 gram-calories per hour. The emanation stored up in the radium is thus responsible for more than two-thirds of the total heat emission from radium. It will be seen later that the decrease to a minimum of the heating effect of radium, after removal of the emanation, is connected with the decay of the excited activity. In a similar way, the increase of the heating effect of the emanation to a maximum some hours after removal is also a result of the excited activity produced by the emanation on the walls of the containing vessel. Disregarding for the moment these rapid initial changes in heat emission, it is seen that the heating effect of the emanation and its further products, after reaching a maximum, decreases at the same rate as that at which the emanation loses its activity, that is, it falls to half value in four days. If _Q__{max.} is the maximum heating effect and _Q_{t}_ the heating effect at any time _t_ later, then $$ \frac {Q_t} {Q_{max}} = e^{–λt} $$ where λ is the constant of change of the emanation. The curve of recovery of the heating effect of radium from its minimum value is identical with the curve of recovery of its activity measured by the α rays. Since the minimum heating effect is 25 per cent. of the total, the heat emission _Q_{t}_ at any time _t_ after reaching a minimum is given by $$ \frac {Q_t} {Q_{max}} = \cdot25 + \cdot75 (1 − e^{–λt}) $$, where _Q__{max.} is the maximum rate of heat emission and λ, as before, is the constant of change of the emanation.