In these experiments, the film of radium bromide was so thin, that only a very small percentage of the α particles was stopped by the radium itself. Assuming that each α particle carries the same charge as an ion, viz. 1·1 × 10⁻¹⁹ coulombs, and remembering that half of the α particles are absorbed in the lower plate, the total number _N_ of α particles expelled per second from one gram of radium bromide (at its minimum activity) can be deduced. In two separate experiments where the amount of radium used was ·194 and ·484 mgrs. respectively, the values of _N_ were in close agreement and equal to 3·6 × 10¹⁰. Now it will be shown later that in radium there are three other products in radio-active equilibrium, each of which probably gives out the same number of α particles as radium itself. If this is the case, the total number of α particles expelled per second from 1 gram of radium bromide _in radio-active equilibrium_ is 4_N_ or 1·44 × 10¹¹. Assuming the composition of radium bromide as RaBr₂, the number per second per gram of radium is 2·5 × 10¹⁰. This number will be found to be in very good agreement with that deduced from indirect data (chapter XIII.). The value of _N_ is of great importance in determining the magnitude of various quantities in radio-active calculations. =94. Mass and energy of the α particle.= It has been pointed out that the α rays from radium and polonium are analogous to the Canal rays of Goldstein, for both carry a positive charge and are difficult to deflect by a magnetic field. The experiments of Wien have shown that the velocity of projection of the canal rays varies with the gas in the tube and the intensity of the electric field applied, but it is generally about ⅒ of the velocity of the α particle from radium. The value of _e_/_m_ is also variable, depending upon the gas in the tube. It has been shown that for the α rays of radium _e_ V = 2·5 × 10⁹ and ---- = 6 × 10³. _m_ Now the value of _e_/_m_ for the hydrogen atom, liberated in the electrolysis of water, is 10⁴. Assuming the charge carried by the α particle to be the same as that carried by the hydrogen atom, the mass of the α particle is about twice that of the hydrogen atom. Taking into consideration the uncertainty attaching to the experimental value of _e_/_m_ for the α particle, if the α particle consists of any known kind of matter, this result indicates that it consists either of projected helium or hydrogen. Further evidence on this important question is given in section 260.