The half-life of ${\mathrm{Ac}}^{222}$ has been determined and an upper limit has been set for the half-life of ${\mathrm{Pa}}^{225}$. An additional short-lived chain collateral to the actinium $4n+3$ natural radioactive family has also been partially identified. This chain decays as follows: ${\mathrm{U}}^{227}$→${\mathrm{Th}}^{223}$→${\mathrm{Ra}}^{219}$→${\mathrm{Em}}^{215}$→${\mathrm{Po}}^{211}$ →${\mathrm{Pb}}^{207}$.

• Received 15 October 1951

DOI:https://doi.org/10.1103/PhysRev.85.429