IT is now well established that the atoms of radium C and thorium C can break up in two distinct ways, i.e., with the expulsion of either an α or a β particle. It is to be expected from the close analogy between the C products of the various radio-active families that actinium C should also show abnormal disintegration, and, further, it might be anticipated that one of the branch products would emit an α particle with great velocity and corresponding long range. We have made experiments to test this point. A source of actinium active deposit was covered with a sheet of mica equivalent to about 5 cm. air in stopping power of α particles, and the whole placed in an exhausted chamber with a zinc sulphide screen about 2 cm. from the source. The numbers of scintillations appearing on the screen per minute for different pressures of the air inside the apparatus were counted, and thus the falling off of the α particles with “range” determined. The results snowed that in addition to the α particles of actinium C with range 5.4 cm., a small number, about 1 in 600, can penetrate as far as about 6.45 cm. Special experiments showed that the long-range α particles could not be due to radium or thorium impurity, and they must therefore be attributed to the expected new branch product.