These estimates of the rate of production of helium were later modified as new and more accurate data became available. In 1905, I measured the charge carried by the α-particles from a thin film of radium. Assuming that each α-particle carried the ionic charge measured by J.J. Thomson, I showed that 6.2 x 10^10 α-particles were expelled per second per gram of radium itself and four times this number when radium was in equilibrium with its three α-ray products. The rate of production of helium calculated on these data was 240 cubic millimetres per gram per year. In the meantime, by the admirable researches of Bragg and Kleeman in 1904, our knowledge of the character of the absorption of the α-particles by matter had been much extended. It had long been known that the absorption of α-particles by matter was different in many respects from that of the β-rays. Bragg showed that these differences arose from the fact that the α-particle, on account of its great energy of motion, was not deflected from its path like the β-particle, but travelled in nearly a straight line, ionizing the molecules in its path. From a thin film of matter of one kind, the α-particles were all projected at the same speed and lost their power of producing ionization suddenly, after traversing a certain definite distance of air. The velocity of the α-particles in this view were reduced by their passage through matter by equal amounts. These conclusions of Bragg were confirmed by experiments I made by the photographic method. As a source of rays, a thin film of radium C, deposited from the radium-emanation on a thin wire, was used. By examining the deflection of the rays in a magnetic field, it was found that the rays were homogeneous and were expelled from the surface of the wire at an identical speed. By passing the rays through a screen of mica or aluminium, it was found that the velocity of all the α-particles were reduced by the same amount and the issuing beam was still homogeneous. A remarkable result was noted. All α-particles apparently lost their characteristic properties of ionization, phosphorescence and photographic action, at exactly the same point while they were still moving at a speed of about 9,000 kilometres per second. At this critical speed, the α-particle suddenly vanishes from our ken and can no longer be followed by the methods of observation at our command. The use of a homogeneous source of α-rays like radium C at once suggested itself as affording a basis for a more accurate determination of the value of _e/m_ for the α-particle and for seeing whether the value was consistent with the view that the α-particle was a charged atom of helium. In the course of a long series of experiments, I proved that the α-particles, whether expelled from radium, thorium or actinium, were identical in mass and must consist of the same kind of matter.