Experiments were also made with very thin layers of radium bromide, when, as we have seen (Fig. 40) a very different shape of curve is to be expected. An example of the results is shown in Fig. 43, curves I., II. and III. Curve I. is obtained from radium bromide which has been heated to drive off the emanation, and curves II. and III. from the same substance several days later, when the emanation was again accumulating. The portion _PQ_, which is absent in the first curve, is probably due to the “excited” activity produced by the emanation. By careful examination of the successive changes in the curves after the radium has been heated to drive off the emanation, it is possible to tell the range of the α rays from each of the different products, and this has been done to some extent by Bragg and Kleeman. It will be seen later that the results here obtained support in a novel way the theory of radio-active changes which has been advanced from data of quite a different character. The inward slope of the curve in Fig. 43 due to the radium indicates that the α particles become more efficient ionizers as their velocity decreases. This is in agreement with observations on the β rays. In some cases Bragg also observed that the α particles are the most efficient ionizers just before they lose their power of ionizing the gas. [Illustration: Fig. 43.] Thus we may conclude from these experiments that the α particles from a simple radio-active substance traverse a definite distance in air, at a definite pressure and temperature, and that the ionization ends fairly abruptly. If the rays traverse a sheet of metal, the effective range of ionization is diminished by a distance corresponding to ρ_d_, where ρ is the density of the material compared with air and _d_ its thickness. The α rays from a thick layer of a simple radio-active substance consist of α particles of different velocities, which have ranges in air lying between 0 and the maximum range. The ionization of the particles per unit path is greatest near the end of its range, and decreases somewhat as we approach the radiant source. A complex source of rays like radium gives out four types of rays, each of which has a different but distinct range. From this theory it is possible to calculate approximately the decrease of current to be observed when sheets of metal foil are placed over a large area of radio-active substance. This is the method that has been employed to obtain the curves of Figs. 35 and 38. Suppose a very thin layer of simple radio-active matter is employed (for example a bismuth plate covered with radio-tellurium or a metal plate made active by exposure to the presence of the thorium or radium emanations) and that the ionization vessel is of sufficient depth to absorb the α rays completely.