During this experiment the dissolved salt was placed in a vessel merely covered with a watch-glass. I made with the same salt two solutions which I kept in sealed tubes for thirteen months; one of these solutions was eight times the strength of the other:— Initial activity of the salt in concentrated solution after evaporation 200 Initial activity of the salt in dilute solution after evaporation 100 The loss of activity of the salt is therefore greater when the amount of solvent is greater, the radio-active energy transmitted to the liquid having a greater volume of liquid to saturate and a greater space to fill. The two specimens of the same salt, which thus had a different initial activity, further increased in activity at very different rates at first; at the end of one day they had the same activity, and the increase of activity now continued in the same manner for both till the limit was reached. When the solution is dilute the loss of activity by the salt is very rapid, as is shown by the following experiments:—Three equal portions of the same radium salt are dissolved in equal quantities of water. The first solution (_a_) is allowed to stand in contact with the air for one hour, and is then evaporated. The second solution (_b_) has a current of air passed through it for one hour, and is then evaporated. The third solution (_c_) is left exposed to the air for thirteen days, and then evaporated to dryness. The initial activity of each of the three salts is:— For portion _a_ 145·2 For portion _b_ 141·6 For portion _c_ 102·6 The limiting activity of the same salt is about 470. We thus see that the greatest part of the effect was produced at the end of one hour. Further, the air current bubbling through solution _b_ for one hour produced little effect. The proportion of salt in solution was about 0·5 per cent. Radio-active energy in the form of an emanation is propagated with difficulty from solid radium in air; it experiences the same resistance to propagation from solid radium in a liquid. When radium sulphate is shaken with water for a whole day, its activity after the operation is practically the same as that of a portion of the same sulphate left exposed to air. On placing the radium salt in a vacuum, all the emanation capable of displacement is withdrawn. However, the radio-activity of a radium chloride kept _in vacuo_ for six days was not sensibly affected by the operation. This experiment shows that the radio-activity of the salt is principally due to the radio-active energy generated within the particles, and which is unaffected by the vacuum. The loss of activity that radium undergoes when in solution is relatively greater for the penetrating rays than for the absorbable rays. The following are examples of this:—