I made several other series of determinations of the same kind, but they do not absolutely agree with one another, although the general character of the curves obtained remains the same. It is difficult to obtain very regular results. It may, however, be remarked that the acquisition of activity requires more than one month for its production, and that the most penetrating rays are the most deeply affected by solution. The initial intensity of the radiation which is able to traverse 3 c.m. of air and 0·01 m.m. of aluminium is only 1 per cent of the limiting intensity, whilst the initial intensity of the total radiation is 21 per cent of the total limiting radiation. A radium salt which has been dissolved and recently evaporated to dryness possesses the same power of causing induced activity (and, consequently, of allowing the escape of an emanation), as a specimen of the same salt which, after having been prepared in the solid state, has remained in this condition long enough to have attained its limiting radio-activity. The radiant activity of these two products is, however, quite different; the former is, for example, five times less active than the latter. _Variations of the Activity of Radium Salts on Heating._ When a radium compound is heated, it gives off an emanation and loses activity. The more intense and the more prolonged the heating, the greater is the loss of activity. Thus, on heating a radium salt for one hour to 130°, it loses 10 per cent of its total radiation; on the other hand, heating for ten minutes to 400° produces no apparent effect. Heating to redness for several hours destroys 77 per cent of the total radiation. The loss of activity on heating is more considerable for the penetrating than for the absorbable rays. Thus, heating for several hours destroys about 77 per cent of the total radiation, but the same amount of heating destroys nearly the whole (99 per cent) of the radiation that traverses 3 c.m. of air and 0·01 m.m. of aluminium. If barium-radium chloride be kept fused for several hours (towards 800°) 98 per cent of the radiation capable of traversing 0·3 m.m. of aluminium is destroyed. The penetrating rays may be considered as no longer in existence after intense and prolonged heating.