THE year I895, in which were published Lenard's experiments on the cathode rays and R6ntgen's discovery of the X-rays, may be said to mark the beginning of a new and mast fruitful period in physical science. The first decade of this period will ever be memorable for great achievements, both in the discovery of new facts and in the framing of hypotheses, which have so materially advanced our knowledge of the constitution of matter. Among the experimental researches of this period none, perhaps, will stand out more conspicuously than those which have brought to light the carriers of radio-activity and which have culminated in the discovery and separation of that most powerfully radio-active element, radimn. It seems hardly within the scope of this report to review, even briefly, the very ntunerous discoveries, made by a host of investigators working in widely different fields, which prepared the way for the experimental inquiries of Professor and Madame Curie; but the starting-point of these researches was the discovery in I896, by the late Henri Becquerel, of those wonderful radiations which their discoverer called uranium rays, and which, under the more fitting name of Becquerel rays, we now regard as a manifestation of radio-activity.
A study of the phosphorescence of certain uranium salts, such as the double sulphate of ammonium or potassium and urarlium, led Mme. Curie to undertake a systematic examination of other substances for Becquerel rays, and she found that thorium compounds also emit them.
Qttantitative determinations of the radio-activity of various uranium and thorium minerals then disclosed the fact that this property was not proportional to their uranium or thorium contents, and that some of the minerals were indeed several times as radio-active as the free elements. Having