IN HER recent visit to America, Madame Curie conferred a special honor upon Vassar College by delivering in the chapel on the evening of May fourteenth the only extended address which she made in this country. In a simple, straightforward way she told the story of her great achievement. One realized how, closely environed by all the great realities of human experience, in the face of tremendous difficulties and with limited resources, she had pursued undaunted her search for truth. The discovery of radium gave Madame Curie immediate distinction among scientists on account of the extremely significant contribution she thereby made to the great ultimate problem of physical science, the constitution of matter. The striking properties possessed by radium gave to its discovery a world-wide interest, all the more intense because of the hope which was inspired by the possible healing qualities of the radiations from this new element. That hope is being realized in large measure. It is therefore fitting that this address should have been given by Madame Curie at Vassar and that it should now be circulated among the members of the college under the foundation in memory of Ellen S. Richards, who devoted her life to the public health. This Edition comes with the following contents: Prefatory Note ii The Discovery Of Radium iii Introduction. 1 Historical. 4 Chapter I. 6 Radio-Activity Of Uranium And Thorium. Radio-Active Minerals. 6 Chapter II. 26 Method Of Research. 26 Chapter III. Radiation Of The New Radio-Active Substances. 51 Chapter IV. 119 Communication Of Radio-Activity To Substances Initially Inactive. 119 Nature And Cause Of The Phenomena Of Radio-Activity. 16