These three ‘radio-active’ substances do not possess identical properties; their rays are unequally absorbed and are differently affected in a magnetic field; moreover radium emits visible rays, while polonium does not. Nor have they the same chemical affinities; polonium belongs to the bismuth group, radium to the barium and actinium to the titanium series. They have not been separated perfectly from their analogues, and consequently their chemical properties and the actual intensity of their physical activities is very imperfectly known. The difficulties of securing even small quantities of crude materials are enormous; Fritz Giesel obtained from one thousand kilograms of raw material only fifteen grams of active compounds, and Mme. Curie, operating on half a ton of the residues of uranium from a chemical manufactory, got about two kilograms of barium chloride rich in radium, but the percentage of active substances in these mixtures is unknown. Radium is spontaneously luminous, and all the bodies emit rays that excite phosphorescence in gems, fluorite and other minerals; they communicate radiant energy to inactive substances, and they exert chemical action, transforming oxygen into ozone and producing changes in the color of glass and of barium platino-cyanid. Through the enterprise and liberality of the Smithsonian Institution, and by the courtesy of Professor Langley, I have enjoyed the opportunity of studying small specimens of these rare and costly substances; they comprised ten grams of ‘radio-active substance’ prepared by a manufacturing chemist of Germany and smaller quantities of ‘radium’ and of ‘polonium’ from Paris. On removing the wrappings of the German specimens in a dark room, they were seen to emit greenish-white light that gave to the enveloping papers a peculiar glow, similar to the fluorescence produced by Röntgen rays. Simple tests of the radium showed that it gave the usual reactions of barium; on boiling it with water it lost its luminosity, but on heating to dull redness this property returned in the dark. It also caused a barium platino-cyanid screen to fluoresce. Experiments to test the actinic power of these bodies gave interesting results; on exposing sections of photographic plates, at distances of five inches, from two to twelve minutes, bands were obtained varying in intensity with the duration of action. By exposing sensitive plates behind negatives to the radiant materials from two to three hours, excellent transparencies were secured; on substituting Eastman’s bromide paper good prints were obtained.