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Excerpt from The Proceedings of the Institute of Medicine of Chicago, 1921, Vol. 3 The twentieth century has been characterized by an entirely different group of experiments and discoveries - discoveries which have appeared at a rate altogether unparalleled in the history of the world. Most of these developments have concerned the giving up of old theories about the atom and the extension of our knowledge of atomic struc ture and atomic behavior. I shall try to give you a picture of what these developments have been and what is our present point of view regarding atoms, their structure and their relations to other atoms and to the phenomenon of radiation. Let me now run over rapidly a list of ten discoveries which I will call the ten most important advances of the last twenty years. The first discovery on my list is the recent verification of the adumbrations of the Greeks regarding the atomic and the kinetic theories - the proof that, as Democritus had imagined 500 B. C., this world does indeed consist, in every part of it, of matter which is in violent motion. Up to within ten years there were not a few distinguished scientists who withheld their allegiance even from these atomic and kinetic theories of matter. The most illustrious of them was Professor Wil helm Ostwald, who at the world's scientific congress in 1902 publicly maintained that the facts of observation could just as well be explained without as with an atomic theory, but in the preface to a new edition of his Outlines of Chemistry he now says frankly: I am convinced that we have recently become possessed of experimental evidence of the discrete or grained nature of matter for which the atomic hypothesis sought in vain for hundreds and thousands of years. The isola tion and counting of gaseous ions on the one hand and on the other the agreement of the Brownian movements with the kinetic hypothesis justify the most cautious scientist in now speaking of the experimental proof of the atomic theo