Admitting that radio-active bodies can be manufactured to order, are we any nearer explaining their mysterious powers? Speculations as to the future history and applications of these wonder-working bodies press upon even the dullest imagination; if a few grams of earth-born material, containing only a small percentage of the active body, emit light enough to affect the human eye and a photographic plate, as well as rays that penetrate with X-ray power, what degree of luminosity, of actinism and of Röntgenism (if the term may be allowed), is to be expected from an hundred weight of the quintessence of energy purified from interfering matter? And to what uses is this light-generating material to be applied? Are our bicycles to be lighted with discs of radium in tiny lanterns? Are these substances to give us the ‘cheapest form of light?’ Are we about to realize the chimerical dream of the alchemists? Seriously, in what direction is profound study of these substances going to lead us? Will it not greatly extend our knowledge of physical manifestations of energy and their correlation? In what corner of the globe will be found the cheap and convenient supply of the raw material yielding the radio-active bodies? Will not chemists be obliged to re-examine much known material by laboratory methods conducted in the dark? Many of us have worked up pounds of pitchblende to extract the uranium oxids, and in so doing have poured down the waste-pipe or thrown into the dust-bin the more interesting and precious bodies. Whatever the future may bring, scientists are deeply indebted to Becquerel and to Mme. and M. Curie for placing in their hands new methods of research and for furnishing a novel basis for speculation destined to yield abundant fruits. DISCUSSION AND CORRESPONDENCE. _WASHINGTON AS AN EXPLORER AND SURVEYOR._