Becquerel found later that all the compounds of uranium as well as the metal itself possessed the same property, and, although the amount of action varied slightly for the different compounds, the effects in all cases were comparable. It was at first natural to suppose that the emission of these rays was in some way connected with the power of phosphorescence, but later observations showed that there was no connection whatever between them. The uranic salts are phosphorescent, while the uranous salts are not. The uranic salts, when exposed to ultra-violet light in the phosphoroscope, give a phosphorescent light lasting about ·01 seconds. When the salts are dissolved in water, the duration is still less. The amount of action on the photographic plate does not depend on the particular compound of uranium employed, but only on the amount of uranium present in the compound. The non-phosphorescent are equally active with the phosphorescent compounds. The amount of radiation given out is unaltered if the active body be kept continuously in darkness. The rays are given out by solutions, and by crystals which have been deposited from solutions in the dark and never exposed to light. This shows that the radiation cannot be due in any way to the gradual emission of energy stored up in the crystal in consequence of exposure to a source of light. =4.= The power of giving out penetrating rays thus seems to be a specific property of the element uranium, since it is exhibited by the metal as well as by all its compounds. These radiations from uranium are persistent, and, as far as observations have yet gone, are unchanged, either in intensity or character, with lapse of time. Observations to test the constancy of the radiations for long periods of time have been made by Becquerel. Samples of uranic and uranous salts have been kept in a double box of thick lead, and the whole has been preserved from exposure to light. By a simple arrangement, a photographic plate can be introduced in a definite position above the uranium salts, which are covered with a layer of black paper. The plate is exposed at intervals for 48 hours, and the impression on the plate compared. No perceptible weakening of the radiation has been observed over a period of four years. Mme Curie[7] has made determinations of the activity of uranium over a space of five years by an electric method described later, but found no appreciable variation during that period.