"The first important discovery in the subject of radio-activity was made in February, 1896, by M. Henri Becquerel, who found that a uranium salt [potassium uranyl sulfate], the double sulphate of uranium and potassium, emitted some rays which gave an impression on a photographic plate enveloped in black paper. These rays were also able to pass through thin plates of metals and other substances opaque to light. ...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 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 is 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."