France, as well as Germany and England, was to take a leading part in furthering the study of radio-activity. In Paris the famous Becquerel family had for three generations devoted themselves to studying phosphorescence. Henri Becquerel, third of the line, said, “I wonder if a phosphorescent substance, such as zinc sulphide, would be excited by X-rays.” He tried the experiment, causing the sulphide to glow with new vigor. From that moment proofs have accumulated that the rays of common phosphorescence such as are emitted by matches, decaying wood and fish, are of kin to the cathode rays which the electrician evokes from any substance whatever when he employs a high-tension current. One day M. Becquerel came upon a remarkable discovery. He noticed that compounds of uranium, whether phosphorescent or not, affected a photographic plate through an opaque covering of black paper, and rendered the adjacent air an electric conductor. Compounds of thorium, similar to those used for incandescent mantles, were found to have the same properties. And here was detected the cause of an annoyance and loss which had long perplexed photographers. Often they had bestowed sensitive paper or plates within wrappers of stout paper, or card, or thick wood, secluded in dark cupboards or drawers. All in vain. At the end of a few weeks or months these carefully guarded surfaces were as much discolored as if they had been for a few minutes exposed, here and there, to daylight itself. All the while each material relied upon as a safeguard had been sending forth a feeble but constant beam; treachery had lurked in the trusted guardian.