<P> I will insist particularly upon the following fact, which seems to me quite important and beyond the phenomena which one could expect to observe: The same crystalline crusts (of potassium uranyl sulfate), arranged the same way with respect to the photographic plates, in the same conditions and through the same screens, but sheltered from the excitation of incident rays and kept in darkness, still produce the same photographic images . Here is how I was led to make this observation: among the preceding experiments, some had been prepared on Wednesday the 26th and Thursday the 27th of February, and since the sun was out only intermittently on these days, I kept the apparatuses prepared and returned the cases to the darkness of a bureau drawer, leaving in place the crusts of the uranium salt . Since the sun did not come out in the following days, I developed the photographic plates on the 1st of March, expecting to find the images very weak . Instead the silhouettes appeared with great intensity...One hypothesis which presents itself to the mind naturally enough would be to suppose that these rays, whose effects have a great similarity to the effects produced by the rays studied by M. Lenard and M. Röntgen, are invisible rays emitted by phosphorescence and persisting infinitely longer than the duration of the luminous rays emitted by these bodies . However, the present experiments, without being contrary to this hypothesis, do not warrant this conclusion . I hope that the experiments which I am pursuing at the moment will be able to bring some clarification to this new class of phenomena . </P> <P> By May 1896, after other experiments involving non-phosphorescent uranium salts, he arrived at the correct explanation, namely that the penetrating radiation came from the uranium itself, without any need for excitation by an external energy source . </P> <P> There followed a period of intense research into radioactivity, including the determination that the element thorium is also radioactive and the discovery of additional radioactive elements polonium and radium by Marie Skłodowska - Curie and her husband Pierre Curie . </P> <P> In 1903, Becquerel shared the Nobel Prize in Physics with Pierre and Marie Skłodowska - Curie "in recognition of the extraordinary services he has rendered by his discovery of spontaneous radioactivity". </P>

When did the book of henery come out