The discovery of X-rays — or ‘unknown radiation' gave scientists a new playground. Henri Becquerel, a French physicist, was one of the those intrigued by the recently-discovered X-radiation and conducted his own experiments to further research the subject. He thought that phosphorescent materials might emit a similar kind of penetrating radiation, just like X-rays. Becquerel's first experiments with uranium salts seemed to be on the right track. The uranium salts were illuminated by bright sunlight, after which they seemingly produced penetrating invisible radiation, imprinting silhouettes on on a photographic plate. But that wasn't really the case.
On March 2, 1896 Becquerel admitted that his hypothesis that the sunlight ‘charged' the uranium salts in some way was false:
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…
After further research into the nature of the uranium salts, Becquerel finally correctly understood the phenomenon: the penetrating radiation was not a product of a reaction of the sample with the sun; it came from the uranium itself. Thus began a new era of physics. For his discovery and for further research with Marie and Pierre Curie, the physicists were awarded a Nobel Prize in Physics in 1903.
Marie Curie perfectly captured the nature of scientific research. She is quoted by Eve Curie Labouisse in the book Madame Curie: A Biography:
I am among those who think that science has great beauty. A scientist in his laboratory is not only a technician: he is also a child placed before natural phenomena which impress him like a fairy tale. We should not allow it to be believed that all scientific progress can be reduced to mechanisms, machines, gearings, even though such machinery also has its beauty. Neither do I believe that the spirit of adventure runs any risk of disappearing in our world. If I see anything vital around me, it is precisely that spirit of adventure, which seems indestructible and is akin to curiosity.
Another interesting fact regarding radioactivity. You probably know that Marie Curie died due to her long-term exposure to radiation. While the new science was fascinating, there was little research into the safety aspect — as such, there was no knowledge of the damaging effect of ionizing radiation and no precautions were taken. The lack of safety by today's standards is really astonishing — for example, Marie Curie carried test tubes with radioactive contents in her pocket. In fact, she and her surroundings were so exposed that to this day Marie Curie's lab papers and even personal cookbook are still radioactive. The papers are stored in lead-lined boxes and can only be read by those wearing special protective gear.