I. History leading up to the Discovery of Radioactivity
Our story starts in the evening of November 8, 1895. This is the day that Wilhelm Conrad Röntgen discovered X-rays. He realized the importance of his discovery at once. He stayed up all night doing experiments and even ate and slept in the laboratory for a time. His "preliminary communication" on X-rays was turned in on December 28, 1895 and published before the end of the year. (An English translation of the original German publication is located here.) On New Year's Day 1896, Röntgen himself mailed copies of his paper and photographs to scientists around the world.
Although there is much more to be said about the discovery of X-rays, that will wait until another day. The next key step on the way to radioactivity happened in Paris on the evening of January 20, 1896; at the weekly meeting of the French Academy of Science. It was there that Henri Becquerel heard Henri Poincaré describe the recent discovery of X-rays and show photographs of living bone that Röntgen had taken.
It appeared that the X-rays were coming from the glass walls of a cathode ray tube that was made fluorescent by the impact of the cathode rays. (By the way, it would not be until April 1897 when J.J. Thomson discovered that the cathode ray was a stream of electrons.) Poincaré wondered out loud during his talk if X-rays were emitted by other luminescent bodies.
(Fluorescence is the emission of light under stimulation by an external energy source. Phosphorescence is the light continuing to be emitted after the external stimulus ceases. Luminescence is the general term for these two specific behaviors.)
Becquerel was the ideal man to follow up on this suggestion. His father had been the leading authority on phosphorescence for many years. Henri himself was an expert in the study of phosphorescence as well as photography. Also, he had already studied the phosphorescence of some uranium compounds.
So, quite naturally, Becquerel brought his expertise to bear on the problem. The working hypothesis he had was: a substance had to luminesce first in order to emit the penetrating radiation that Röntgen had found. So the stage is set for the discovery of radioactivity.
Becquerel started on his search the very next day, studying several different phosphorescent substances, but with no results. He believed, however, that he would have success in using uranium compounds. In his Nobel Prize lecture of 1903, he said:
". . . notwithstanding the negative experiments with other bodies, I placed great hopes in experimentation with uranium salts, whose phosphorescence I had formerly studied, following the works of my father. Those bodies, which emit and absorb a whole series of harmonic luminous radiations, seem to have a particularly remarkable molecular constitution, at least from the point of view of absorption and phosphorescence."
Many historians of science are under the impression that the selection of uranium was fortuitous, but, as Becquerel himself says, he believed he could suceed with uranium.
Why then did he first experiment with other materials? Why was it not until late February that he did his first uranium experiments? Simple. He had loaned out all his working samples of uranium, so he had nothing to work with! He had to wait for their return and when the samples came back, it was not too soon after that that radioactivity was discovered.
II. The Discovery of Radioactivity
Bequerel's technique was to wrap a photographic plate in black paper, lay the uranium-containing mineral on top and expose the set to sunlight, causing the mineral to glow. He then would develop the plate and examine it for exposure caused by radiation which could penetrate the black paper.
At the February 24, 1896 meeting, he claimed success. He reported that his uranium (after being made to luminesce) emitted rays which fogged the photographic plate. He placed coins and other objects under the crystals and the shapes were reproduced on the photographic plate.
Now, the critical moment approaches. On February 26 and 27, Becquerel made some more experimental setups with crystals on top of photographic plates wrapped (actually double wrapped) in black paper. However, it was cloudy and Becquerel felt he needed strong sunlight. So everything went into the drawer to wait for a better day.
Here is the critical moment. It is now Sunday, March 1, 1896. He decides to develop the plates which have been sitting in the drawer without exposing then to the sunlight. In fact, there isn't any sun on March 1. There was complete cloud cover all day long. Don't believe me? Look at the record. The question also lingers, what was he doing in his lab on Sunday?
Becquerel, of course, found that the plates were fogged by the uranium crystal and that "activation" by the sunlight was not needed. Here is a photo of the discovery plate:
He was a very good scientist and he immediately recognized he had an important discovery. The uranium was emmitting mysterious rays by itself and didn't need the sunlight to get started.
So, he announced his discovery of radioactivity (so named by Marie Curie about 4 years later) the next evening. So it turns out, in the end, he did have some results to discuss. (An English translation of the original French reports by Becquerel on Feb. 24 and March 2 is at Carmen Guinta's classic papers web site.)
On to the discovery of alpha and beta radiation by Rutherford.