On April 30, 1897, Joseph John (J.J.) Thomson (1856-1940) announced that cathode rays were negatively charged particles which he called 'corpuscles.' He also announced that they had a mass about 1000 times smaller than a hydrogen atom, and he claimed that these corpuscles were the things from which atoms were built up. Later in 1897, he wrote:
"...we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much farther than in the ordinary gaseous state: a state in which all matter-that is, matter derived from different sources such as hydrogen, oxygen, etc.-is of one and the same kind; this matter being the substance from which the chemical elements are built up." (J.J. Thomson (1897). "Cathode Rays," Philosophical Magazine 44, 295.)
He had leaped to the conclusion that the particles in the cathode ray (which we now call electrons) were a fundamental part of all matter. This was reaching quite far beyond what he had actually discovered. As he was to recall much later:
"At first there were very few who believed in the existence of these bodies smaller than atoms. I was even told long afterwards by a distinguished physicist who had been present at my  lecture at the Royal Institution that he thought I had been `pulling their legs.' " (J.J. Thomson (1936). Recollections and Reflections. G. Bell and Sons: London. p. 341.)
Thomson's corpuscle hypothesis was not generally accepted, even by British scientists, until he spoke of it again in 1899. By this time, George Francis FitzGerald (1851-1901), an Irish physicist, had suggested that Thomson's 'corpuscles' making up the cathode ray were actually free electrons. In fact, this suggestion was published as a commentary to the publication of Thomson's April 30, 1897 lecture in which he first announced his results. Thomson himself continued to use the term corpuscle until 1913.
Other people had measured the e/m ratio or suggested that the cathode rays were composed of particles, but Thomson was the first to say that the cathode ray was a building block of the atom. It was a risky thing, but he was proved right and for his courage he is remembered as the discoverer of the electron.
Walter Kaufmann deserves special mention before leaving this subject. In 1897, he had better data than Thomson and had it months before him. However, Kaufmann was a follower of a philosophy called "positivism," championed at that time by Ernst Mach (whose name is used today to signify the speed of sound - Mach one; twice the speed of sound - Mach two, and so on). Positivism allowed explanations of events which were based on sensory experience only. Since submicroscopic particles were not seen by the human senses, but rather inferred from the data, Kaufmann could not bring himself to the "corpuscle hypothesis" that Thomson announced. So it was that Kaufmann missed out on a great discovery and become a footnote to history. By the way, Kaufmann was convinced by 1901 of the electron's existence and became a leading experimenter working to determine more about it.