Tsiolkovsky's first scientific study dates back to 1880–1881. He wrote a paper called "Theory of Gases," in which he outlined the basis of the kinetic theory of gases, but after submitting it to the Russian Physico-Chemical Society (RPCS), he was informed that his discoveries had already been made 25 years earlier. Undaunted, he pressed ahead with his second work, "The Mechanics of the Animal Organism". It received favorable feedback, and Tsiolkovsky was made a member of the Society. Tsiolkovsky's main works after 1884 dealt with four major areas: the scientific rationale for the all-metal balloon (airship), streamlined airplanes and trains, hovercraft, and rockets for interplanetary travel.