The main elements of the Mendeleev periodic system was discovered in the XIX and XX century. This is due to the fact that the majority of them in a natural form is extremely rare, and to find them (or get), you need a certain level of technological development. Each element has its own history of discovery. Perhaps the most instructive - helium, because until the late 1930s, scientists could not conclusively disprove the assumption that somewhere in the universe there may be chemical elements that do not exist on Earth. If it were true, would be called into question one of the basic principles of modern science, according to which all we know at the moment the laws of nature have acted and will act always and at all points of the universe (this is the Copernican principle).
A key role in the history of the discovery of helium played Norman Lockyer, the founder of one of the world's foremost scientific publications - journal Nature. In preparation for the release of the magazine, he met with the London scientific establishment, and became interested in astronomy. It was a time when, inspired by the discovery of the Bunsen-Kirchhoff, astronomers began to study the spectrum of light emitted by stars. Lockyer himself managed to make a number of important discoveries - in particular, it is the first to show that sunspots are cooler than the rest of the solar surface, and the first to point out the presence of the Sun of the outer shell, calling it the chromosphere. In 1868, exploring the light emitted by atoms in prominences - huge plasma emission from the surface of the Sun - Lockyer noticed several previously unknown spectral lines (see. Spectroscopy). Attempts to get the same line in the lab have failed, from which Lockyer concluded that he had discovered a new chemical element. Lockyer named it helium, from the Greek helios - «The Sun».
Scientists have wondered how to treat the appearance of helium. Some suggested that the interpretation of the spectra of prominences was a mistake, but this viewpoint is less supporters, as a growing number of astronomers were able to observe the line Lockyer. Others have argued that on the Sun there are elements that do not exist in the world - which, as already mentioned, contrary to the main provisions of the laws of nature. Still others (they were a minority) felt that someday He will be found on Earth.
At the end of 1890 Lord Rayleigh and Sir William Ramsay conducted a series of experiments that led to the discovery of argon. Ramsay remade his installation to use it to explore the gases given off by uranium-bearing minerals. The spectrum of these gases Ramsay discovered unidentified lines and several colleagues sent samples for analysis. After receiving a sample, Lockyer immediately recognized the line that more than a quarter century ago, he watched in the sunlight. The mystery was solved by helium gas, of course, is to the sun, but it also exists here on Earth. In our time, this gas is best known in everyday life as a gas to inflate the balloons and airships (see. Graham's Law), and science - thanks to its use in cryogenics technology to achieve ultra-low temperatures.