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53.  A Luminous Jet

     How many states of matter are there? Modern physicists have counted up no less than seven. Three of them are widely known: gas, liquid, and solid. Strictly speaking, we practically never encounter any others in our daily life. Chemistry has also contented itself with these three for centuries. And only during the last decade has it begun to take an interest in the fourth state of matter, plasma.

    Plasma is also a gas, if you like, but not an ordinary one. Besides neutral atoms and molecules it contains ions and electrons. An ordinary gas also contains ionized particles, and the higher its temperature the more of them it contains. Therefore there is no distinct boundary line between an ionized gas and plasma. But it is conventionally considered that a gas has turned into plasma when it begins to display the principal properties of the latter, say, high electrical conductivity.

    Paradoxical as it may seem at first glance, plasma is the master in the universe. The matter of the Sun and the stars, as well as the gases of outer space are in the plasma state. All this is natural plasma. On Earth it has to be prepared artificially, in special apparatuses called plasmotrons. In them various gases (helium, hydrogen, nitrogen, argon) are converted to plasma by means of an electric arc. The luminous plasma jet is compressed by the narrow channel of the plasmotron nozzle and by a magnetic field, so that a temperature of several tens of thousands of degrees develops in it.

    Chemists had long dreamt of such temperatures, because the role of high temperatures for many chemical processes can hardly be overestimated. Now this dream has come true: a new branch of chemistry known as plasmochemistry, or the chemistry of "cold" plasma, has been born.

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