Complex sentence: The Geissler tube is an evacuated glass cylinder with an electrode at each end, containing rarefied gasses such as neon, argon, or air; mercury vapor or other conductive liquids; or ionizable minerals or metals, such as sodium.
Split into simpler sentences: The Geissler tube was an evacuated glass cylinder with an electrode at each end. A Geissler tube contains one or more of the following rarefied (thinned) gasses, such as neon, argon, or air; mercury or other conductive liquids; or ionizable minerals or metals, such as sodium.