The neon sign is an evolution of the earlier Geissler tube, which is an electrified glass tube containing a “rarefied” gas (the gas pressure in the tube is well below atmospheric pressure). When a voltage is applied to electrodes inserted through the glass, an electrical glow discharge results. Geissler tubes were quite popular in the late 1800s, and the different colors they emitted were characteristics of the gases within. They were, however, unsuitable for general lighting; the pressure of the gas inside typically declined in use. The direct predecessor of neon tube lighting was the Moore tube, which used nitrogen or carbon dioxide as the luminous gas and a patented mechanism for maintaining pressure; Moore tubes were sold for commercial lighting for a number of years in the early 1900s.