Light and view were given by means of four thick lenticular glass scuttles, two pierced in the circular wall itself, the third in the bottom, the fourth in the top. These scuttles then were protected against the shock of departure by plates let into solid grooves, which could easily be opened outward by unscrewing them from the inside. Reservoirs firmly fixed contained water and the necessary provisions; and fire and light were procurable by means of gas, contained in a special reservoir under a pressure of several atmospheres. They had only to turn a tap, and for six hours the gas would light and warm this comfortable vehicle. There now remained only the question of air; for allowing for the consumption of air by Barbicane, his two companions, and two dogs which he proposed taking with him, it was necessary to renew the air of the projectile. Now air consists principally of twenty-one parts of oxygen and seventy-nine of nitrogen. The lungs absorb the oxygen, which is indispensable for the support of life, and reject the nitrogen. The air expired loses nearly five per cent. of the former and contains nearly an equal volume of carbonic acid, produced by the combustion of the elements of the blood. In an air-tight enclosure, then, after a certain time, all the oxygen of the air will be replaced by the carbonic acid-- a gas fatal to life. There were two things to be done then-- first, to replace the absorbed oxygen; secondly, to destroy the expired carbonic acid; both easy enough to do, by means of chlorate of potassium and caustic potash.