Amongst the many purposes for which caustic soda is used is the preparation of oxygen and hydrogen. Water, to which a little sulphuric acid has been added, is split up by a current into oxygen and hydrogen, as we saw in Chapter V. This method may be used for the preparation of these two gases on a commercial scale, but more usually a solution of caustic soda is used as the electrolyte. If the oxygen and hydrogen are not to be used at the place where they are produced, they are forced under tremendous pressure into steel cylinders, and at a lantern lecture these cylinders may be seen supplying the gas for the lime-light. Although the cylinders are specially made and tested for strength, they are covered with a sort of rope netting; so that if by any chance one happened to burst, the shattered fragments of metal would be caught by the netting, instead of flying all over the room and possibly injuring a number of people. A large quantity of hydrogen is prepared by this process for filling balloons and military airships. CHAPTER XXIV THE RÖNTGEN RAYS In the chapter on electricity in the atmosphere we saw that whereas air at ordinary pressure is a bad conductor, its conducting power increases rapidly as the pressure is lowered. Roughly speaking, if we wish to obtain a spark across a gap of 1 inch in ordinary air, we must have an electric pressure of about 50,000 volts. The discharge which takes place under these conditions is very violent, and it is called a “disruptive” discharge. If however the air pressure is gradually lowered, the discharge loses its violent character, and the brilliant spark is replaced by a soft, luminous glow.