Now I go to another point: having obtained the gas, I must set fire to it. It is important to note that the temperature required to set fire to different gases varies with the gas. For instance, I will set free in this bottle a small quantity of gas, which fires at a very low temperature. It is the vapour of carbon disulphide. See, I merely place a hot rod into the bottle, and the gas fires at once. If I put a hot rod into this bottle of coal gas, no such effect results, since coal gas requires a very much higher temperature to ignite it than bisulphide of carbon gas. I want almost--not quite--actual flame to fire coal gas. But here is another gas, about which I may have to say something directly, called marsh gas (the gas of coal-mines). This requires a much higher temperature than even coal gas to fire it. I want you to understand that although all gases require heat to fire them, different gases ignite at very different temperatures. Bisulphide of carbon gas, _e. g._, ignites at a very low temperature, whilst marsh gas requires a very high temperature indeed for its ignition. You will see directly that this is a very important fact. Sulphur gas ignites fortunately at a fairly low temperature, and that is why sulphur is so useful an addition to the wood splint by which to get fire out of the tinder-box.