Before I pass on, I want to refer to the beautiful burner that I have here. It is the burner used by the Whitechapel stall-keepers on a Saturday night (Fig. 30). (Fig. _a_ is an enlarged drawing of the burner.) Just let me explain the science of the Whitechapel burner. First of all you will see the man with a funnel filling this top portion with naphtha (_c_). Here is a stop-cock, by turning which he lets a little naphtha run down the tube through a very minute orifice into this small cup at the bottom of the burner (_a_). This cup he heats in a friend's lamp, thereby converting the liquid naphtha, which runs into the cup, into a gas. So soon as the gas is formed--in other words, so soon as the naphtha has been sufficiently heated--the naphtha gas catches fire, the heat being then sufficient to maintain that little cup hot enough to keep up a regular supply of naphtha gas. When the lamp does not burn very well, you will often see the man poking it with a pin. The carbon given off from the naphtha is very disposed to choke up the little hole through which the naphtha runs into the cup, and the costermonger pushes a pin into the little hole to allow the free passage of the naphtha. That, then, is the mechanism of this beautiful lamp of the Whitechapel traders, known as Halliday's lamp. 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. [Illustration: Fig. 31.]