consume all the particles of carbon, whereas without the draft produced by the chimney the supply of oxygen is insufficient to ignite all the carbon, which then escapes in the form of smoke or soot. [Illustration: _Fig. 212._ _Fig. 213._] It must not, however, be hastily assumed that if the flame does not give out a bright light, therefore the combustion is not complete. As has already been stated, the light of the gas flame is due to the presence of burning particles of solid carbon, which is set free by the combustion of the hydrogen with which it is combined. After it is separated from the hydrogen it immediately assumes a solid form. If the coal gas is mixed with a sufficient quantity of air before it is burned, the oxygen in the latter will be in such intimate contact with the former that the difference of affinity of oxygen for the carbon and hydrogen does not come into play, and as there is enough oxygen for all, the carbon is burnt before it is set free, and as there are then no solid particles in the flame, there is no light. This is illustrated by a “Bunsen burner,” fig. 212, which is much used in chemical laboratories. It consists of a small tube or burner, _a_, which is placed inside of another larger tube _b_. The latter has holes, _c_, _c_, a little below the top of the small tube. The current of gas escaping from the small tube produces what is called an _induced current_ of air in the large tube. This air enters through the holes _c_, _c_, and is mixed with the gas in the tube _b_, and the mixture is burned at _d_. The flame from such a burner gives hardly any light, but the heat is intense, as is shown if a metal wire is held in it for a few seconds, as it will very soon glow with heat. QUESTION 380. _What important difference is there in the structure of the flame of a Bunsen burner and that of an ordinary gas-burner or candle?_