It will be remembered that the active agent in the reduction of the ore is the carbonic oxide gas formed by the incomplete combustion of the carbon of the fuel; or what comes to the same thing, the absorption by carbonic acid first produced of another proportion of carbon. The carbon oxide robs the iron oxide of its oxygen to become itself changed into carbonic acid. In reality however the action is more complex than this in its chemical relations; for instance, metallic iron will under certain circumstances act conversely on carbonic acid, and rob it of half its oxygen. The net result of the reactions between carbon, iron, iron oxide, and these gases depends mainly upon the temperature and pressure and upon the relative quantities of each substance present. In the gases escaping from the blast furnace there is always a large quantity (nearly one-third) of carbonic oxide. At the blast furnaces in work during the first half of our century the combustible gases were allowed to burn to waste as they issued from the top of the furnace, in the manner shown in Fig. 17, and at night the flames used to form a weird and striking feature in the prospect of an iron-smelting region. Instead of allowing the escaping gases to burn to waste, it became the practice about 1860, and so continues, to draw them off and burn them under steam boilers or use their flames for heating the blast. An effective method of withdrawing the gases is shown in Fig. 21, which is a section through the upper part of a smelting furnace, with the “cup and cone” arrangement. The mouth of the furnace is covered by a shallow iron cone _a_, open at the bottom, into which fits another cone _b_, attached to a chain _c_, sustained by an arm of the lever _d_, which is firmly held in position by the chain _e_, and is also provided with a counterpoise _f_. When the mouth of the furnace is thus closed, the gases find an exit by the opening _g_, seen behind the cones, and leading into a downward passage, through which they are drawn by the draught of a tall chimney to the place where they are burnt. The charge for the furnace is filled into the hopper _a_, and at the proper time the chain, _e_, is slackened when the weight of the material resting on the suspended cone overcomes that of the counterpoise, and the charge slides down over the surface of the cone _b_, which is immediately drawn up again by the counterpoise, so that the opening into the air is at once closed. [Illustration: FIG. 21.—_Cup and Cone._ ]