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COLD    AND    HOT    BLAST    VS.    COMBUSTION.               71
TAIiLK   14.
	Volume.	Weight.
Nitrogen.. ............. . ........	7O   IQ	
Oxygen ...................... . .................................     ....: ...	20. 81	27 or
	IOO . OO	100.00
As the blast is forced into a furnace or cupola, the oxygen combines, with the carbon of the fuel and produces carbonic acid gas, which is two atoms of oxygen'to one of carbon. This gas, in passing upward, takes np more carbon and is gradually converted into carbonic oxide,1 a gas which so.on gains supremacy in lowering the high temperature necessary to liquidize ores or metals. By considering that a state of carbonic acid is necessary to liquidize, and that car-bonoxkle alone will not heat metals to a red hot color, we are in a position to fairly comprehend the difference in •degrees of temperature which ascending gases must have in reducing ores in, a furnace or melting iron in a cupola. It is said that one unit of carbon passing to the state of carbonic oxide only yields 2400 heat units centigrade, but when it becomes carbonic acid, 5,600 additional heat units are evolved, further illustrating the difference in temperature 'which the two states of carbon can create.
The existence of carbonic oxide is essential in the blast furnace for the reduction of ores to produce iron, but not in remelting iron. In the cupola the less carbonic oxicle gas, the greater the economy, and, to decrease this gas, upper tuyeres are sometimes utilized. These supply additional oxygen to the escaping carbon and convert it back more to carbonic acid gas andn used is simply in the convenience, and the use of clay is avoided. The iron and slag-holes of a furnace are sometimes lowered or raised from their original positions by reason of a furnace filling up with chilled iron, but if this can be avoided by tapping the