Page:Popular Science Monthly Volume 29.djvu/245

Rh as is the case in ninety-nine times out of a hundred, they are shameful waste, and more than waste, for they entail a great cost for their removal. The town of Leeds pays about fourteen thousand pounds a year for the scavenging of the streets and the emptying of ash-pits. Nearly every house in Leeds supplies in the way of cinders at least twice as much ash-pit refuse as it might do, were the fireplaces properly constructed. The ash-pit refuse of Leeds is burned in a "destructor," and the cinders in the refuse provide not only heat enough for its reduction to a mineral residue, but spare heat for driving two sixty-horse-power engines, and for consuming a reasonable amount of pigs, etc., killed by or on account of disease.

These three great evils, evils affecting not only individuals, but the community, waste of fuel and heat, production of soot, production of cinders, are a direct result of the violation of the correct principles in fireplace construction.

Let us next inquire what are the principles which promote good combustion in an open fireplace—i. e., what are the conditions which are essential to enable fuel to give out to a room "good money's worth in heat." That such a result may be obtained, fuel must burn well but not rapidly. Two things in combination are essential to the combustion of fuel—a supply of oxygen, and a high temperature—i. e., plenty of heat around the fuel. If fuel be burned with a hot jacket around it, a very moderate amount of oxygen will sustain combustion, and, if the supply of oxygen be moderate, combustion is slow. Burn coal with a chilling jacket around it, a rapid conductor like iron, and it needs a fierce draught of oxygen to sustain combustion, and this means rapid escape of actual heat, and also of potential heat in unburned gases and smoke, up the chimney. This is the key to the whole position; this is the touchstone by which to test the principles of fireplace construction.

Few people probably realize the exact conditions of combustion, which may be well illustrated from the process of manufacture of coal-gas. In coal we have three kinds of constituents: One mineral, incombustible, seen in the ash residue, which for good coal amounts to barely three per cent. The second, volatile, and which, under the influence of heat becoming gaseous, appears in an open fire as tall flame and smoke, and, where combustion is imperfect, produces soot. The third constituent is carbon or charcoal, familiarly known as coke or cinder, and when burning gives a short, shallow, bluish flame. The carbon and the volatile portions can be raised to a high temperature, and still will not burn unless oxygen be brought into contact with them.

In the manufacture of gas, coal is raised to a high temperature, and the gases are driven off by roasting the coal in an oven from which air, i. e., oxygen, is shut out. The gases are conducted away, cooled, purified, and stored for future use in a gasometer; the