Page:Popular Science Monthly Volume 29.djvu/246

234 carbon and mineral residue, being non-volatile, is cooled down before being exposed to the air, and is sold as coke. Here we have a striking proof of the fact that high temperature in fuel does not of itself involve combustion. If air were admitted to the red-hot coke, or to the gases as they escape in their heated condition from the furnace, they would burn. But when coke has become cold, and the gases are cold, as in a gasometer, no amount of oxygen will of itself start combustion.

The deduction from all this is, that complete oxidation, i. e., good combustion, is possible only when the fuel and gases are at a high temperature, and that high temperature of fuel does not produce combustion until oxygen is introduced: therefore we can have a high temperature of fuel, without rapid combustion, provided we control and limit the supply of oxygen. If we have thoroughly grasped these elementary facts, we shall be in a position to understand the points to be aimed at in the construction of a fireplace.

My attention was first directed to the question of waste of fuel at the time of the coal-famine some twelve years ago. I read in the "Times," and acted upon the suggestion, to economize coal by inserting an iron plate on the grid under the fuel so as to cut off all draught through the fire. This undoubtedly induced slow combustion, and economized fuel, but the fire was dull, cold, and ineffective. The plan was abandoned. It taught me, however, the fact that combustion could be controlled by cutting off the under-draught, but I did not then see why combustion was spoiled. The reason was that the under surface of the fire was chilled, and the fuel lost its incandescence owing to the rapid loss of heat through the iron toward the open-hearth chamber. To some persons even now "slow-combustion stoves" are an abomination, and are supposed to by synonymous with bad combustion.

The next stage in my fireplace education was the adoption of the Abbotsford grate. I thereby learned that the reason why an Abbotsford grate was an advance upon the iron plate lay in the fact that the solid fire-brick bottom stored up heat and enabled the fuel to burn more brightly resting upon a hot surface—not upon a cooling iron plate. But Abbotsford grates, and the other class of grates with solid fire-brick bottoms, the Parson's grates, have disadvantages. They are apt to become dull and untidy toward the end of the day, and do not burn satisfactorily with inferior coal. There is a better thing than a solid fire-brick bottom, and that is the chamber under the fire closed in front by an "Economizer."

The history of the next, the most important stage of my fireplace education, was as follows:

Some five years ago I made, somewhat accidentally, the discovery that the burning of coal in an ordinary fireplace could be controlled and retarded by the adoption of a very simple and inexpensive