All About Engines

334 All About Engines overheating. But within the last two years a new process has been devised which is interesting because it has arisen out of investigations in pure science, and because it enables a greater proportion of the heat produced by the burning fuel to be communicated to the water in the boiler. In order to understand this invention let us con- sider two facts. Two gases, such as hydrogen, coal gas, or producer gas, or Mond gas, or blast-furnace gas, and air or oxygen, can be exploded by means of a flame, or a spark, or a hot wire, but a certain tem- perature is necessary to effect this. Slow combus- tion, however, goes on at temperatures below that at which explosion takes place, and the rate at which this slow combustion goes on depends upon the nature of the surface of the vessel in which the gases are enclosed, or upon the presence of certain sub- stances in the mixture. In the presence of certain (generally porous) substances, combination may be so rapid on the surface of the material that the heat produced causes explosion. The first fact, then, is that certain substances promote extremely rapid com- bustion. The second fact is best illustrated by an ordinary Bunsen burner, or a gas stove, or any gas burner in which some air is mixed with the gas before burn- ing. In these burners the air openings are always so proportioned that the air which enters at the base is insufficient for complete combustion, the remainder being obtained by the flame from the atmosphere above the burner. If the air holes be stopped up