But Mr. Siemens believes, and adduces some evidence to prove, that the dissociation point is not a constant and definite temperature for a given compound; it depends entirely upon whether solid or foreign surfaces are present or not.  These it is which appear to be an efficient cause of dissociation, and which, therefore, limit the temperature of flame.  In the absence of all solid contact, Mr. Siemens believes that dissociation, if it occur at all, occurs at an enormously higher temperature, and that the temperature of free flame can be raised to almost any extent.  Whether this be so or not, his radiating flames are most successful, and the fact that large quantities of steel are now melted by mere flame radiation speaks well for the correctness of the theory upon which his practice has been based.

Use of Small Coal.—­Meanwhile, we may just consider how we ought to deal with solid fuel, whether for the purpose of making gas from it or for burning it in situ.  The question arises, In what form ought solid fuel to be—­ought it to be in lumps or in powder?  Universal practice says lumps, but some theoretical considerations would have suggested powder.  Remember, combustion is a chemical action, and when a chemist wishes to act on a solid easily, he always pulverizes it as a first step.

Is it not possible that compacting small coal into lumps is a wrong operation, and that we ought rather to think of breaking big coal down into slack?  The idea was suggested to me by Sir W. Thomson in a chance conversation, and it struck me at once as a brilliant one.  The amount of coal wasted by being in the form of slack is very great.  Thousands of tons are never raised from the pits because the price is too low to pay for the raising—­in some places it is only 1s. 6d. a ton.  Mr. McMillan calculates that 130,000 tons of breeze, or powdered coke, is produced every year by the Gas Light and Coke Company alone, and its price is 3s. a ton at the works, or 5s. delivered.

The low price and refuse character of small coal is, of course, owing to the fact that no ordinary furnace can burn it.  But picture to yourself a blast of hot air into which powdered coal is sifted from above like ground coffee, or like chaff in a thrashing mill, and see how rapidly and completely it might burn.  Fine dust in a flour mill is so combustible as to be explosive and dangerous, and Mr. Galloway has shown that many colliery explosions are due not to the presence of gas so much as the presence of fine coal-dust suspended in the air.  If only fine enough, then such dust is eminently combustible, and a blast containing it might become a veritable sheet of flame. (Blow lycopodium through a flame.) Feed the coal into a sort of coffee-mill, there let it be ground and carried forward by a blast to the furnace where it is to be burned.  If the thing would work at all, almost any kind of refuse fuel could be burned—­sawdust, tan, cinder heaps, organic rubbish of all kinds.  The only condition is that it be fine enough.