When night came only one of the two frames was in place, and it was obvious, as Jack said, that "another half day must be wasted on such work" before they could begin mining again. But that evening the Doctor dug two bushels of coal out of the farthest end of the shaft, built a special fire, placed the coal on it, and carefully covered it with earth.
"What are you doing, Doctor?" asked his crony, Tom.
"I'm making a coke oven, Tom," he replied. "I want to see how our coal will coke."
"But I don't understand about coke," answered Tom. "Why is it that when you burn most of the substance out of coal it will make a hotter fire than with all its combustible materials in it?"
"That isn't quite the case, Tom," answered the Doctor. "What we do in making coke is chiefly to expel the gas from the coal and to roast out the sulphur, which seriously interferes with the making of sufficient heat to smelt iron. Some coal gets burnt up in the process; some makes an indifferent and nearly worthless coke; while some makes a coke that would melt the heart of a miser. Now, as I told you the other night, I am convinced as a geologist, that a little further in our mine we shall come to coal so free from sulphur that we can smelt iron with it without making coke of it at all. But it is always preferable to make coke of it, and so I'm trying to see what sort of coke our coal will make. Of course we haven't come to the real coal yet, but I can tell a good deal by what we have now. We'll let my little coke oven roast all night and in the morning I'll know a great deal more than I do now. But if you have any question in your mind as to the gas making capacity of this coal, I'll remove it at once."
With that he went to the camp fire, seized a blazing brand and applied it to the little mound of earth under which he had buried his coal. Instantly the whole outside of the mound was aflame.
"That's the gas," said the Doctor. "You see there's plenty of it, even in the imperfect coal that we've reached. It will burn out presently and meantime its heat will help roast my coal into coke."
After supper the boys again plied the Doctor with questions concerning coal. Tom began it by saying:
"You told us the other evening, Doctor, that the value of a bed of coal depends upon many things besides its location and its accessibility to market. What are those things?"
"Thickness, for one thing," answered the Doctor, "and that is a point in which our mine excels. You see coal seams are of every thickness, from that of a knife blade to beds 100 feet through. Those last are very rare, however. In this country the seams vary from knife blade thickness to about nine or ten feet. Now, in working a coal mine the men, of course, must have room to stand up in the shaft, so that wherever the vein is less than six feet thick a good deal of rock or earth must be removed so as to give sufficient height to the mine. It costs as much to remove the rock or earth as to handle a like amount of coal, and the stuff is worthless. So you see it is greatly more profitable to work a thick than a thin vein. Indeed there are very few veins under three or four feet thick that it pays to work at all. Our deposit here appears to be about nine feet thick, and that means much to us.