Having finished this part of his work, Jack went into the woods near the camp and prepared a large quantity of "fat" pine for burning. Piling this in the shallow pit, and heaping it two or three feet above the level ground, he took the shovel and covered the pile with earth to a depth of a foot or more, leaving a single opening through which he could set fire to the mass. His object was, by smothering the flames in this way, to make the fat, resinous pine burn slowly, creating a roasting heat under the earth, and thus, as it were, melting the tar out of the pine. If he had not covered the wood with earth, it would have blazed up and burned to smoke, resin and all, making no tar at all.
When all was ready the pile was set on fire, and as soon as it had caught well, Jack covered the single opening with earth, and the mound smoked like a volcano. Pretty soon a little stream of smoking-hot tar began trickling through the cane-tube into the deep pit.
Night had now come on, and the smoke from the tar-kiln, catching the light from the camp-fire, glowed with a peculiar red color, and gave a picturesque air of strangeness to the camp.
"You've started a young volcano, Jack," said Charley, as he looked at the smoking mound.
"Yes. An improvement on Crusoe," said Ned; "he had no volcano on his island. But what a quantity of smoke the thing does make. It looks as if more material came out of the mound in that way than you put into it in the shape of wood."
"Yes, and so a gallon of water will fill a big room if you make it into steam."
"What is smoke anyhow?" asked Charley.
"It is composed of several things," answered Ned, "but chiefly of carbon. Indeed, all that you can see in smoke is carbon."
"Then why doesn't it burn?"
"It would if it were kept in the fire long enough; but the light vapors that rise from the fire carry the particles of carbon with them, and so they get out of the fire before they are burned. The smoke is simply so much wasted fuel, and many plans have been made to save it in factories where the cost of fuel is great."