"O F wood used for fuel it is customary to distinguish two kinds, hard and soft. At the head of the former class stand the different species of oak, notably the common oak scattered all over France, and the evergreen or holm oak peculiar to the south. This last is called evergreen because it does not shed its foliage in winter, but continues green the year through. Among the soft woods are the poplar, willow, plane-tree, and pine.
"The way in which these two kinds of wood burn is quite different in the two instances. Soft wood, suitably dried, takes fire readily and gives much flame with a heat that is quick but of short duration. It is convenient for use in the kitchen, where it is often necessary to obtain a prompt and intense heat, for example in roasting fowl on the spit and in cooking with the frying-pan. Furthermore, soft wood is very useful for kindling other and less combustible fuel, such as hard wood, coal, and charcoal. But for use in open fireplaces, when it is desired to keep a fire going all day long, soft wood is by no means economical, because it burns up so quickly. The best fuel in this case is oak, which burns slowly and yields a large, compact mass of coals that retain their heat for hours at a time, especially if care is taken to cover them partly with ashes."
"I have noticed the difference you speak of," said Marie. "Willow and poplar burn to ashes in a few minutes, leaving hardly any coals; but oak gives a fire that lasts and at the same time leaves a mass of glowing coals."
"If we had to keep up a fire with dry twigs and willow splinters," remarked Claire, "we should be kept busy all day throwing on wood, whereas three or four sticks of oak last a long time."
Emile here interposed with a question. "How is charcoal made," he asked, "such as is used for cooking?"
"Charcoal is made from wood," was the reply. "Its superiority to wood is that it burns almost without smoke and without flame, a very valuable quality where cleanliness is desired, as it is in our kitchens. More than that, charcoal yields a heat that is equable and lasting, thus dispensing with the need of careful watching. The best charcoal is obtained from the best wood, that is to say from oak, especially evergreen oak. I will now describe its mode of manufacture as followed by charcoal-burners in the heart of a forest.
"Upon a plot of ground beaten hard and level there is first built a sort of chimney made of logs planted vertically in the soil, and around this chimney the wood destined to be converted into charcoal is piled in tiers, one on top of another, but with openings left at the base for admitting air. The whole is covered with a layer of earth and sod, leaving exposed only the central chimney and the air-holes at the base. Finally, the mass is set on fire by means of dry brushwood."
"I should think," said Jules, "that the whole pile would burn up and leave nothing but ashes, a dead loss to all concerned."
"By no means, replied Uncle Paul. "Since air can reach the burning mass only with difficulty, combustion is slow and the wood is but half consumed. Besides, if the fire should burn too briskly the attendants would make haste to stop up with sod some or, if necessary, all of the vents at the base of the pile. As soon as the pile is thought to be one mass of glowing coals, the fire is smothered with earth and the structure is left to cool off. When this process is complete the work of demolition is undertaken, and in place of wood in its primitive state there is found nothing but charcoal. Some few fragments there may be that are not thoroughly charred, and they are recognized by their reddish color. They are the half-burnt pieces we find now and then in our charcoal."
"I know them," said Claire, "they make such a disagreeable smoke; and I always take them out of the stove as quickly as I can and throw them into the fireplace."
"That is right," returned Uncle Paul. "And now let us direct our attention to another kind of coal, the coal that is dug out of the earth and that comes to us from the coal-mines. Coal of this sort is of vegetable origin, no less than the charcoal whose mode of manufacture in the heart of the forest you now understand. But charcoal-burners do not make this other coal; it is found all made in the bowels of the earth, at great depths bellows the surface."
"But how," asked Claire, "can coal from deep down in the earth come from vegetable matter growing only on the surface?"
"To explain that to you in full is out of the question, because your knowledge is still too elementary; but I can at least give you some idea of the natural processes involved.
"Let us suppose the existence of great forests of luxuriant growth, forests that man can never penetrate with his instruments of destruction. The trees fall under the weight of years and go to decay at the foot of other and younger trees, forming at last a layer of matter half carbonized by the action of the elements. One generation of trees succeeds another, and the layer grows thicker, so that after centuries it attains the thickness of a meter or more. Imagine now a succession of violent earthquakes which break up the surface of the earth, pushing up mountains where before there have been plains, and making plains where there have been mountains. Imagine, further, that as a result of these changes of level the sea is displaced and forced to forsake, wholly or partly, its old bed for a new one; conceive of this new sea as covering the shattered remains of old forests with mud and sand that eventually become hardened and converted into thick beds of rock; and, finally, picture to yourselves the sea as at last, in consequence of still further upheavals of the earth's surface, forsaking the new bed it had found for itself and seeking still another, leaving behind it a continent of dry land. Thus you will have all the essential facts that you require in order to understand the presence of coal in the interior of the earth."
"But those frightful upheavals that you speak of," put in Jules, "those great changes that make continents of seas and seas of continents—have they ever really taken place?"
"A science called geology," replied Uncle Paul, "teaches us that these events actually occurred as I have described them, but so long ago that man was not yet in existence.
"There was a time, for example, when this corner of the
globe that bears