The wood which smolders on the hearth and in the stove is charcoal in the making.  Formerly wood was piled in heaps, covered with sod or sand to prevent access of oxygen, and then was set fire to; the smoldering wood, cut off from an adequate supply of air, was slowly transformed into charcoal.  Scattered over the country one still finds isolated charcoal kilns, crude earthen receptacles, in which wood thus deprived of air was allowed to smolder and form charcoal.  To-day charcoal is made commercially by piling wood on steel cars and then pushing the cars into strong walled chambers.  The chambers are closed to prevent access of air, and heated to a high temperature.  The intense heat transforms the wood into charcoal in a few hours.  A student can make in the laboratory sufficient charcoal for art lessons by heating in an earthen vessel wood buried in sand.  The process will be slow, however, because the heat furnished by a Bunsen burner is not great, and the wood is transformed slowly.

A form of charcoal known as animal charcoal, or bone black, is obtained from the charred remains of animals rather than plants, and may be prepared by burning bones and animal refuse as in the case of the wood.

Destructive Distillation.  When wood is burned without sufficient air, it is changed into soft brittle charcoal, which is very different from wood.  It weighs only one fourth as much as the original wood.  It is evident that much matter must leave the wood during the process of charcoal making.  We can prove this by putting some dry shavings in a strong test tube fitted with a delivery tube.  When the wood is heated a gas passes off which we may collect and burn.  Other substances also come off in gaseous form, but they condense in the water.  Among these are wood alcohol, wood tar, and acetic acid.  In the older method of charcoal making all these products were lost.  Can you give any uses of these substances?

54.  Matter and Energy.  When wood is burned, a small pile of ashes is left, and we think of the bulk of the wood as destroyed.  It is true we have less matter that is available for use or that is visible to sight, but, nevertheless, no matter has been destroyed.  The matter of which the wood is composed has merely changed its character, some of it is in the condition of ashes, and some in the condition of invisible gases, such as carbon dioxide, but none of it has been destroyed.  It is a principle of science that matter can neither be destroyed nor created; it can only be changed, or transformed, and it is our business to see that we do not heedlessly transform it into substances which are valueless to us and our descendants; as, for example, when our magnificent forests are recklessly wasted.  The smoke, gases, and ashes left in the path of a raging forest fire are no compensation to us for the valuable timber destroyed.  The sum total of matter has not been changed, but the amount of matter which man can use has been greatly lessened.