a test tube containing carbon dioxide, it is promptly extinguished, because carbon dioxide cannot support combustion; if a stream of carbon dioxide and water falls upon a fire, it acts like a blanket, covering the flames and extinguishing them. The value of a fire extinguisher depends upon the amount of carbon dioxide and water which it can furnish. A fire extinguisher is a metal case containing a solution of bicarbonate of soda, and a glass vessel full of strong sulphuric acid. As long as the extinguisher is in an upright position, these substances are kept separate, but when the extinguisher is inverted, the acid escapes from the bottle, and mixes with the soda solution. The mingling liquids interact and liberate carbon dioxide. A part of the gas thus liberated dissolves in the water of the soda solution and escapes from the tube with the outflowing liquid, while a portion remains undissolved and escapes as a stream of gas. The fire extinguisher is therefore the source of a liquid containing the fire-extinguishing substance and further the source of a stream of carbon dioxide gas.
51. Carbon. Although carbon dioxide is very injurious to health, both of the substances of which it is composed are necessary to life. We ourselves, our bones and flesh in particular, are partly carbon, and every animal, no matter how small or insignificant, contains some carbon; while the plants around us, the trees, the grass, the flowers, contain a by no means meager quantity of carbon.
Carbon plays an important and varied role in our life, and, in some one of its many forms, enters into the composition of most of the substances which are of service and value to man. The food we eat, the clothes we wear, the wood and coal we burn, the marble we employ in building, the indispensable soap, and the ornamental diamond, all contain carbon in some form.
52. Charcoal. One of the most valuable forms of carbon is charcoal; valuable not in the sense that it costs hundreds of dollars, but in the more vital sense, that its use adds to the cleanliness, comfort, and health of man.
The foul, bad-smelling gases which arise from sewers can be prevented from escaping and passing to streets and buildings by placing charcoal filters at the sewer exits. Charcoal is porous and absorbs foul gases, and thus keeps the region surrounding sewers sweet and clean and free of odor. Good housekeepers drop small bits of charcoal into vases of flowers to prevent discoloration of the water and the odor of decaying stems.
If impure water filters through charcoal, it emerges pure, having left its impurities in the pores of the charcoal. Practically all household filters of drinking water are made of charcoal. But such a device may be a source of disease instead of a prevention of disease, unless the filter is regularly cleaned or renewed. This is because the pores soon become clogged with the impurities, and unless they are cleaned, the water which flows through the filter passes through a bed of impurities and becomes contaminated rather than purified. Frequent cleansing or renewal of the filter removes this difficulty.
Commercially, charcoal is used on a large scale in the refining of sugars, sirups, and oils. Sugar, whether it comes from the maple tree, or the sugar cane, or the beet, is dark colored. It is whitened by passage through filters of finely pulverized charcoal. Cider and vinegar are likewise cleared by passage through charcoal.
The value of carbon, in the form of charcoal, as a purifier is very great, whether we consider it a deodorizer, as in the case of the sewage, or a decolorizer, as in the case of the refineries, or whether we consider the service it has rendered man in the elimination of danger from drinking water.
53. How Charcoal is Made. Charcoal may be made by heating wood in an oven to which air does not have free access. The absence of air prevents ordinary combustion, nevertheless the intense heat affects the wood and changes it into new substances, one of which is charcoal.
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.
The principle just stated embodies one of the fundamental laws of science, called the law of the conservation of matter.
A similar law holds for energy as well. We can transform electric energy into the motion of trolley cars, or we can make use of the energy of streams to turn the wheels of our mills, but in all these cases we are transforming, not creating, energy.
When a ball is fired from a rifle, most of the energy of the gunpowder is utilized in motion, but some is dissipated in producing a flash and a report, and in heat. The energy of the gunpowder has been scattered, but the sum of the various forms of energy is equal to the energy originally stored away in the powder. The better the gun is, the less will be the energy dissipated in smoke and heat and noise.
CHAPTER V
FOOD
55. The Body as a Machine. Wholesome food and fresh air are necessary for a healthy body. Many housewives, through ignorance, supply to their hard-working husbands and their growing sons and daughters food which satisfies the appetite, but which does not give to the body the elements needed for daily work and growth. Some foods, such as lettuce, cucumbers, and watermelons, make proper and satisfactory changes in diet, but are not strength giving. Other foods, like peas and