Very likely a fire must be built in the cook stove. In order that chemical combination may take place, the conditions must be right. The stove is so constructed that a current of air can pass from under the grate through the fire box, and funnel, to the chimney, and we must arrange that this air current shall not be unduly obstructed, for fuel will not burn without oxygen.

Substances differ greatly as to the ease or difficulty with which they may be made to burn, or in chemical terms, with which they may be made to unite with oxygen. The temperature to which a substance must be heated before it will take fire is called the kindling point. We therefore place light materials, like shavings, pitch-pine chips, or paper on the grate, twisting the paper and arranging all in such a way that oxygen has free access to a large surface; upon this we place small sticks of wood, piling them across each other for the same reason, and on this, in turn, hard wood or coal. The large stick of wood or the coal cannot be kindled with a match, but the paper or shavings can, and these in burning will heat the wood until it takes fire which then will kindle the coal.

Argon

Kindling Point

To kindle the fire, we unthinkingly light a match. The burning of the match repeats the same principle we have described. The match is made by dipping the ends of small sticks of wood into melted sulphur, a substance more easily kindled than wood. When the sulphur is dried, the match is tipped with a preparation of phosphorus. Phosphorus has such a low kindling temperature that friction of the match against any rough surface heats it sufficiently to set it on fire. In burning, this sets fire to the sulphur and this, in turn, kindles the wood. Paraffine now has replaced sulphur.

The products (substances formed) of the burning match are oxide of phosphorus, oxide of sulphur, and carbon dioxide and water from the carbon and hydrogen of the wood. As our coal fire burns, we have two different oxides of carbon formed - carbon monoxide composed of one part carbon and one part oxygen, and carbon dioxide having two parts oxygen to one of carbon. The carbon monoxide formed in the lower part of the fire rises through the burning coals, takes up more oxygen at the top of the fire and forms carbon dioxide. The blue flames seen over a hard coal fire are caused by carbon monoxide burning. Carbon dioxide does not burn, since in this form the carbon holds as much oxygen as possible. The drafts and dampers so regulate the supply of oxygen that the fire may burn rapidly or slowly and that the harmful products of combustion may be carried out of the house by way of the chimney.

Chemistry of a Hatch

Products of Combustion

Carton Monoxide

It might be thought that with the millions of human beings and animals and countless fires constantly using oxygen and giving off carbon dioxide, that the atmosphere would soon consist of a large proportion of carbon dioxide. Nature has wonderfully provided for this. Carbon dioxide, which is the waste matter of animals, is one of the foods of plants. Thus the trees of the forest and the shrubs and plants of the garden are continually taking in the carbon dioxide and giving out pure oxygen, so that the carbon dioxide is kept at about three or four parts in 10,000 of air.

As has been said, wood consists mainly of the substances, carbon, oxygen, hydrogen, and nitrogen, together with other substances in small amounts. The growing tree has taken these simple substances from the air and earth and stored them up in a complex form as wood.

The chemist calls the simple substances out of which different things are made, elements. Carbon, oxygen, nitrogen, sulphur, phosphorus, silver, gold, copper, iron, lead, tin, mercury, zinc, aluminum are the chemical elements familiar to most people. When the wood is burned, or oxidised, its elements are made into new combinations, but in the burning no substance is destroyed. Some of the new products are invisible, it is true, but that they exist may be proved in many ways.

One of the fundamental laws of chemistry is the Law of Conservation of Matter (substance). This may be stated as follows: The weight of all the products made in a chemical action is exactly equal to the weight of all the substances used. That is, the weight of the dry wood plus the weight of the oxygen required to burn it, equals the combined weight of carbon dioxide, water, and ashes produced. Matter can neither be destroyed nor created - it can only be changed or transformed. Scientists have reason to believe that there is just the same amount of oxygen, nitrogen, sulphur, iron and of all the other elements in the universe at the present moment as there was at the beginning of things.

A familiar form of nearly pure carbon is charcoal. It is made by heating wood for a time with a very small amount of air. The volatile parts of the wood are driven off, leaving the carbon. The old fashioned method of making charcoal is shown in Fig. II, where the burning of part of the wood gave the heat necessary for the making of the charcoal. At the present time, most charcoal is made by the destructive distillation of hard wood in iron stills; the products being charcoal, crude wood alcohol, crude acetic acid, together with gas and wood tar, which last are burned to give the heat for the process.

Charcoal is a porous substance and has the power of absorbing into its pores gases and even particles of coloring matter. A few pieces of charcoal added to the water in which flowers are standing, or plants growing, help to keep the water sweet by absorbing the impurities. Boneblack, a very finely powdered animal charcoal, is used to decolorize liquids. If it is mixed with a dark syrup, for instance, and the mixture violently shaken, the color will be absorbed and filtration will give a nearly colorless syrup.

Coal is formed in almost every country on the earth, but the United States has the largest amount. It was originally wood and other carbonaceous material, once a part of living organism at a date of perhaps millions of years ago. During these years, the earth's crust has been subjected to slow upheavals and depressions, so that in some places, what was originally at the surface has been covered with thousands of feet of earthy matter, or possibly by the ocean. Under enormous pressure, the plants have been subjected to heat from the earth's interior. This is destructive distillation on the largest scale.