ELEMENTS
One feature of the work of the chemist is to separate compound bodies into their simple constituents. These constituents he also endeavors to dissociate; and if this cannot be done by any means known to him, then the thing must be regarded as a simple substance. Such simple bodies are called elements.
Definition. An element then may be defined as a simple substance, which cannot by any known process be transformed into anything else; that is, no matter how it is treated, it still remains chemically what it was before. Gold, silver, copper, iron, platinum, carbon, phosphorus, calcium, oxygen, hydrogen, nitrogen, and chlorin are examples of elements. Once it was believed that there were but four elements in the world—earth, air, fire, and water. Then it was learned that these were not elements at all, but compounds, and the number of elements increased, until now sixty-eight are admitted to be simple primary substances. Some of these may in the future be proven to be compounds. Sulphur is at present in the doubtful list.
Oxygen. Oxygen is an element. It is an invisible gas, without taste or smell. It is the most abundant substance in the world, and an exceedingly active agent, entering into nearly all chemical changes and forming compounds with all known elements except one—fluorin. It is a necessity of life and of combustion.[4] It constitutes about two thirds of the weight of our bodies and one fifth of the weight of the air.
Hydrogen. Hydrogen is a gas. It is the lightest substance known. It unites with oxygen to form water, and, as will be seen later, enters into the composition of the human body.[5]
Nitrogen. Nitrogen is also a gas, but, unlike oxygen, is an inactive element. It supports neither fire nor life. It is not poisonous, however, for we breathe it constantly in the atmosphere, where its office is to dilute the too active oxygen. A person breathing it in a pure state dies simply from lack of oxygen.
Carbon. Carbon is a solid and an important and abundant element. It is known under three forms: diamond, graphite, and charcoal. The diamond is nearly pure carbon. Graphite (the "black-lead" of lead-pencils), coal, coke, and charcoal are impure forms of it. Carbon is combustible; that is, it burns or combines with oxygen. In this union carbonic acid is formed, and there is an evolution of heat, and usually, if the union be rapid and intense enough, of light. It is the valuable element in fuels, and in the body of man it unites with the oxygen of the air, yielding heat, to keep the body warm, and energy or muscular strength for work (Prof. Atwater). The carbonic acid formed in the body is given out by the lungs and skin.
Other Elements. There are many other elements about which it would be interesting to note something, such as calcium and phosphorus (found abundantly in the bones), magnesium, sulphur, sodium, iron, etc. Samples of these may be obtained to show to pupils, and descriptions given and experiments made, at the discretion of the teacher. Of the four most abundant elements of the body and of food,—oxygen, carbon, hydrogen, and nitrogen,—it is extremely important that some study be made, and if the apparatus can be procured, that it be of an experimental nature rather than simply descriptive.[6]