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CHEMISTRY

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CHEMISTRY

solid substance disappears and products which are largely gases are formed; the burning of a candle, where the fat or wax combines with oxygen from the air to form water-vapor and carbon dioxid; the rusting of iron, where a brown earthy substance is produced by the slow action of oxygen and moisture; the destruction of sugar by heating, where water-vapor and pungent gases come off and charcoal, a form of carbon, is left behind; the slaking of quick-lime, where water combines with the lime and much heat is produced; the action of metallic zinc in a solution of lead acetate, where zinc changes place with lead and a lead-tree is formed, together with soluble zinc acetate. The most careful experiments have shown that no gain or loss of total weight takes place during any chemical change. Matter cannot be destroyed, therefore, nor can it be produced in any circumstances whatever. This fact leads to the statement that there is a law like this in regard to energy. Physical changes are distinguished from those that are chemical in being merely changes of condition. For example, when a piece of glass is heated to redness it becomes soft, but it remains unchanged chemically and has the same composition and properties after it has cooled; when water is changed to steam by heating or to ice by cooling, it undergoes no chemical change, for the steam and ice may be readily turned into water again; when a piece of sulphur is crushed to powder the change is merely physical, for every one of the small particles is still sulphur; when common salt is dissolved in water it undergoes a physical change, for it may be regained unchanged by boiling off the water.

Chemistry has to do with the composition of all substances; not only those that occur naturally in the earth as minerals, or are produced by plants and animals, but those that are prepared artificially by chemical changes. Analytical chemistry deals with finding out what is contained in substances. This is qualitative analysis when only the identity of the constituents is sought, while it is quantitative analysis when their quantities are determined. A vast amount of research has shown that the innumerable objects that have been analyzed contain comparatively few kinds of matter or elements. As far as we know, each element contains only one thing, and all the evidence goes to show that it is impossible to change any element into another, even to the slightest extent. At the present time seventy-six elements are recognized by chemists, as follows:

Gaseous non-metallic elements. Argon, chlorine, fluorine, helium, hydrogen, krypton, neon, nitrogen, oxygen and xenon.

Other non-metallic elements. Boron, bromine, carbon, iodine, phosphorus, selenium, silicon and sulphur.

More common  or important  metallic ele-

ments. Aluminium, antimony, arsenic, barium, bismuth, cadmium, calcium, chromium, cobalt, copper, gold, iron, lead, lithium, magnesium, manganese, mercury, sodium, nickel, platinum, potassium, silver, strontium, thorium, tin, titanium and zinc.

Rarer or less important metallic elements. Beryllium, caesium, erbium, gadolinium, gallium, germanium, indium, iridium, lanthanum, molybdenum, neodymium, niobium, osmium, palladium, praseodymium, rhodium, rubidium, ruthenium, samarium, scandium, tantalum, tellurium, thallium, thulium, tungsten, uranium, vanadium, ytterbium, yttrium and zirconium.

Most of the elements included in the last list are exceedingly rare, and are found only in minerals which occur in small quantities and in but few places. The elements given in the other lists vary enormously in their abundance. About one fifth of the atmosphere, exactly eight ninths of pure water and nearly one half of the earth s crust are made up of oxygen. Silicon, which occurs in combination with oxygen as quartz and in silicates, is next to oxygen in abundance, while calcium, the metal of limestone, and aluminium, the metal of clay, occur in large quantities. Magnesium, iron, potassium and sodium are also very important constituents of rocks; carbon, oxygen, hydrogen and nitrogen make up the greater part of plants and animals; but calcium, phosphorus, potassium, sulphur, iron and a few other elements are also required by living things in larger or smaller quantities. Plants get their carbon from the carbon dioxide of the air; hydrogen and oxygen are taken in by the roots in the form of water, and from the soil also are taken the nitrogen and the other elements that plants require. Animals obtain their nourishment directly or indirectly from plants, so that they contain no elements that are not found in vegetable matter. The term, organic chemistry, originally referred to the chemistry of the substances produced by plants and animals, and it was formerly supposed that these substances could be produced only by living organisms. Many of the products of life, however, such as alcohol, some of the sugars, indigo, oil of winter-green and many others, have now been made artificially, so that there is no necessity for classifying these products by themselves. For convenience, however, substances containing carbon, the characteristic element of living things, are still called organic, and they include a vast number of artificial substances that do not occur in nature. The chemistry of all substances that do not contain carbon is called inorganic chemistry.

All substances that are not elements are either mixtures or chemical compounds, containing two or more elements. These two classes are to be distinguished by the fact that compounds do not vary in composition, while mixtures may vary greatly. Examples