An Elementary Study of Chemistry | Page 5

liquid. The substance is, therefore, a mechanical mixture.
If now a new portion of the mixture is placed in a dry test tube and carefully heated in the flame of a Bunsen burner, as shown in Fig. 3, a striking change takes place. The mixture begins to glow at some point, the glow rapidly extending throughout the whole mass. If the test tube is now broken and the product examined, it will be found to be a hard, black, brittle substance, in no way recalling the iron or the sulphur. The magnet no longer attracts it; carbon disulphide will not dissolve sulphur from it. It is a new substance with new properties, resulting from the chemical union of iron and sulphur, and is called iron sulphide. Such substances are called chemical compounds, and differ from mechanical mixtures in that the substances producing them lose their own characteristic properties. We shall see later that the two also differ in that the composition of a chemical compound never varies.
[Illustration: Fig. 3]
DEFINITION: A chemical compound is a substance the constituents of which have lost their own characteristic properties, and which cannot be separated save by a chemical change.
~Elements.~ It has been seen that iron sulphide is composed of two entirely different substances,--iron and sulphur. The question arises, Do these substances in turn contain other substances, that is, are they also chemical compounds? Chemists have tried in a great many ways to decompose them, but all their efforts have failed. Substances which have resisted all efforts to decompose them into other substances are called elements. It is not always easy to prove that a given substance is really an element. Some way as yet untried may be successful in decomposing it into other simpler forms of matter, and the supposed element will then prove to be a compound. Water, lime, and many other familiar compounds were at one time thought to be elements.
DEFINITION: An element is a substance which cannot be separated into simpler substances by any known means.
~Kinds of matter.~ While matter has been grouped in three classes for the purpose of study, it will be apparent that there are really but two distinct kinds of matter, namely, compounds and elements. A mechanical mixture is not a third distinct kind of matter, but is made up of varying quantities of either compounds or elements or both.
~Alchemy.~ In olden times it was thought that some way could be found to change one element into another, and a great many efforts were made to accomplish this transformation. Most of these efforts were directed toward changing the commoner metals into gold, and many fanciful ways for doing this were described. The chemists of that time were called alchemists, and the art which they practiced was called alchemy. The alchemists gradually became convinced that the only way common metals could be changed into gold was by the wonderful power of a magic substance which they called the philosopher's stone, which would accomplish this transformation by its mere touch and would in addition give perpetual youth to its fortunate possessor. No one has ever found such a stone, and no one has succeeded in changing one metal into another.
~Number of elements.~ The number of substances now considered to be elements is not large--about eighty in all. Many of these are rare, and very few of them make any large fraction of the materials in the earth's crust. Clarke gives the following estimate of the composition of the earth's crust:
Oxygen 47.0% Calcium 3.5% Silicon 27.9 Magnesium 2.5 Aluminium 8.1 Sodium 2.7 Iron 4.7 Potassium 2.4 Other elements 1.2%
A complete list of the elements is given in the Appendix. In this list the more common of the elements are marked with an asterisk. It is not necessary to study more than a third of the total number of elements to gain a very good knowledge of chemistry.
~Physical state of the elements.~ About ten of the elements are gases at ordinary temperatures. Two--mercury and bromine--are liquids. The others are all solids, though their melting points vary through wide limits, from c?sium which melts at 26° to elements which do not melt save in the intense heat of the electric furnace.
~Occurrence of the elements.~ Comparatively few of the elements occur as uncombined substances in nature, most of them being found in the form of chemical compounds. When an element does occur by itself, as is the case with gold, we say that it occurs in the free state or native; when it is combined with other substances in the form of compounds, we say that it occurs in the combined state, or in combination. In the latter case there is usually little about the compound to suggest that the element is present in it; for we have seen that elements lose their own