The great variety of solids, liquids, and gaseous substances that are used in one form or another in every-day industrial operations may be divided into mixtures, compounds, and elements.

Fig. 66. - Taking Test Borings of Pig Iron. Various parts of the bar are drilled and the borings thus obtained are mixed and analyzed.

When two or more substances are put together, the result is called a mixture. While the mixture may differ in some ways from each of the substances that compose it, no new compound is formed, and the original substances may be separated by mechanical means. We can mix substances in any proportion. Gunpowder, for example, is a mixture of sulphur, carbon, and saltpeter. Each one of these ingredients may be separated from the others. Water, for example, will separate the saltpeter.

A compound, the smallest part of which is called a molecule, is a substance composed of two or more special substances called elements, which are combined in definite proportions. The new substance formed as the result is generally unlike either of the elements which compose it. For example, by passing an electric current through a mixture of 2 parts of hydrogen and 16 parts of oxygen, both of which are gases, water, a liquid, is formed.