A mineral is an inorganic body (that is, one in which no parts are formed for special purposes), possessed of a definite chemical composition, and usually of a regular geometric form. It may seem at first glance that the last part of this definition is not correct, but there is reason to believe that all mineral substances may, under favorable circumstances, assume crystalline forms. Water and air are minerals. Other liquids and gases are included in the term, but as we have had already something to say of these latter substances, we shall, for the purposes of this article, use the word earth in the popular sense; namely, inorganic matter, which at ordinary temperature is solid. All materials are classified into ELEMENTS AND COMPOUNDS. By an element is meant a substance which has never been resolved into parts, and conversely, one that can not be produced by the union of two or more substances. There is some difference of opinion as to their number. It is usually given as sixty-four. There are a great many compounds. Nature seems to delight in surprising us by the simplicity of the means employed in producing marvelous results. As the mind of Milton combined the twenty-six letters of our alphabet to form “Paradise Lost,” so the Infinite arranged and re-arranged the elements to form the sublime poem of creation. Fifty-one of the elements are metals, and thirteen metalloids; gold is a familiar example of the former, and sulphur of the latter. A few, like hydrogen and oxygen, are gases; two are liquids; quicksilver and bromine: the greater number exist as solids. But few of them are found native, _i. e._, chemically uncombined with other substances. In the fierce heat of former ages they were mixed as in a mighty crucible, and few escaped the power of affinities thus engendered. Gold and copper are sometimes found pure, but even they, more frequently than otherwise, exist fused with other substances. Compounds are of three classes—acids, bases and salts. Sand is a specimen of the first, lime of the second, and clay of the third. _Fixedness_ is a characteristic of mineral compounds, yet they are by no means incapable of change; certain influences come in to promote it, of which the following are the most important—heat, solution, friction and percussion. Two gases, oxygen and hydrogen, may remain side by side for years uncombined, but a single spark will cause them to rush together with terrific energy.