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ELEMENTS   OF   CAST   IRON.                          203
kind of atoms. An atom is the smallest sub-division of matter which cannot be divided. Every atom is exactly like every other atom of the same kind and is, as a rule, incapable of independent existence. Atoms unite to form molecules, which are the smallest particles of matter capable of independent existence to retain the properties of a mass, and which is any form of matter appreciable to the senses. Molecules can be formed of one or different kinds of atoms. Where molecules are formed of different kinds of atoms, the mass is called a compound. There are now about seventy different kinds of atoms or elements, among which are classed carbon, iron, manganese, phosphorus, silicon, and sulphur. Table 130, at the close of this work, gives the chemical and atomic weights of various elements.
One method of distinguishing the metallic elements or atoms from the non-metallic ones is as follows: Solutions of compounds are sometimes decomposed by an electric current. That element which will go to the positive pole is said to be the electro-negative or non-metallic, while that element which goes to the negative pole is said to be electro-positive or metallic. This division of elements among iron workers is more generally understood in being classed as metals and metalloids, the latter being limited to inflammable non-metallic elements, and which as a rule are lighter, bulk for bulk, than metals. With this conception of the elements, we can consider iron, "manganese, and silicon as being metals, while the carbon, sulphur and phosphorus would be classed as metalloids. While this classification may be accepted, it is for convenience, with founders especially, considered that the term. about 5 per cent, shop scrap, the pig rangingan-