The object of this work is to point out those distinctions so clearly and in popular language that those who do not claim to be scientific may determine specimens for themselves; in other words, to furnish _a key for the ready determination of all the useful minerals within the United States_.[1] Two hundred and forty-four mineral species have been found within the bounds of the Union. Of these only one-third are of any use to the practical man. These eighty have certain general characters in common, but always some specific differences. The object is to divide them into groups, as the botanist divides the plants, and then to separate the individuals by some properties or features peculiar to each. Only those minerals are mentioned which are useful: any specimen, therefore, which does not fit any of the descriptions given, may be considered of no special value. By the term “color,” is meant the color of a fresh fracture, for the exposed surface often misrepresents the true aspect. Exact color is not meant, but “red” stands for reddish, “yellow” for yellowish, “white” for a light gray up to the perfectly transparent. “Magnetic” means that the specimen disturbs the needle of a compass, or that a magnet will take up fine particles. A mineral is “opaque” if the light will not pass through either the edges or a thin fragment. A “translucent” mineral is either clear as crystal or only allows light to pass dimly through a thin portion. “Effervescence” is the bubbling produced by the escape of a gas, as in soda-water. “Gravity” is the weight compared with that of an equal bulk of water. In the majority of cases the specimen can be determined without it; but there may be several doubtful cases which can be settled only by obtaining the gravity. This is done by first weighing a fragment of the mineral in a small apothecary or jeweler’s balance, reckoning it in grains. Then by a thread suspend it below one of the scales in a tumbler of water, taking care that the specimen is covered with water and does not touch the sides. Subtract the weight in grains as it hangs in the water from the first weight, and divide the first weight by the difference: the result is the gravity. Five per cent. should be allowed for impurities. Where exactness is not required, the gravity of a specimen may be judged by comparing it with well-known substances. Thus, The gravity of anthracite coal is about 1.5 The gravity of brick is about 1.8 The gravity of clay is about 2.0 The gravity of marble and glass is about 2.5 The gravity of slate is about 2.8 The gravity of cast-iron is about 7.0 The gravity of copper is about 9.0 The gravity of lead is about 11.0 If the gravity of a mineral is 1.5, a cubic inch of it will weigh about ¾ ounce; if 2., 1 oz.; if 2.5, 1¼ oz.; if 3., 1½ oz.; if 4., 2 oz.; if 5, 2½ oz., etc.