Gravity of Minerals by
Jolly and Beam Balance

By Means of a Jolly Balance: The balance of Jolly, by which the specific gravity is measured through the stretching of a spiral wire spring. From the spring is suspended two small metal pans (c and d), one above the other. The apparatus is so arranged that the lower pan (d) is always immersed in a beaker of water which, resting upon the adjustable platform B, can be placed at any required height. On the side of the upright a, which faces the spiral wire, there is a mirror with a graduated scale engraved upon it. The position of the balance is determined by means of a small which serves as an indicator. They eye is brought into such a position that the bead exactly covers its image in the mirror, and its position is then determined by means of the scale.

Three readings must be taken: first, simply the position of the balance with the lower pan in the water, x; second, its position when the mineral is placed in the upper pan, y; and third, its position when the mineral is in the lower pan and covered with water, z. the platform B with the beaker of water, z. the platform B with the beaker of water must be properly adjusted fro each of these readings so as to always have the lower pan immersed in the water. The expression x – y will give a number representing the weight of the mineral in air, while x – z will yield a number corresponding to its weight in water. From these values the specific gravity of the mineral can be calculated as described above.

By Means of a Beam Balance: This is a very convenient and quite accurate method of determining specific gravity. The balance illustrated in Fig. 196 was devised by S.L. Penfield, who describes its operation as follows: “The beam of wood is supported on a fine wire, or needle, at b and must swing freely. The long arm bc is divided into a decimal scale, commencing at the fulcrum b; the short arm carriers a double and the other in water. A piece of lead on the short arm serves to almost balance the long arm, arm, and, being empty, the beam is brought to a horizontal position, marked upon the upright, near c, by means of a rider d. A number of counterpoises are needed, which do not have to be of any specific denomination, as it is heir position on the beam and not their actual weight which is recorded. The beam being adjusted by means of the rider d, a fragment of the mineral is placed in the upper pan and a counterpoise is chosen, which, when placed near the end of the long arm, will bring it into a horizontal position. The weight of the mineral in air is given by the position of the counterpoise on the scale. The mineral is next transferred to the lower pan, and the same counterpoise is brought nearer the fulcrum b until the beam becomes again horizontal, when its position gives the weight of the mineral in water”. From these two values the specific gravity of mineral can be calculated.