Specific gravity

The specific gravity of a mineral is its weight compared with the weight of an equal volume of water, and is therefore the expression of how many times as heavy as water the mineral is. For instance the specific gravity of pyrite is 5.1, which is saying it is 5.1 times as heavy as water. In a pure mineral the specific gravity is constant, and an important factor in making final determinations. As ordinarily obtained, a piece of pure mineral is weighed in air, which value may be called x. It is then immersed in water and again weighed, and this value is called y. The difference between the weight in air and that in water is the weight of an equal volume of water. Then we have the following formula:

specific gravity =

x-y

Various balances have been devised for making these measurements, but any balance which will weigh small objects accurately, may be adapted to specific gravity work, by hanging a small pan under the regular weighing pan. When using this balance, care is taken to see that the lower pan is always submerged in water, even while the mineral is being weighed in air, so that when weighed in water in the lower pan, the weight of this lower pan has already been considered.