Density and relative density (a term which replaces “specific gravity”) are important terms we shall often encounter in coal preparation. We must clearly understand their meaning before we consider different methods of washing coal.
Probably we have all asked the question, “Which is the heavier, a ton of lead or a tone of feathers?” We know that both are the same mass, but when the question was first put to us we may have had doubts; our first reaction is that lead is obviously heavier than feathers. Yet we have a very small piece of lead which is lighter than a large bulk (or volume) of feathers.
In considering relative quantities of materials, therefore, it is not enough simply to state the mass, or to say that one material is heavier than another. We must also state the volume of the materials we are considering. Applying this idea to our example, we note that lead is heavier than the same volume of feathers; or that a ton of feathers would occupy much more volume than a ton of lead, although they would weigh the same.
Let us now consider the meaning of weight, mass and volume.