What is density? Going back to the ton of lead and the ton of feathers, we find the ton of lead occupies much less space than the ton of feathers. It is much more compact, or much more dense. We say, in fact, that it has a higher density.
Consider a piece of sponge rubber. Its mass depends on the amount of material in it; its volume depends on the amount of space it occupies. If we squeeze the rubber tightly – so that it becomes much smaller in volume – and then reweigh it, we shall find there has been no change in mass. This is because the mass of the rubber depends on the amount of material it contains, which has not changed; we have merely squeezed the rubber so that the material has become compact.
In compacting the sponge rubber, we have concentrated its mass into a smaller volume; we have made it more dense. The mass of material in a given volume is called the density of the body. For density to have a clear meaning, so that substance may be compared, the mass per unit volume is quoted. A unit of volume is one cubic measurement, such as 1 cubic meter, 1 cubic centimeter, etc. Hence the density of any material is the mass of unit volume of that material, expressed in cubic measurement. The density of water is 1 gram per millimeter; the density of clean coal is about 1.3 grams per cubic centimeter. These statements mean that 1 milliliter of water weighs 1 gram and 1 cubic centimeter of clean coal weighs 1.3 grams.
Referring now to the lead and feathers example, we can say that lead is more dense than feathers. Therefore, 1 cubic meter of lead weighs more than 1 cubic meter of feathers.