What do we mean when we say: as heavy as lead and as light as a feather? It is clear that a grain of lead will be light, while a mountain of feathers has considerable mass. Those who use such comparisons have in mind not the mass of a body but the density of the material of which it consists.

The mass of a unit volume of a body is called its density. It is evident that a grain of lead and a massive block of lead have the same density.

In denoting density, we usually indicate how many grams (g) a cubic centimetre (\(\mathrm{cm^{3}}\)) of the body weighs—after this number we place the symbol  \(\mathrm{g/cm^3}\). In order to determine the density, the number of grams must be divided by the number of cubic centimetres; the solidus in the symbol reminds us of this.

Certain metals are among the heaviest materials—Osmium whose density is equal to 22.5 \(\mathrm{g/cm^3}\), Iridium (22.4), Platinum (21.5), Tungsten and Gold (19.3). The density of Iron is 7.88, that of Copper 8.93.

The lightest metals are Magnesium (1.74), Beryllium (1.83) and Aluminium (2.70). Still lighter bodies should be sought among organic materials: various sorts of wood and plastic may have a density as low as 0.4.

It should be stipulated that we are dealing with continuous bodies. If there are pores in a solid, it will of course be lighter. Porous bodies—cork, foam glass—are frequently used in technology. The density of foam glass may be less than 0.5, although the solid matter from which it is made has a density greater than 1 \(\mathrm{g/cm^3}\). As all other bodies whose density is less than 1 \(\mathrm{g/cm^3}\), foam glass floats superbly on water.

The lightest liquid is liquid hydrogen; it can only be obtained at extremely low temperatures. One cubic centimetre of liquid hydrogen has a mass of 0.07 g. Organic liquids—alcohol, benzine, kerosene—do not differ significantly from water in density. Mercury is very heavy—it has a density of 13.6 \(\mathrm{g/cm^3}\).

And how can the density of gases be characterized? For gases, as is well known, occupy whatever volumes we let them. If we empty gas-bags with the same mass of gas into vessels of different volumes, the gas will always fill them up uniformly. How then can we speak of density?

We define the density of gases under so—called normal conditions—a temperature of 0 °C and a pressure of 1 atm. The density of air under normal conditions is equal to 0.00129 \(\mathrm{g/cm^3}\), of chlorine 0.00322 \(\mathrm{g/cm^3}\). Gaseous hydrogen, just as the liquid one, holds the record: the density of this lightest gas is equal to 0.00009 \(\mathrm{g/cm^3}\).

The next lightest gas is helium; it is twice as heavy as hydrogen. Carbon dioxide is heavier than air by a factor of 1.5. In Italy, near Naples, there is a famous “canine cave”; carbon dioxide continually exudes from its lower part, hangs low and slowly escapes from the cave. A person can enter this cave without difficulty, but such a stroll will end badly for a dog. Hence the cave’s name.

The density of gases is extremely sensitive to external conditions-pressure and temperature. Without an indication of the external conditions, the values of the density of gases have no meaning. The densities of liquids and solids also depend on temperature and pressure, but the dependence is considerably weaker.