One half of the earth has therefore a greater density than the other.
One of the most remarkable of Airy's researches was his determination of the mean density of the earth.
According to the law of Avogadro, equal volumes of different gases under the same conditions of temperature and pressure contain equal numbers of molecules; therefore, since the density depends upon the number of molecules present in unit volume, it follows that for a comparison of the densities of gases, the determinations must be made under coincident conditions, or the observations reduced or re-computed for coincident conditions.
Owing to difference of density the oil and water in the anticlines separate into two layers, the upper consisting of oil which fills the anticlines, while the water remains in the synclines.
The solution obtained may be evaporated in vacuo until it attains a density of 1.46 when, if partially saturated with potassium hydroxide and filtered, it yields crystals of potassium pentathionate, K 2 S 5 0 6.3H 2 0.