Additional evidence as to the structure of the molecule was discussed by **Avogadro** in 1811, and by Ampere in 1814.

The weight contained in a molecule of hydrochloric acid, thus differing from **Avogadro** who chose the weight of a hydrogen molecule.

Equal changes in temperature and pressure occasion equal changes in equal volumes of all gases and vapours - **Avogadro** deduced the law: Under the same conditions of temperature and pressure, equal volumes of gases contain equal numbers of molecules; and he showed that the relative weights of the molecules are determined as the ratios of the weights of equal volumes, or densities.

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.

The density gives very important information as to the molecular weight, since by the law of **Avogadro** it is seen that the relative density is the ratio of the molecular weights of the experimental and standard gases.

AMEDEO **AVOGADRO**, CONTE DI QUAREGNA (1776-1856), Italian physicist, was born at Turin on the 9th of June 1776, and died there on the 9th of July 1856.