[1] In practice, the chemist has to continually deal with gases, and gases are often collected over water; in which case a certain amount of water passes into vapour, and this vapour mixes with the gases. It is therefore most important that he should be able to calculate the amount of water or of _moisture in air and other gases_. Let us imagine a cylinder standing in a mercury bath, and filled with a dry gas whose volume equals _v_, temperature _t_°, and pressure or tension _h_ mm. (_h_ millimetres of the column of mercury at 0°). We will introduce water into the cylinder in such a quantity that a small part remains in the liquid state, and consequently that the gas will be saturated with aqueous vapour; the volume of the gas will then increase (if a larger quantity of water be taken some of the gas will he dissolved in it, and the volume may therefore he diminished). We will further suppose that, after the addition of the water, the temperature remains constant; then since the volume increases, the mercury in the cylinder falls, and therefore the pressure as well as the volume is increased. In order to investigate the phenomenon we will artificially increase the pressure, and reduce the volume to the original volume _v_. Then the pressure or tension will be greater than _h_, namely _h_ + _f_, which means that by the introduction of aqueous vapour the pressure of the gas is increased. The researches of Dalton, Gay-Lussac, and Regnault showed that this increase is equal to the maximum pressure which is proper to the aqueous vapour at the temperature at which the observation is made. The maximum pressure for all temperatures may be found in the tables made from observations on the pressure of aqueous vapour. The quantity _f_ will be equal to this maximum pressure of aqueous vapour. This may be expressed thus: the maximum tension of aqueous vapour (and of all other vapours) saturating a space in a vacuum or in any gas is the same. This rule is known as _Dalton's law_. Thus we have a volume of dry gas _v_, under a pressure _h_, and a volume of moist gas, saturated with vapour, under a pressure _h_ + _f_. The volume _v_ of the dry gas under a pressure _h_ + _f_ occupies, from Boyle's law, a volume _vh_/_h_ + _f_; consequently the volume occupied by the aqueous vapour under the pressure _h_ + _f_ equals _v_-_vh_/(_h_ + _f_), or _vf_/(_h_ + _f_). Thus the volumes of the dry gas and of the moisture which occurs in it, at a pressure _h_ + _f_, are in the ratio _f_ : _h_. And, therefore, if the aqueous vapour saturates a space at a pressure _n_, the volumes of the dry air and of the moisture which is contained in it are in the ratio (_n_-_f_) :