The preceding ratio gives the maximum quantity of water which can be held in a gas, and the degree of moisture shows what fraction of this maximum quantity occurs in a given case, when the vapour does not saturate the space occupied by the gas. Consequently, if the degree of moisture equals 50 p.c.—that is, half the maximum—then the volume of dry gas at 760 mm. is equal to the volume of dry gas at 760 mm. multiplied by h - 0·5f / 760 , or, in general, by h - rf / 760 where r is the degree of moisture. Thus, if it is required to measure the volume of a moist gas, it must either be thoroughly dried or quite saturated with moisture, or else the degree of moisture determined. The first and last methods are inconvenient, and therefore recourse is usually had to the second. For this purpose water is introduced into the cylinder holding the gas to be measured; it is left for a certain time so that the gas may become saturated, the precaution being taken that a portion of the water remains in a liquid state; then the volume of the moist gas is determined, from which that of the dry gas may be calculated. In order to find the weight of the aqueous vapour in a gas it is necessary to know the weight of a cubic measure at 0° and 760 mm.

Knowing that one cubic centimetre of air in these circumstances weighs 0·001293 gram, and that the density of aqueous vapour is 0·62, we find that one cubic centimetre of aqueous vapour at 0° and 760 mm. weighs 0·0008 gram, and at a temperature t° and pressure h the weight of one cubic centimetre will be 0·0008 × h / 760 × 273 / 273 + t . We already know that v volumes of a gas at a temperature t° pressure h contain v × f / h volumes of aqueous vapour which saturate it, therefore the weight of the aqueous vapour held in v volumes of a gas will be v x 0·0008 × f / 760 × 273 / 273 + t Accordingly, the weight of water which is contained in one volume of a gas depends only on the temperature and not on the pressure. This also signifies that evaporation proceeds to the same extent in air as in a vacuum, or, in general terms (this is Dalton's law), vapours and gases diffuse into each other as if into a vacuum. In a given space, at a given temperature, a constant quantity of vapour enters, whatever be the pressure of the gas filling that space. From this it is clear that if the weight of the vapour contained in a given volume of a gas be known, it is easy to determine the degree of moisture r = p / v × 0·0008 × 760 / t × 273 + t / 273 .