DALTON'S LAW. In a mixture of gases the volumes of the individual gases are equal to one another and to the volume of the containing vessel (e.g., in a flask of 500 cu.cm. capacity, containing a mixture of oxygen and nitrogen, the volume occupied by the oxygen is 500 cu.cm. and that occupied by the nitrogen 500 cu.cm.) but the pressure exerted by each gas (its partial pres sure) is proportional to the quantity (mass) of that gas present in the mixture. The sum of the partial pressures of the individual gases is the total pressure exerted by the mixture.