[21] The solubility, or co-efficient of solubility, of a substance is determined by various methods. Either a solution is expressly prepared with a clear excess of the soluble substance and saturated at a given temperature, and the quantity of water and of the substance dissolved in it determined by evaporation, desiccation, or other means; or else, as is done with gases, definite quantities of water and of the soluble substance are taken and the amount remaining undissolved is determined. [Illustration: FIG. 16.--Bunsen's absorptiometer. Apparatus for determining the solubility of gases in liquids.] The solubility of a gas in water is determined by means of an apparatus called an _absorptiometer_ (fig. 16). It consists of an iron stand _f_, on which an india-rubber ring rests. A wide glass tube is placed on this ring, and is pressed down on it by the ring _h_ and the screws _i i_. The tube is thus firmly fixed on the stand. A cock _r_, communicating with a funnel _r_, passes into the lower part of the stand. Mercury can be poured into the wide tube through this funnel, which is therefore made of steel, as copper would be affected by the mercury. The upper ring _h_ is furnished with a cover _p_, which can be firmly pressed down on to the wide tube, and hermetically closes it by means of an india-rubber ring. The tube _r r_ can be raised at will, and so by pouring mercury into the funnel the height of the column of mercury, which produces pressure inside the apparatus, can be increased. The pressure can also be diminished at will, by letting mercury out through the cock _r_. A graduated tube _e_, containing the gas and liquid to be experimented on, is placed inside the wide tube. This tube is graduated in millimetres for determining the pressure, and it is calibrated for volume, so that the number of volumes occupied by the gas and liquid dissolving it can be easily calculated. This tube can also be easily removed from the apparatus. The lower portion of this tube when removed from the apparatus is shown to the right of the figure. It will be observed that its lower end is furnished with a male screw _b_, fitting in a nut _a_. The lower surface of the nut _a_ is covered with india-rubber, so that on screwing up the tube its lower end presses upon the india-rubber, and thus hermetically closes the whole tube, for its upper end is fused up. The nut _a_ is furnished with arms _c c_, and in the stand _f_ there are corresponding spaces, so that when the screwed-up internal tube is fixed into stand _f_, the arms _c c_ fix into these spaces cut in _f_. This enables the internal tube to be fixed on to the stand _f_. When the internal tube is fixed in the