This apparatus has the form shown in Fig. 21. It consists of a U-tube, the limbs of which are bent close together, and placed in front of a millimetre scale. The bend of the tube is filled with oil or other suitable liquid, _e.g._ bromonaphthalene. If it is desired to measure the dissociation pressure of, say, a salt hydrate, concentrated sulphuric acid is placed in the flask _e_, and a quantity of the hydrate, well dried and powdered,[163] in the bulb d. The necks of the bulbs _d_ and _e_ are then sealed off. Since, as we have learned, suspended transformation may occur, it is advisable to first partially dehydrate the salt, in order to ensure the presence of the second solid product of dissociation; the value of the dissociation pressure being independent of the degree of dissociation of the hydrate (p. 86). The small bulbs _d_ and _e_ having been filled, the apparatus is placed on its side, so as to allow the liquid to run from the bend of the tube into the bulbs _a_ and _b_; it is then exhausted through _f_ by means of a mercury pump, and sealed off. The apparatus is now placed in a perpendicular position in a thermostat, and kept at constant temperature until equilibrium is established. Since the vapour pressure on the side containing the sulphuric acid may be regarded as zero, the difference in level of the two surfaces of liquid in the U-tube gives directly the dissociation pressure of the hydrate in terms of the particular liquid employed; if the density of the latter is known, the pressure can then be calculated to cm. of mercury. [Illustration: FIG. 21.] * * * * * {92} CHAPTER VI SOLUTIONS Definition.--In all the cases which have been considered in the preceding pages, the different phases--with the exception of the vapour phase--consisted of a single substance of definite composition, or were definite chemical individuals.[164] But this invariability of the composition is by no means imposed by the Phase Rule; on the contrary, we shall find in the examples which we now proceed to study, that the participation of phases of variable composition in the equilibrium of a system is in no way excluded. To such phases of variable composition there is applied the term _solution_. A solution, therefore, is to be defined as _a homogeneous mixture, the composition of which can undergo continuous variation within certain limits_; the limits, namely, of its existence.[165]