From the foregoing it will be evident that the amount of acetic acid required to produce a homogenous solution, will depend on the relative amounts of chloroform and water from which we start, and can be ascertained by joining the corner C with the point on the line AB representing the total composition of the initial binary system. The point where this line intersects the boundary curve aKb will indicate the minimum amount of acetic acid which, under these particular conditions, is necessary to give one homogeneous solution.
Retrograde Solubility.—As a consequence of the fact that acetic acid distributes itself unequally between chloroform and water, and the critical point K, therefore, does not lie at the summit of the curve, it is possible to start with a homogeneous solution in which the percentage amount of acetic acid is greater than at the critical point, and to pass from this first to a heterogenous and then again to a homogenous system merely by altering the relative amounts of chloroform and water. This phenomenon, to which the term retrograde solubility is applied, will be observed not only in the case of chloroform, water, and acetic acid, but in all other systems in which the critical point lies below the highest point of the boundary curve for heterogeneous systems. This will be seen from the diagram, Fig. 85. Starting with the homogeneous system represented by x, in which, therefore, the concentration of C is greater than in the critical mixture (K), if the relative amounts of A and B are altered in the direction xx′, while the amount of C is maintained constant, the system will become heterogeneous when the composition reaches the point y, and will remain
heterogeneous with changing composition until the point y′ is passed, when it will again become homogeneous. If the relative concentration of C is increased above that represented by the line SS, this phenomenon will, of course, no longer be observed.
Relationships similar to those described for chloroform, water, and acetic acid are also found in the case of a number of other trios, e.g. ether, water, and alcohol; chloroform, water, and alcohol.[[322]] They have also been observed in the case of a considerable number of molten metals.[[323]] Thus, molten lead and silver, as well as molten zinc and silver, mix in all proportions; but molten lead and zinc are only partially miscible with one another. When melted together, therefore, the last two metals will separate into two liquid layers, one rich in lead, the other rich in zinc. If silver is now added, and the temperature maintained above the freezing point of the mixture, the silver passes for the most part, in accordance with the law of distribution, into the upper layer, which is rich in zinc; silver being more soluble in molten zinc than in molten lead. This is clearly shown by the following figures:—[[324]]
| Heavier alloy. | Lighter alloy. | ||||
| Percentage amount of | Percentage amount of | ||||
| Silver. | Lead. | Zinc. | Silver. | Lead. | Zinc. |
| 1.25 | 96.69 | 2.06 | 38.91 | 3.12 | 57.97 |
| 1.71 | 96.43 | 1.86 | 45.01 | 3.37 | 51.62 |
| 5.55 | 93.16 | 1.29 | 54.93 | 4.21 | 40.86 |
The numbers in the same horizontal row give the composition of the conjugate alloys, and it is evident that the upper layer consists almost entirely of silver and zinc. On allowing the mixture to cool slightly, the upper layer solidifies first, and can be separated from the still molten lead layer. It is on this behaviour of silver towards a mixture of molten lead and zinc that the Parkes's method for the desilverization of lead depends.[[325]] If aluminium is also added, a still larger proportion of silver passes into the lighter layer, and the desilverization of the lead is more complete.[[326]]
