266 REPORT — 1901. 



the solubility of mixtures, each of the equilibrators is then added 

 and the liquid stirred during twenty-four hours, when the density is again 

 determined. To ascertain whether the necessary equilibrators are all 

 present some of the solid is microscopically examined ; and to leave no 

 room for doubt a few c.c. of the solution are left in contact with a clear 

 crystal of each equilibrator ia a test tube at 25° during twenty -four hours 

 to see if this remain unaltered. The solution having been analysed is 

 then again stirred during a further period, more of each equilibrator being 

 added, and the tests and analyses are repeated ; if the results agree, the 

 solution is regarded as saturated. For minor details, often of consider- 

 able importance, the original publications must be consulted. The best 

 test of saturation is to maintain the solution in contact with a sharply 

 defined crystal of an equilibrator : should this remain unaltered, the 

 solution is in equilibrium with it. It may seem that the precautions 

 described are exaggerated, but experience shows that this is not the case, 

 a curious lag in the formation of a compound being often met with which 

 prevents the attainment of equilibrium — indeed, this is one of the chief 

 difficulties in such inquiries. 



The Graj)hic Exjjression of the Results. 



Case I. — As a typical simple case, a solution containing the chlorides 

 of Sodium and Potassium may be taken ; these salts neither give rise to 

 double salts, nor are they capable of existing in various hydrate forms. 

 On evaporating at a constant temperature a solution containing, say, 

 equal molecular quantities of the two chlorides, the solution will first 

 become saturated with the less soluble — viz., KCl— and this will separate 

 as the solution becomes concentrated. Subsequently the solution 

 becomes saturated with Sodium chloride as well as with Potassium 

 chloride ; from this point onwards, two solid equilibrators being present, 

 further concentration will cause the separation of both salts in constant 

 proportions and the solution will gradually evaporate without altering in 

 composition. To construct the diagram, therefore, three determinations 

 are necessary— viz., the composition of the solutions saturated with 

 (a) NaCl, (6) KCl, (c) both NaCl and KCl. 



It is convenient to express the solubility as the proportion which the 

 number of molecules of dissolved salt bears to 1000 molecules of water. 



If the solubilities of the pure substances are plotted on rectangular 

 co-ordinates, that of the one as influenced by the other will be represented 

 by a point inside the rectangle. In the following diagram the line AC 

 represents the change in the amount of Sodium chloride in the saturated 

 solution as the amount of Potassium chloride increases, whilst BC gives 

 the change in the amount of Potassium chloride in the saturated solution 

 as the amount of Sodium chloride increases. This diagram therefore 

 expresses the composition of all possible solutions containing both Sodium 

 and Potassium chlorides at 25° ; obviously : 



(1) All solutions falling on the line acb are saturated with the one 

 or the other salt, and with both at the point c, whilst (2) unsaturated 

 solutions are represented by the region inside the figure oacb and 

 (3) supersaturated solutions by the region outside acb. 



It is important to bear in mind that, as the diagram shows, on pro- 

 ceeding from the origin o towards any point on the line acb, the 

 solution rejnains upst^turatpd until thfit line iis reached. At points 



