88 



SCIENCE PROGRESS. 



In the case of dilute solutions the depression of the 

 freezing point below that of the pure solvent is found to be 

 directly proportional to the percentage weight of dissolved 

 metal and inversely proportional to its atomic weight. But 

 if we continue to increase the strength of the solution a 

 concentration will be reached at which the solvent is 

 saturated with the dissolved metal. A further addition of 

 this metal will then have no effect on the freezing point of 

 the solution ; we shall have reached the eutectic state. 

 The freezing point of such a mixture is as steady as that 

 of a pure substance; the whole mass solidifies at a perfectly 

 constant temperature, and if there is no excess of either 

 metal the resulting solid will be remarkably homogeneous. 

 Many cases of this kind have been studied. 



If we plot horizontally the percentage of the dissolved 

 metal B, and the depression of the freezing point be plotted 

 vertically downwards, we can by adding weighed quan- 

 tities of B to the mixture, and determining the freezing 

 point after each addition, get a series of freezing points 

 which will lie on the broken line L M N (fig. 3). The 



FIG. 3 



X 



/ 



P / 



N 



Per csrrt 



o/B 



point L is the freezing point of the pure solvent A, 

 and the horizontal line M N gives the constant freezing 

 point of the eutectic alloy, a temperature which is inde- 

 pendent of the excess of B that may be present. II B is 

 sparingly soluble in A the line L M will be very nearly 



