506 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



may crystallise out first according to the composition of the 

 solution. 



" A mixture of nitrate of potash and muriate of lime 

 yields also a result, in which the influence of the proportions 

 is still more marked^ because the two least soluble salts 

 which can be formed, the nitrate of potash and the muriate 

 of potash, differ but little in this property : either of these 

 salts may also be obtained at the first crystallisation by a 

 little variation in the proportions of the nitrate of potash 

 and muriate of lime" (Chemical Statics, I. 70-71) : 



2 KNO 3 + CaCl 2 = 2KC1 + Ca(NO 3 ) 2 . 



If potassium nitrate is in excess, the potash will crystallise 

 out as nitrate, but an excess of calcium chloride will displace 

 the equilibrium towards the right-hand side of the equation 

 and cause the potash to separate as chloride. 



(c) One salt is least soluble in the cold, but another in the hot, 

 solution. 



" The solubility of salts varies by a difference of temper- 

 ature ; but this does not follow the same progression in all 

 of them. In some it acquires a considerable augmentation 

 by an elevation of heat ; in others it remains nearly the same. 

 This condition, which determines the separation of salts, 

 may therefore produce different effects according to the 

 thermometric state ; hence it happens, that salts whose 

 solubility is nearly equal at one degree of heat, may, never- 

 theless, be easily separated, by producing a great change in 

 the temperature, and, by making the effect of the propor- 

 tions, and that of the difference of solubility, predominate 

 alternately." 



" Nitrate of potash and muriate of soda furnish us with 

 a striking example of this effect. Near the freezing-point, 

 nitrate of potash has much less solubility than muriate of 

 soda ; but it is considerably increased by heat, and that of 

 muriate of soda very little ; so that the solubility of the latter, 

 which was only about half that of the nitrate of potash, 

 comes to a degree at which it is equal, and finally at the 

 boiling point becomes nearly eight times less. By boiling 

 the mixture, therefore, the muriate of soda is made to 



