xix BALANCED ACTIONS 507 



crystallise at a high temperature, and then, by cooling, the 

 nitrate of potash, is crystallised : the proportion of each salt is 

 diminished alternately, and by repeated crystallisations they 

 are both entirely separated" (Chemical Statics, I. 71-72). 

 Here the action 



NaNO 3 + KC1^KNO 3 + NaCl 



can be brought to completion in the direction of the upper 

 arrow by crystallising out, alternately, nitrate of potash at 

 the freezing-point and common salt at the boiling-point. If 

 the common salt were allowed to accumulate, it would, by 

 the law of mass-action, check the formation of potassium 

 nitrate from sodium nitrate and potassium chloride, with 

 the result that one of these salts (potassium chloride) 

 might crystallise out and carry the action backwards in 

 the direction of the lower arrow. 



An action of this kind actually occurs when a solution 

 containing sulphate of potash and nitrate of soda is 

 evaporated. The sulphate of potash, which is the least 

 soluble of the four salts, 



2 NaNO 3 + K 2 SO 4 ^2KNO 3 + Na 2 SO 4 , 

 crystallises first. 



" But when the proportion of the first shall be diminished 

 by the crystallisation, nitrate of potash will also be obtained, 

 because the water remaining at this period is incapable of 

 holding in solution the quantity of this salt which may be 

 formed, and because the sulphate of potash, on its part, is 

 rendered more soluble by the action of the other salt : this 

 result might have been obtained from the commencement 

 of the crystallisation by augmenting the proportion of nitrate 

 of soda" (Chemical Statics, I. 70). 



Action of acids on insoluble salts. When a strong acid 

 acts on an insoluble salt of a weak acid, the insolubility of 

 the salt greatly assists the weaker acid in its effort to retain 

 possession of the base. 



"In fact, if oxalic acid is added to a salt with base of 





