CH. xix BALANCED ACTIONS 499 



with a sufficient quantity of water to dissolve the saline 

 mass, and I set the liquid to crystallise : the salt which I 

 obtained proved to be very pure nitre, crystallised partly in 

 needles and partly in small cubic crystals ; I set it to drain 

 on grey paper . . . ; the nitre thus drained, showed just the 

 same phenomena as nitre prepared by the combination of 

 nitric acid with fixed alkali" 1 (Mem. Math. Phys'.^ 1774, 

 VI. 231-236; paper read before the Paris Academy of 

 Sciences, Dec. 23, 1760). 



Thus under one set of conditions, sulphuric acid may 

 expel nitric acid from its salts, but under other conditions 

 nitric acid expels sulphuric acid. 



Reversible actions. This reversal of the action may be 

 shown by writing two equations, thus : 



May ow found that: 



2 KNO 3 + H 2 SO 4 -> K 2 SO 4 + 2HNO 8 



Nitre + SU S dC gave *^~ + nitric acid 



Bourne found that : 



K 2 SO 4 + 2HNO 3 -> 2 KNO 8 + H 2 SO 4 



These two equations may then be combined into a single 

 equation in which arrows pointing in opposite directions 

 show the direction taken by the action under different 

 conditions, thus : 



Hot 



2KNO 3 + H 2 SO 4 = K 2 SO 4 + 2HNO 3 



Cold 



Bergman's tables of affinity (1775). Mayow's idea that 

 a " closer union " existed between fixed than between volatile 

 acids and alkalis was extended to series of acids and bases 

 in the " tables of affinity " drawn up by Geoffrey in 1718, by 

 Stahl in 1720, and finally by Bergman in 1775 (A Disserta- 

 tion on Elective Attractions, tr. T. Beddoes, London, 1785). 

 In these tables one acid or base was selected and the 

 different bases or acids were arranged under it in the order 

 1 Potassium carbonate. 



K K 2 



