xix BALANCED ACTIONS 501 



In the moist way : 

 2 NH 3 + PbQ 2 + 2 H 2 O -> 2 NH 4 C1 + Pb(OH) 2 



Ammonia + 



lead . ammonium lead 



chloride ' chloride hydroxide 



In the dry way : 

 2 NH 4 C1 + Pb(OH) 2 -> 2 NH 3 + PbCl 2 + 2H 2 O 



Ammonium , lead lead 



chloride ->' hydroxide lves ammonia + ch , oride + water 



Combining these equations, we may write : 



Moist 



2 NH 3 + PbCl 2 + 2H 2 O ^ 2 NH 4 Cl + Pb(OH) 2 . 



Dry 



Berthollet (1799) investigates the laws of chemical 

 affinity, Bergman had recognised the influence of tem- 

 perature on the order of affinity, and had shown that this 

 order was altered considerably when substances were ignited 

 together instead of being merely mixed in solution or 

 in presence of water. To Berthollet belongs the credit 

 of directing attention to the influence on chemical action 

 of the quantities of the interacting substances, an influence 

 which is now described as MASS-ACTION. 



In his Researches into the Laws of Chemical Affinity 

 (read in Cairo in 1799; published 1801 ; tr. M. Farrell, 

 London, 1804) he criticises Bergman's method in the 

 following terms : 



" He prescribes then, for determining the elective affinity 

 of two substances, to try if one of them can remove the 

 other from its combination with a third, and vice versa. He 

 takes it for granted, that that body which has removed 

 another from its combination, cannot, in like manner, be 

 expelled by that other, and that both experiments will 

 concur to prove that the first has a greater elective affinity 

 than the second. He adds at the same time, that it may be 

 necessary to employ six times as much of the decomposing 

 substance as would be necessary to saturate immediately 

 the substance with which it tends to combine." 



" The doctrine of Bergman is founded entirely on the 



