122 Dr. J. J. Hood on the 



inverse meaning to that attached to coefficient of affinity by 

 Guldberg and Waage in their tract Sur les Affinites chimigues. 

 For convenience it will be termed the basic power, or strength 

 of a particular base, a relative magnitude to be determined by 

 the process of precipitation ; but it will remain for future 

 experiments to give it a more definite meaning. 



Suppose, then, that to a solution of sulphates or other salts 

 of two metals a precipitant is added less in amount than requi- 

 site for complete precipitation ; the salt which offers the greater 

 resistance to the action of the precipitant, and for which the 

 magnitude e is the greater, will suffer less change than the 

 second salt in the solution. If the element time be introduced, 

 the rates of change or of formation of the precipitates vary 

 inversely as the basic strengths, for the greater the value of e 

 the slower the rate of precipitation. 



No account is taken of the precipitation of so-called basic 

 salts, but simply the total action of the precipitant on the 

 salt-solution, whatever may be the nature of the material pre- 

 cipitated. The secondary actions that might possibly occur 

 between the products of the change are also neglected ; such, 

 for instance, as the action of the precipitate a obtained from 

 the salt A reacting on B to precipitate ft and re-form A; and, 

 conversely, /3 acting on A to form a and change into B. 



Let the masses of the two salts in the solution be A and B, 

 to which a precipitant C is added, insufficient for complete 

 precipitation. The rates of formation of the precipitates may 

 be formulated on the usual assumption that they are propor- 

 tional to the products of the active materials existing in the 

 system at any time. The amounts of A and B that remain 

 unchanged being A— x and B — ?/, and since for the forma- 

 tion of x a quantity of the precipitant p has been utilized, and 

 for y a quantity g, the amount of C that still remains capable 

 of effecting further precipitation is G—p—g, the rates of 

 change are 



! = I(B- y )(C-^- ? ), 



where e and e f are the basic powers. 



Since p=.ot.x and g=/3y, and the total mass of the preci- 

 pitant necessary for complete precipitation Aa + B/3, if the 



~ , Aa + B/3 M ,. ' , 



fraction C be , these equations become 



