No. 582] SIGNIFICANCE OF INTERNAL CONDITIONS 347 



1. The Law of Mass Action 

 The mass law or law of mass action expresses the re- 

 lationship between the molecular concentration and the 

 speed of the reaction. The concentration of the substance 

 is usually expressed in terms of the number, either whole 

 or partial, of gram molecular weights or gram molecules 

 present in one liter of the solution. On the basis of Avo- 

 gadro's law, there is the same number of molecules of 

 sodium chloride in a gram molecule (58.4 grams) of 

 sodium chloride as there are molecules of cane sugar in a 

 gram molecule (342 grams) of cane sugar or molecules 

 of oxygen in a gram molecule (32 grams) of oxygen. If 

 equal fractions of the gram molecular weights of two dif- 

 ferent substances are each dissolved in a liter of water, 

 there will be the same number of molecules in each of 

 the two solutions. 



Let two substances, A and B, be present in a solution 

 in the concentrations (expressed in gram molecules) of 

 c A and c B , respectively, at any stage of the reaction 

 A + B C + D and let the temperature remain constant 

 throughout the action. The speed (S) of the forward ac- 

 tion expressed in gram molecules of A and B transformed 

 in unit time is denned by the relation c A Xc B XF^S 

 where F is the affinity constant. As the reaction pro- 

 ceeds, c A and c B and, hence S, steadily decrease, since A 

 and B are being continually used up. S may therefore be 

 taken at any time as the quantity of A and B which would 

 be transformed in the unit of time if the concentration 

 o A and c B were maintained at a constant value by the con- 

 tinual addition of new substance. F is the measure of 

 the intrinsic activity (affinity) which is the driving force 

 in the reaction, and is independent of the concentration. 

 If unit concentrations are taken, c A = c B = l and F —S. 

 The activity, F, is thus represented numerically by the 

 number of gram molecules transformed in unit of time 

 when each reacting substance is present in unit concentra- 



