94 CHEMICAL STATICS 



When any two or more substances, for example, A and B react 

 to form other substances A' and B', equiHbrium is reached, i.e., no 

 further change occurs in the composition of the system, when the 

 reaction 



A-\-B:f±A'-^B' 



proceeds at exactly the same velocity from left to right as it does* 

 from right to left. Since the velocity with which a chemical 

 reaction proceeds varies directly as the active masses of the react- 

 ing substances (Guldberg and Waage's law) the velocities of the 

 two opposing reactions gradually approach each other as the 

 reaction proceeds, until they become equal, when the reaction 

 apparently ceases. According to the relative magnitude of the 

 proportionality-factors expressing the ratio of mass to velocity of 

 each of the reactions concerned, equilibrium may occur so far to 

 the left or right that, with our limited precision of measurement, 

 the reaction appears to be complete or not to occur at all as the 

 case may be, or equilibrium may occur when the active masses of 

 the reacting components on either side of the equation are nearly 

 equal; and, of course, every type of equilibrium intermediate be- 

 tween these extremes is found to occur. If, after the attainment 

 of equilibrium, one of the reacting components be wholly or par- 

 tially removed from the system the reaction will recommence more 

 or less rapidly, restoring a certain proportion of the abstracted 

 component, and attaining a fresh station of equilibrium deter- 

 mined by the new relative masses of the components. 



If, now, after the attainment of equilibrium in such a reaction as: 



Casein + HCl ^ Casein hydrochloride 



(Insoluble) (Soluble) (Insoluble or sparingly soluble) 



even if only a small proportion of free HCl and casein exist in the 

 system, and HCl be removed by trituration with water, the re- 

 action will recommence, now proceeding from right to left, and 

 restore a proportion of the HCl which has been washed away. If 

 trituration be prolonged and the velocity-constant (proportionality 

 between mass and velocity) for the decomposition of the casein 

 hydrochloride be tolerably high, it is easy to understand how the 

 compound might ultimately be completely decomposed and 

 deprived of its combined acid. Similarly, the acid components of 

 substances so universally admitted to be chemical compounds as 

 mercuric sulphate and cuprous chloride can be completely removed 



