CHEMICAL EQUILIBRIUM 235 



increased. The introduction of an excess of hydrogen chloride 

 would have had precisely the same effect. 



An Experimental Illustration. A reaction in which the 

 effects of different concentrations were carefully studied by Glad- 

 stone (1855) affords a good illustration. If ferric chloride and 

 ammonium thiocyanate are mixed in aqueous solution, a liquid 

 containing the soluble, blood-red ferric thiocyanate is produced. 

 The compound radicals are (NH 4 ) and (CNS), and the action is 

 a simple double decomposition : 



FeCl 3 + 3NH 4 CNS^Fe(CNS) 3 + 3NH 4 C1. 



The action is a reversible one, and the mixture is homogeneous, 

 i.e., there is no precipitation. Now, if the two just-named salts 

 are mixed in very dilute solution in the proportions required by the 

 equation, say by adding 20 c.c. of a decinormal solution of each 

 salt to several liters of water, a pale-reddish solution is obtained. 

 When this is divided into four parts, and one is kept for reference, 

 the addition of a little of a concentrated solution of ferric chloride 

 to one jar, and of ammonium thiocyanate to another, will be 

 found to deepen the color by producing more of the ferric thio- 

 cyanate. On the other hand, mixing a few drops of concentrated 

 ammonium chloride solution with the fourth portion will be found 

 to remove the color almost entirely, on account of its influence in 

 favoring the backward change. 



The Law of Molecular Concentration. The general prin- 

 ciple discussed and illustrated in this section may be called the 

 law of molecular concentration, and may be stated as follows : In 

 every chemical change the activity, and therefore the speed 

 of the action, is proportional to the molecular concentration of each 

 interacting substance. This holds whether the action is reversible 

 or not. 



The molecular concentration is expressed, numerically, for each 

 substance, in terms of the number of moles (gram-molecular 



