508 JOHN JOHNSTON AND PAUL NIGGLI 
the form of gelatinous silica) liberated; but yet at a very high 
temperature, as in the process of glazing earthenware, silicic anhy- 
dride in presence of water vapor is capable of decomposing sodium 
chloride with expulsion of hydrochloric acid. This behavior is not 
at first sight dependent on solubility relations; but it is perfectly 
analogous if we consider that volatility may be looked upon as 
solubility in a vacuum as solvent. We can therefore extend the 
statement of the preceding paragraph and say that the factor which 
determines the result of heating together a number of substances 
at high temperatures is lack of solubility or of volatility, the sub- 
stance which tends to appear being that which under the particular 
equilibrium conditions is least soluble or least volatile. 
THE LAW OF MASS ACTION AND THE REACTION CONSTANT TG 
Consider the reversible chemical action 
A+BsC+D 
where the letters represent single molecules of the substances as in 
ordinary chemical formulae. Then according to the law of mass 
action the state of equilibrium of the above system is determined 
by the equation’ 
[A] [B]_ 
[C] [D] 
where the symbols [A], [B], etc., represent the “active masses” at 
equilibrium of the respective substances; K is a constant, the value 
of which depends solely on the external conditions (e.g., tempera- 
ture) but is independent of the total amount of material present. 
When the reaction takes place in a wholly gaseous system, the 
active mass of each substance is proportional to its molecular con- 
centration or partial pressure. When the system is an aqueous 
«When the reaction is nA+mB <s pC+gqD (that is, where more than one 
molecule of any (or all) of the substances enters into the reaction) the expression for 
the reaction constant is 
IN n B m e 
[AP"BI" _ 
[C]?[D}? 
In this case attention must be paid to the unit of concentration employed except when 
m-+n=p-+gq, as the numerical value is then dependent on the unit of concentration 
employed. 
