516 JOHN JOHNSTON AND PAUL NIGGLI 
such as hibschite, kaolin, pyrophyllite, nontronite, all closely 
related to the kaolin formula R,ALSi,0,. Now the reaction between 
complex silicates and water is not necessarily simple; for instance, 
the reaction may be due to hydrolysis of the first products of decom- 
position of the silicate. Moreover, atmospheric carbon dioxide 
might play a part in the reaction. In this connection it is well to 
observe that it is incorrect and useless to speak of the solubility of a 
mineral in water, unless the mineral dissolves as a whole; just as 
it is useless to speak of the solubility of a mefal in an acid. We 
may speak of solubility only when the relations between the various 
components are the same in the solution as in the solid, or when it 
is possible to recrystallize the original silicate from the solution. 
Otherwise we are dealing, not with a process of solution, but with a 
decomposition. 
