[MILLER & KENRICK] IDENTIFICATION OF BASIC SALTS 41 
These conditions are:—(a) The quantity of water must be large in 
comparison with that of the second and of the third component; (b) The 
fourth component must remain in the solution and not enter into the 
composition of the precipitate; (c) In each member of a series of experi- 
ments there must be a constant ratio between the amounts of the fourth 
component and of the water in the system. 
If the water is present in large excess, (condition a) the mass of the 
solution will be much greater than that of the precipitate, and no reac- 
tion occurring in the system can have more than a very slight effect on 
the total mass of the water contained in the solution. In reactions of 
the first type (during which the composition of the solution remains 
unaltered), the quantities of the other components entering or leaving 
the solution must be still smaller than that of the water; so that re- 
actions of this type leave the total mass of the solution (practically) 
unaltered. 
If now a fourth component be added, the total amount of which 
in the solution is unaffected by the reaction (condition b), this shght 
variation in the quantities of the other components in the solution will 
cause such a very slight change in the composition of the latter that the 
effect of the change in checking the progress of the reaction may safely 
be neglected; so that the presence of the fourth component, under these 
conditions, will not interfere with the efficiency of reactions of the 
first type in keeping down the number of phases to three. 
Reactions of the second type (involving changes in the composition 
of the solution) are of course just as possible when the solution contains 
a fourth component as when it does not. In order, however, that the 
solutions over a given pair of basic salts should reach exactly the same 
composition in different experiments, it is obviously necessary that the 
concentration of the fourth component in the solution should be the 
same from case to case. This is provided for by condition c. In experi- 
ments on the action of caustic potash on solutions of bichloride of 
copper, for example, H,0, CuO, and CuCl, may be selected as compo- 
nents, with KCl as the “fourth component” of the preceding para- 
graphs; and condition ¢ requires that if the quantity of potash added 
to a given volume of the copper solution should vary from case to case, 
enough KCl must be added to keep the total amount of potassium per 
cubic centimetre of water the same in each experiment. 
