THE ORIGIN OF IGNEOUS ROCKS. 
155 
that they do not exist in a molten magma in fixed molecular 
compounds that correspond to definite mineral molecules, 
but are in effect uncombined and free to form such combi¬ 
nations as circumstances may impose. 
From the great variability in the proportions of the sim¬ 
ple oxide molecules in the different varieties and facies of 
rocks belonging to one group, and from the absence of all 
stoichiometric ratios between these oxides, it is equally evi¬ 
dent that in the process of differentiation the simple oxide 
molecules shift about independently of one another to a 
great extent, though certain tendencies toward association 
must not be overlooked and the little understood force 
termed “ mass action ” must be taken into account. 
Since the foregoing conclusion was written Prof. F. W. 
Clarke has called the writer’s attention to the steadily accu¬ 
mulating evidences that are leading chemists to the opinion 
that many salts when in solution are dissociated to a greater 
or less extent. 
The paper by Arrhenius* on the dissociation of matter 
dissolved in water notices the exceptions that are found to 
exist to the law of osmotic pressure as stated by van’t Hoff, 
and suggests that they correspond to the exceptions to Avo- 
gadro’s law of gaseous pressure, which are due to the disso¬ 
ciation of gases. They are both additive in character, and 
Arrhenius argues that in such cases the salts in solution are 
more or less dissociated. 
Following this line of argument it is found that a salt in 
solution may be partly dissociated and partly combined, and 
that the extent of dissociation is greater as the solution is 
more dilute, and that it varies with the nature of the salt, 
and is most noticeable with strong bases and strong acids. 
It is also pointed out that the lowering of the freezing point 
of a solution is an additive effect wdiich indicates a state of 
more or less dissociation. 
This view of the character of solutions appears to be in 
* Arrhenius (S.) fiber die Dissociation der in Wasser gelosten Stoffe. 
Zeitschr. phys. Chemie. 8°. Leipzig, 1887, pp. 631-648. 
