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IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
ternal pressures, then they will be soluble in each other in all pro- 
portions. In other words, when a molecule of solute is under the 
same forces when surrounded by molecules of its own kind as 
when it is surrounded by molecules of solvent, then solution 
will take place; but when these forces are altogether different, 
solubility will not take place. Thus he explains the immiscibility 
of benzene in water as being due to the great difference in the in- 
ternal pressure between the two components. The miscibility of 
alcohol and water, he claims, is due to the similarity of the in- 
ternal pressures of the two liquids but is influenced also by their 
polarity. 
The internal pressure of any component can be determined by 
three or four different methods and while concordant results are 
not obtained, the order of arrangement is in general the same. 
Hildebrand has the support of Bradford who comes to the same 
conclusion, although his mode of approach is somewhat different. 2 
When, however, we note a number of cases, no such universal 
agreement as these men claim between solubility and internal pres- 
sure seems to occur, for illustrations can be found where two com- 
ponents have nearly the same internal pressure and yet are not 
soluble in each other. Again components can be found which are 
miscible in all proportions but their internal pressures are by no 
means alike. Such cases are not in harmony with the theory and 
hence we can not depend on this principle to solve the problem of 
solubility. 3 
POLARITY 
Let us now consider the effect of polarity on solubility. By pol- 
arity we mean the condition or state of being polar, i.e. the mole- 
cules of a substance have the same property as magnets in the 
sense that they can attract other molecules of like properties. Thus 
if we have a polar substance, its molecules will attract each other 
and association will take place. 
In Table I are given the solubilities in ccs. of C0 2 per cc. of the 
solvent listed. The results in Tables I, II, and III are due to Just 4 
and were taken at 20° C. The chemical formulae and the structural 
formulae of the solvents also are given. For simplicity the H’s are 
omitted in the structural formulae. 
2 Bradford, Phil. Mag. 38, p. 696 (1919). 
3 For a further discussion of this and related theories of solubility the reader 
should consult Phys. Rev. May, 1922. 
4 Just, Zeit. fiir Phys. Chem. 37, p. 342 (1901). 
