188 BOTANICAL GAZETTE [SEPTEMBER 
The findings of BERKELEY and HARTLEY were discussed by 
CALLENDAR (13), who offers a plausible explanation for the high 
pressures observed. He suggests that the solute probably enters 
into a molecular complex with the solvent, forming hydration 
compounds. This chemical combination of solute with solvent 
reduces the number of free molecules of water in the solvent, 
decreases therefore the vapor tension, or, what amounts to the same 
thing, increases the osmotic pressure; for, as CALLENDAR says: 
“There is a definite and simple relation between vapor pressure and 
osmotic pressure which has been verified by Lord BERKELEY and 
HARTLEY for strong solutions.’ 
The degree of hydration is represented by a in CALLENDAR’S 
discussion, and he believes a=5 in the case of the sucrose solution 
used by BERKELEY; and in the case of KAHLENBERG’S (23) observa- 
tions on the rise of boiling-points in concentrated solutions of NaCl, 
the curve agrees with a hydration value of a=6 up to a concen- 
tration in which there are 6 molecules of NaCl present for every 
100 molecules of water (CALLENDAR 13, p. 493, diagram p. 492). 
CALLENDAR assumes that the molecular complex formed by the 
hydration is a definite chemical compound at any given concentra- 
tion. In this he does not agree with Jones and Bassett (22), 
who claim to have shown that such hydration compounds are 
indefinite, and may vary from a few to many molecules of water to 
each molecule of solute in any given concentrated solution. 
If the compounds formed were indefinite, it would not be pos- 
sible to explain the increased osmotic pressure of strong solutions by 
hydration; but CaLLENDaR points out that the determinations of 
JONEs and his co-workers are erroneous at various places, and their 
conclusions untrustworthy. The writer can only express it as his 
opinion that CALLENDAR’s assumption is probably the correct one, 
that the hydration compounds are definite, and that the osmotic 
pressure of concentrated solutions depends partly upon the hy dra- 
tion value of the molecular complex formed. 
If this opinion is correct, then Drererici’s and RACIBORSKI'S 
figures for thé osmotic pressure of saturated NaCl solutions, which 
are based on vapor pressure determinations, are much nearer the 
truth than Brown’s figures for the same solution. Although the 

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