1913] SHULL—SEMIPERMEABILITY OF SEED COATS 187 
pressures agreeing with the gas laws. But curves representing the 
pressures in higher concentrations show that the osmotic pressure 
increases in strong solutions much more rapidly than the molecular 
concentration. 
Formerly it was customary to calculate the osmotic pressure on 
the basis of solution volumes. Morse, FRASER, and others, how- 
ever, found a better agreement between increase of concentration 
and osmotic pressure when the concentration is referred to a liter 
of solvent rather than to a liter of solution. RENNER (31) has 
recently given an excellent summary of the literature on the calcula- 
tion of osmotic pressures, and holds that this change from liter of 
solution to liter of solvent will bring into agreement the pressures 
obtained by plasmolytic and those obtained by cryoscopic methods, 
the former having been in error. 
Although making this change brings agreement between 
theoretical and observed pressures for more concentrated solutions 
than was true with the old method of making up molecular solutions, 
the observed pressures are still considerably above the pressures 
demanded by our theories, after all these corrections and improve- 
ments in method have been made. 
The relation of the actual pressures found by BERKELEY and 
HartLey to the pressures demanded theoretically by both methods 
of calculating pressure from concentration is shown graphically by 
PuiLip (27, p. 53) by means of curves. RENNER also shows a 
similar diagram in his recent paper but does not mention PHILIP’s 
discussion. There is little doubt of the correctness of the pressures 
found by the direct methods of measurement employed by 
BERKELEY and Hartiey, as their results have been confirmed 
recently by Trouton (36), who uses an entirely new method for 
the direct measurement. Ether will not take up as much water 
from a solution as it will from pure water. He determines the 
pressure necessary to force as much water from a solution of 
sucrose into ether as the ether will take up normally from pure 
water (1.05 per cent). The pressure required to force the water 
content of ether up to 1.05 per cent from a solution of sucrose con- 
taining 600 grams per liter was 80 atmospheres. BERKELEY’S result 
was 81 atmospheres with the same strength of sucrose solution, an 
agreement well within the limits of error. 
