Physical and Chemical Papers. 69 



is lower. This imbibition continues until the vapor pressure of 

 the seeds and that of the solution over which they lie are equal. 

 This establishes equilibruim of forces, and no further change 

 occurs. The higher the vapor pressure of the solution, the 

 larger the amount of adjustment which occurs; and the lower 

 the vapor pressure, the smaller the adjustment necessary for 

 attainment of equilibrium relations. 



Above the concentrated acids, where the vapor pressure of 

 the solutions is low. the seeds lose moisture because their own 

 vapor pressure is higher. They continue to lose moisture 

 until the vapor pressure of the seeds is reduced to that of the 

 solution below them, after which no change occurs, as the 

 equilibrium is established at that point. The loss is greatest 

 over the strongest acid, and is successively less as the vapor 

 pressure increases by dilution. 



Between these two extremes there is a point w^hich is easily 

 established where the seeds neither gain nor lose in moisture 

 because the vapor pressure of the seed colloids and that of the 

 solution are equal at the start. 



Using the original weight of the seeds as a base line, and 

 calling percentage of gain in weight positive, and percentage 

 of loss in weight negative, a curve can be constructed, con- 

 necting the equilibrium points in the series. This curve cuts 

 the base line at the point of natural equilibrium. In order to 

 determine the original internal force of the seeds, all that is 

 necessary now is to determine the osmotic pressure of the 

 sulphuric acid over which seeds neither gained nor lost in 

 weight. 



This is done from the data by using the vapor-pressure 

 formula for determining osmotic pressure given in all the 

 physical chemistries, and securing the necessary values from 

 the physical chemical tables of Landolt-Bornstein. In one such 

 determination the internal force of an air-dry seed was found 

 to be more than 900 atmospheres, which is not in serious dis- 

 agreement with the values found by the osmotic-pressure 

 method. A difference of one per cent in the hygroscopic mois- 

 ture of the seeds with which the two tests mentioned were 

 carried out would make much more than 100 atmospheres of 

 difference in the internal force of the seed. Since the differ- 

 ence between the two tests is only about 50 atmospheres, a 

 fraction of one per cent difference in the original moisture 



