Physical and Chemical Papers. 67 



about 36 calories. Basing his calculation on the known quan- 

 tity of heat liberated by water on condensation by pressure, 

 he estimates that this heat production would require at least 

 6150 atmospheres of pressure. Similar surface forces have 

 been suggested for soils. Thus Mitscherlich, by using an ice 

 calorimeter, has measured the heat of wetting in soils, and 

 finds that the quantity of heat liberated varies with the physi- 

 cal character of the soil, the finer soils yielding larger amounts 

 of free energy. Brown and Smith explain the fact that bac- 

 teria can continue the nitrification process in frozen soils by 

 suggesting that these surface forces in soils may prevent the 

 freezing of the water nearest the soil particle. They state 

 that the force of surface condensation has been estimated at 

 from 6000 to 25,000 atmospheres. 



However, forces of such magnitude as these do not occur in 

 ordinary air-dry seeds, for they already contain as hygroscopic 

 moisture the water which would be taken up with such tre- 

 mendous force if they were absolutely dry. But they do give 

 us a clear idea of the power with which the seed retains the 

 last part of its hygroscopic moisture, for this must be a force 

 opposite and equal to that with which wetting occurs. 



The discovery of perfectly semipermeable seed coats in bar- 

 ley by Brown in 1907, in wheat by Schroder in 1909, and in 

 Xanthium by myself in 1912, laid open the way to measuring 

 the ordinary internal forces of seeds by osmotic solutions. 

 Such measurements were made by the writer a couple of years 

 ago for the seeds of several species of Xanthium, using. satu- 

 rated lithium and sodium chloride solution, and more dilute 

 solutions of the latter salt to provide the external forces. 

 Seeds were placed in solutions of various concentrations, and 

 allowed to come to moisture equilibrium, which was indicated 

 by constancy in the weight of the seeds. The osmotic pres- 

 sure, calculated by physical chemical methods, gave the ex- 

 ternal force, and therefore the equal internal force of the seeds 

 when they had come to equilibrium with that strength of 

 solution. 



While the forces were by no means so large as those indi- 

 cated for oven-dried matter, still they were found to be of 

 considerable magnitude. In saturated solutions of sodium 

 chloride the seeds were able to crystallize the salt out of solu- 

 tion by withdrawal of pure water from its hydration com- 

 pounds. It was found that an air-dry seed of the cocklebur. 



