igi6] SHULL— SOILS 23 



sents relative lowering of the vapor pressure of the solution as com- 

 pared with that of water. His formula actually gives the relative 



lowering. Nernst, however, uses for this factor in the 



equation, and with concentrated solutions it is quite a different 

 thing. In a concentrated solution, where the relative lowering of 

 the vapor pressure is 0.75, Nernst's formula will give a value for 

 the osmotic pressure 4 times as great as that given by Walker's 

 formula. 



In the third English edition of Nernst he gives an equation 

 for this calculation which he claims gives a very exact value for 

 the osmotic pressure from vapor pressure. His equation is as 

 follows: 



_ 0.0821.7". 1000 5 , b 



P = — — 77 — - . In - 



M p< 



The physical chemists claim that this formula should hold in so 

 far as it includes the factors involved. But even this exact 

 formula does not take care of the change in volume occurring on 

 dilution of the acid, nor for the heat of dilution, which is very 

 considerable in the strong acid solutions over which equilibrium 

 of moisture vapor was obtained in these experiments. 



I have chosen to use Walker's formula because it actually 

 gives the relative lowering of the vapor pressure, as it is supposed 

 to do. However, it is perfectly clear from this discussion that the 

 osmotic pressure of sulphuric acid of any given concentration, 

 especially of high concentration, cannot be measured accurately 

 by any single vapor pressure formula suggested to date. 



The measurements made over sulphuric acid give values, there- 

 fore, which are merely suggestive. They indicate that the internal 

 forces of air-dry seeds of all kinds are very high, and confirm the 

 high values obtained by the osmotic solution method. But the lat- 

 ter method only, I believe, can be relied upon for the measuring 

 of the internal forces of seeds until the vapor pressure relations are 

 more perfectly understood. 



The main conclusion in regard to these seed measurements may 

 be stated briefly thus : It is possible, with seeds having a perfectly 

 semipermeable coat, to measure the water-attracting internal forces 



