1916] SHVLL— SOILS 9 



tested were suspended in shallow paper baskets a few millimeters 

 above the surface of the acid, the baskets being hung on cotton 

 threads fastened to the corks with carna-uba wax. All metallic 

 condensers were thus avoided. After the seeds were carefully 

 weighed and arranged, the bottles were sunk in a trough of running 

 water to prevent any considerable changes in temperature. Con- 

 densation effects due to change of temperature could not occur 

 except over water, for Mitscherlich (21) has shown that even 

 10 per cent sulphuric acid will prevent deposition of dew in deter- 

 mining hygroscopic coefficients of soils. It may be questioned 

 whether the inclosed space actually reaches the vapor pressure of 

 the solution, for, as Hilgard (17) points out, it is most difficult 

 to secure complete saturation in the case of water vapor. How- 

 ever, the space of air to be brought into equilibrium with the 

 solution vapor pressure in these experiments is very small, and it 

 seems probable that the whole system of liquid, air, and seed 

 comes to an equilibrium pressure in the time of the experiment, 

 except possibly in the case of water. After allowing 15 days for 

 reestablishment of equilibrium by the seeds, the point of no change 

 was determined by weighing. 



The osmotic pressure of the sulphuric acid is roughly deter- 



mined by the use of the vapor pressure formula P=—r~. -^r=-, 



] M ' 



in which / is the vapor pressure of pure water at the temperature 

 of the experiment, f 1 the vapor pressure of the acid, M the molec- 

 ular weight of the solvent's vapor, T the absolute temperature, 

 S the density of the sulphuric acid, and R the gas constant. The 

 osmotic pressure (P) is given in grams per square centimeter, and 

 must be reduced to atmospheres. This formula has been developed 

 for dilute solutions and does not hold accurately for high concen- 

 trations, but there are at present no data on which to base more 

 accurate estimations. The boiling-point method yields a result 

 close to that given by this formula for sulphuric acid, as will be 

 shown later. 



The earliest soil measurements were made with no. 2/0 sand. 

 Seeds of known weight were packed firmly in sand of known 

 water content in paraffined wire baskets, and allowed to come to 



