RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 109 



be stirred with a glass or suitable wooden rod to expel all air bubbles. Thawing may 

 be completed within 15 or 20 minutes at most, and the possibility of chemical change 

 is thus very greatly reduced. When thawing is complete, -the thermometer is inserted, 

 the tube is placed in the freezing mixture, and the material allowed to reach a tem- 

 perature about 1° below its freezing point. Solidification is then brought about, as 

 has been stated, by turning the thermometer backward and forward a few times to 

 create a slight disturbance in the pulp. It has been found in practice that much more 

 .satisfactory results are obtained if the material is thus allowed to undercool about 1° 

 than when solidification is brought about with less undercooling. In the latter case 

 the mercury rises to the freezing point much more slowly and the determination of 

 this point is consequently more difficult. 



Osmotic pressure in soils. 



Although it is possible to remove the soil solution from the soil 

 and to determine its osmotic pressure by the freezing-point method, 

 this will fall far short of the desired end, which is to determine how 

 the water behaves in the presence of the capillary forces and ad- 

 sorption tendencies of the soil particles and colloids. As has been 

 suggested in the introductory paragraphs to this chapter, these in- 

 fluences may run parallel with the influences of dissolved salts in 

 the soil water. 



The freezing-point determinations for moist soils are so similar in 

 method to those for plant pulps that it seems unnecessary to describe 

 them here in detail. The reader is especially referred to the descrip- 

 tion given by Bouyoucos (107) . It would seem that the fundamental 

 consideration in testing a given soil at various moisture contents is 

 to have samples very evenly wetted. This is accomplished by placing 

 the samples in a moisture-tight jar and, after thorough shaking, allow- 

 ing it to stand one or more days, so that the moisture is evenly dis- 

 tributed and, has ample opportunity to be adsorbed. While it is 

 possible to use measured amounts of water in wetting the soil, it is 

 probably safer procedure to take moisture samples at the same time 

 that samples are taken from the jar for freezing tests. 



Vapor transfer in soils. — The vapor transfer method is the only 

 other method of osmotic determination which appears feasible for 

 soils, and where time is not an important element, it is believed to be 

 preferable to the freezing-point method because of the possibility 

 of treating soils in their natural states. It should be understood, how- 

 ever, that the value of the method is as yet theoretical rather than 

 proven. 



The work of Alway (103) and Hilgard (125) on the hygroscopic 

 coefficient of soils has already been mentioned, with the suggestion 

 that since the moisture boxes used could not completely prevent the 

 vapor from escaping, their results are not indicative of those to be 

 expected when vapor pressures are maintained in true equilibrium 



