>i6] 



SHULL— SOILS 



15 



made at Chicago. The hygroscopic moisture of the seeds was 

 uniformly near 5.5 per cent in Kansas, 1 while at Chicago the 

 hygroscopic moisture was rarely less than 7 per cent, usually above. 

 The equilibrium point would naturally run higher, therefore, in a 

 drier climate. However, it is not claimed that the discrepancy 

 would entirely disappear if repeated with seeds of 

 the same moisture content. 



There seems to be little doubt as to the high 

 osmotic strength of the sulphuric acid, for the 

 boiling-point method of measuring osmotic press- 

 ure gives a value in close agreement with those 

 just given. The boiling point of 53 per cent H 2 S0 4 

 is 128. 5 C. If for each rise of 0.52 C. we may 

 assume an osmotic pressure equal to one mole of 

 dissolved substance (Hober, Phys. Chem. p. 19), 

 this strength of sulphuric acid should have an 

 osmotic pressure of 1227 atmospheres. 



At all events, it is safe to con- 

 clude that air-dry seeds possess a very 

 high internal attraction for water 



100% 



845 



73 66 57.5 54 50 



39 35 26.5 



H 2 



Fig. 3. — Curves of moisture equilibrium of seeds suspended over sulphuric acid: 

 a, Ricinus communis; b, Triticum sativum; c, Oryza saliva; abscissae, strength of 

 sulphuric acid; ordinates, moisture intake by seeds in percentage of air-dry weight. 



which at the initial moment of intake is but little short of 1000 

 atmospheres. Owing to uncertainty of the figures for sulphuric 

 acid, only those obtained by direct contact with osmotically 

 active solutions will be used as a standard for the following soil 

 experiments : 



1 Seeds sent to Washington were dried in the vacuum oven by Mr. A. B. Camp- 

 bell. Upper seeds averaged 5.48 per cent of the absolute dry weight, and lower 

 seeds averaged 5. 72 per cent of their absolute dry weight. 



