4 BOTANICAL GAZETTE [jdly 



The physicists Young, Dupre, and Lord Rayleigh have esti- 

 mated the surface forces of finely divided absolutely dry matter 

 at from n,ooo to 25,000 atmospheres. Lagergren's (20) estimate 

 for charcoal fine enough to have 4 sq. m. of internal surface per 

 gram was 6150 atmospheres. 



It is probable that this method would give results too high for 

 soils, for, as Muntz and Gaudechon (22) have shown, there are 

 other sources of heat release than mere compression when absolutely 

 dry soil and water are mixed. Heat of solution, dilution, and 

 hydration may make considerable errors in estimates of surface 

 forces by this means. The statement frequently made that the 

 force of surface condensation in soils runs from 6,000 to 25,000 

 atmospheres, as by Cameron (10), and by Brown and Smith (5), 

 is based upon the discussion previously mentioned. 



B. Compression method. — Rodewald (27) has used - a different 

 method in measuring the forces on the surface of starch particles, 

 which has the advantage of being a direct method; that is, the 

 forces of compression are measured by the amount of compression 

 produced instead of by the amount of heat produced. He found 

 that 1 gm. of oven-dry starch absorbed 0.326 gm. of water in becom- 

 ing saturated. But while the starch swelled, the swelling did not 

 amount to as much as the volume of water absorbed. In other 

 words, there was a volume loss due to compression of the water. 

 The amount of volume loss was 0.0432 cc, and if we refer this to a 

 gram of water, the volume loss is equal to 0.1325 cc. per gm. The 

 compression coefficient of water is calculated by Wullner to be 

 0.00004659 cc. per gm. for each atmosphere of pressure exerted. 

 This would give a pressure of 2821 atmospheres for the compres- 

 sion actually obtained if we refer the compression solely to the 

 water involved. 



By a slightly different method of calculating the force of com- 

 pression Rodewald obtained a result of 2523 atmospheres, which 

 is not referred to the water alone, but to the whole system of starch 

 and water. He thinks that the close agreement shows that water 

 alone is involved, or that starch happens to have about the same 

 coefficient of compressibility as water. 



The low value obtained by Rodewald as compared with the 

 values for inorganic bodies, Patten thinks is due to the fact that 



