4 BOTANICAL GAZETTE [JULY - 
The physicists Younc, Dupré, and Lord RAytricuH have esti- 
mated the surface forces of finely divided absolutely dry matter 
at from 11,000 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 Mttntz 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 SitH (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 RopEWALD as compared with the 
values for inorganic bodies, PATTEN thinks is due to the fact that 
