25 
Soil No. 516. — Here again it was not possible to determine exactly 
the optimum point, but 26 per cent water is probably very close to this 
stage. The effect of increasing the moisture content is to decrease 
cohesion to a certain point, followed by an increase, then descending 
as the optimum moisture content is approached. Potassium sulphate 
decreases cohesion while the rest of the salts apparently increase this 
property in varying degree. 
Soil No. 428. — The effect of varying the moisture content of this 
soil differs from that observed with the other soils in that the de- 
crease in cohesion is regular and rapid as a result of increasing the 
moisture content up to the optimum point. The effect of salts is to 
increase the cohesion of this type of soil. This fact was found to be 
true in every instance. Owing to a lack of sufficient soil, ammonium 
sulphate and sodium nitrate were not used. 
Throughout this work care was exercised to subject each cup of soil 
to the same procedure. Penetrations were made at equal distances 
from the center of the cup, the weight was allowed to fall through 
equal heights, and similar methods used throughout. Even with 
these precautions it was difficult to obtain closely agreeing results 
from a clay soil, but very concordant results were obtained from 
the other types. 
APPARENT SPECIFIC GRAVITY. 
Closely related to cohesion and bearing directly on the swelling of 
soils on wetting and shrinking and cracking upon drying is the appar- 
ent specific gravity, the relation between the weight of a soil and the 
volume it occupies. This property has been supposed to be at a 
minimum at the optimum moisture content of the soil. Like all 
physical properties it is subject to modification and is more or less 
affected by the same factors that affect the cohesive power. 
The same apparatus used for penetration experiments was used for 
the determination of the apparent specific gravity. The data were 
obtained by dividing the weight of the soil in the container by its 
volume. The results are given in Table XI. 
Table XI. — Effect of salts and moisture content on apparent specific gravity. 
SOIL NO. 573. 
Check. 
Potassium 
Calcium 
Superphos- 
Ammonium Sodium 
Sodium 
sulphate. 
oxid. 
phate. 
sulphate. nitrate. 
carbonate. 
Ap- 
Ap- 
Ap- 
Ap- 
Ap. 1 
Ap- 
Ap. 
Moist- 
parent Moist- 
parent 
Moist- 
parent 
Moist- 
parent Moist- parent Moist- 
parent 
Moist- 
parent 
ure 
SRf- 
ure 
spe- 
ure 
spe- 
ure 
spe- 
ure 
spe- 
ure 
spe- 
ure 
sp^e- 
con- 
cific 
con- 
cific 
con- 
cific 
con- 
cific 
con- 
cific 
con- 
cific 
con- 
ciflc 
tent. 
grav- 
tent. 
grav- 
tent. 
grav- 
tent. 
grav- 
tent. 
grav- 
tent. 
grav- 
tent. 
grav- 
ity. 
ity. 
ity. 
ity. 
ity. 
ity. 
ity. 
Perct. 
Perct. 
Perct. 
Perct. 
Perct. 
Perct. 
Perct. 
13.43 
0. 7017 
16.67 
.6745 
io-ei 6.6807 
is. 85 
0.6842 
15.55 
0.6868 
15.43 
0.6587 14.55 
0.6693 
16.94 
0.6605 
20.65 
. 6359 19. 14 
.6305 
17.62 
.6605 
17.60 
.6544 
17.87 
.6394! 18.05 
.6394 
19.26 
.6412 
23.14 
.6000 21.94 
.5021 
20. 4C 
.6254 
21.22 
.6193 
20.54 
.6061 
21.10 
.5894 
21.70 
.6079 
26.11 
24.72 
.5438 
22.96 
.5842 
23.02 
.5710 
23.15 
.5674 
23.78 
.5587 
24.07 
.5649 
