PHYSICAL PROPERTIES OF HAWAII SOILS 
37 
either colloidal fraction, indicating a higher adsorptivity of the non- 
colloidal fraction in this soil than in the colloidal fraction. 
Table 16. — Hygroscopicity of coarse and fine fractions of the colloidal material 
as compared with that of the original soil 
Organic 
matter 
Water vapor adsorbed per gram of 
material 
Soil No. 
Original 
soil 
Colloids 
Coarse 
fraction 
Fine 
fraction 
(ultra-clay) 
12 
Per cent 
15.75 
13.36 
Gram 
0.395 
.350 
Oram 
0.388 
.389 
Gram 
0.324 
21... 
.352 
In conclusion, therefore, it seems inadvisable to attempt to trans- 
late physical properties into particle size with soils of high organic 
content, such as are found in humid tropical regions. The writer is 
of the opinion that the determination of the various fractions in the 
mechanical analysis and the determination of the magnitude of a 
significant physical property, such as water-vapor adsorption car- 
ried out on a soil, will adequately describe the quantitative colloidal 
properties of that soil, even though a part of the colloidal property 
be due, for some reason, to the fine silt fraction for instance. If to 
these determinations is added the chemical analysis of the clay frac- 
tion, an approximate qualitative picture may be obtained of the frac- 
tion chiefly responsible for these colloidal properties. It appears, 
however, that with a highly heterogeneous substance, such as soil, 
possessing a number of compounds both organic and inorganic in 
widely varying proportions not only in different soils but sometimes 
in the different fractions of the same soil, the expression of a certain 
physical property in terms of particle size may sometimes become 
misleading. 
COMPARISON OF THE HYDROMETER AND A MODIFIED PIPETTE 
METHOD OF MECHANICAL ANALYSIS 
Bouyoucos (5, tf, 7) proposed the use of a hydrometer for meas- 
uring soil-suspension concentrations. In an early publication (6, p. 
480) on the subject, he proposed a rapid method for the determina- 
tion of the sand, silt, and colloid fraction of soil. In this method the 
hydrometer reading is taken of the soil suspension after 1 and 15 
minutes of settling time, and from these readings the three fractions 
are computed. The author pointed out that in this method the col- 
loid fraction includes a part of the fine silt fraction. In his subse- 
quent article (7, p. 235) , Bouyoucos proposed the use of the hydrom- 
eter for a detailed mechanical analysis in which any number of class 
divisions might be made. Hydrometer readings are taken at differ- 
ent intervals according to Stokes' law of settling particles, and 
from the readings the particular fraction in question is calculated. 
The method and its different modifications are too well known to 
need describing here in detail, hence only a resume is given. 
