34 
BULLETIN 62, HAWAII EXPERIMENT STATION 
water-vapor adsorption. The adsorption of water vapor was de- 
termined in accordance with the findings of Middleton (26, p. 435), 
who noted the effect of the various factors upon the adsorption en- 
tering into this determination. Two to three grams of soil or colloid 
was placed in shallow aluminum dishes, and these in turn were 
placed in a desiccator over 3.3 per cent by weight of sulphuric acid. 
From 20 to 28 samples were used at the same time, and 100 cubic cen- 
timeters of sulphuric acid. The adsorption of the colloids and of 
their corresponding soils was always determined together. The 
desiccator was evacuated by a vacuum pump to about 28.5 inches 
mercury — about 50 millimeters absolute pressure — placed in a ther- 
mostat which was regulated to 30° C, and kept there for six days. 
At the end of that time adsorption was considered as complete. The 
dishes were immediately covered, weighed, and dried at 105° for 
18 hours, and then weighed again. Table 15 gives the adsorptivities 
obtained and the percentage of colloids calculated therefrom. The 
results are shown graphically in Figure 12. 
X 
\ 
St 
1 a 
Table 15.- 
1 &OZL 
W//////H COLLO/0 
I 
i 
1 
t 
\ 
t 
r 
* 
1 

P 
t 
P 
/ 
t 
/ 
\ 
i 
,._ 
/ 
/ 
/ 
/ 
4 
/ 
■ 
■ 
l 
/ 
/ 
/2 2/ 3/ 
37 42 48 
SO/A. WO. 
sff S7 ss eff g& 7e 
Figure 12. — Water vapor adsorbed by soil and extracted colloid 
-Water vapor adsorption in Hawaii soils and colloids as a measure 
of colloidal content 
Soil No. 
Adsorp- 
tion per 
gram of 
soil (a) 
Adsorp- 
tion per 
gram of 
colloid 
(b) 
Com- 
puted 
percent- 
age of 
colloids 
in soil 
Amount 
of 
colloids 
by 
pipette 
method 
Soil No. 
Adsorp- 
tion per 
gram of 
soil (a) 
Adsorp- 
tion per 
gram of 
colloid 
(b) 
Com- 
puted 
percent- 
age of 
colloids 
in soil 
Amount 
of 
colloids 
by 
pipette 
method 
-Jxioo 
?X100 

2 
Oram 
0.133 
.208 
.216 
.237 
.178 
.226 
.186 
.097 
Oram 
0.236 
.271 
.221 
.240 
.219 
.208 
.183 
.122 
Per cent 
56.4 
76.7 
97.7 
98.7 
81.3 
108.6 
101.6 
79.5 
Per cent 
51.5 
66.6 
15.0 
23.9 
49.3 
23.1 
32.5 
30.1 
56 
Gram 
0.093 
.120 
.296 
.256 
.188 
.148 
Oram 
0.124 
.138 
.331 
.266 
.260 
.223 
Per cent 
75.0 
87.0 
89.4 
96.2 
72.3 
66.4 
Per cent 
48.0 
6 
57 — 
49.1 
12 
65 
35.1 
21 
66 
30.6 
31 
69 
48.9 
37 
76 - 
25.8 
Average. . 
48 
.184 
.217 
! 
