May io, 1924 Factors Influencing Binding Power of Soil Colloids 501 
In making a briquette the piston was pushed to the bottom of the cylinder 
with a wooden stick, the material was introduced into the cylinder, and the caps 
put in place. The valve on the pump was closed and the pressure gradually 
raised to 2,000 pounds, as indicated by the gauge. After the pressure had been 
applied a sufficient length of time, it was released by opening the valve and the 
caps were removed. Then the valve was closed and the briquette pushed out of 
the cylinder (as shown in fig. 2) by operating the hydraulic pump, and removed. 
All briquettes which were used for compression tests were made 25 mm. high. 
In order to determine the amount of material necessary to make a briquette 25 
mm. high, a preliminary briquette containing 20 gm. of the material was made 
up. The height of this briquette was measured with a micrometer and the 
amount of material necessary to make a 
briquette 25 mm. high calculated by simple 
proportion. This preliminary briquette was 
then used to determine the amount of 
moisture in the material at the time the 
briquettes were made. 
GENERAL CONSIDERATIONS 
At the beginning of this work, it was 
found very difficult to get determinations 
of the breaking strength of any one given 
material which could be duplicated at 
two different times. The briquettes in a 
given series made up at one time would 
give very different results from the same 
series made up at another time. In tracing 
out the causes of the discrepancies there 
were found to be four main factors gov¬ 
erning the compressive strength of the 
briquettes, as follows: 
(a) The amount of moisture present in 
the material when the briquettes were 
made. 
( b ) The treatment of the material before 
the briquettes were made. 
(c) The pressure used in making up the 
briquettes. 
(d) The method of drying the briquettes. 
When these factors were taken into consideration the results could be duplicated 
within 5 per cent of the average load. 
MOISTURE CONTENT 
Dry briquettes of a given material varied in breaking strength depending on 
the moisture content of the material. By varying the moisture content and 
keeping the dry weight of the material constant a point was found where the 
briquette reached a minimum size. This indicated that the density of the mate- 
rial was greatest at this point. For the purpose of this investigation, the moisture 
content at this point was called the critical moisture content. Since the diame¬ 
ters of the briquettes were held constant by the size of the opening in the cylinder, 
the differences in the size of the briquettes were indicated by their heights. 
| +-25mm+ 
Fig. 2—Cylinder for molding briquettes 
