20 
BULLETIN 1452, U. S. DEPARTMENT OF AGRICULTURE 
All Ostwald type pipette was not well adapted for the determi- 
nation of the viscosity of concentrated soil colloids, since the mate- 
rial adhering to the glass obscured the flow ; hence use was made of 
a 100 cubic centimeter pipette which had an opening of such size that 
50 cubic centimeter. of water was delivered in about 18 seconds. The 
pipette was placed in an air thermostat at 25° C. and filled to the 
mark. The time required to deliver 50 cubic centimeters into a grad- 
uated flask was noted. The sols were prepared in the same manner 
as the dilute sols, no deflocculating agent being used. 
Determinations were made of suspensions containing varying con- 
centrations of solid. It was found that, after a concentration suffi- 
cient to give a relative viscosity of 1.4 was reached, the viscosity 
increased very rapidly with a slight increase in concentration. At a 
relative viscosity just above 2 the flow became irregular. A further 
slight increase in concentration apparently brought the colloids into 
the region of zero fluidity, which Bingham (7) suggests is where true 
viscous flow ceases and plastic flow begins. The different colloids 
were therefore compared on the basis of the concentrations required 
to give a relative viscosity of 1.9, which was about the highest point 
where viscosity could be determined accurately with the particular 
pipette used. 
Table 10 shows the grams of colloid per 100 grams of suspension 
which were required to give a relative viscosity of 1.9 with the par- 
ticular pipette employed. 
Table 10. — Concentrations of soil colloids required to produce a relative vis- 
cosity of 1.9 
Kind of colloid 
Concen- 
tration of 
solid 
Kind of colloid 
Concen- 
tration of 
solid 
Fallon 
Per cent 
11.2 
11.9 
13.1 
13.8 
Per cent 
13.8 
Susquehanna 
16.4 
Sassafras 
Huntington.. ... 
19.0 
Sharkey 
Judging from the concentrations at which the soil colloids approach 
zero fluidity, they are evidently not markedly lyophile, as compared 
with starch, gelatin, and agar, which under certain conditions form 
jellies at concentrations of 0.5 to 2 per cent (49). 
There is considerable variation in the concentrations at which the 
different soil colloids have a relative viscosity of 1.9. These varia- 
tions do not parallel differences in the viscosity of 2 per cent suspen- 
sions, but they show some correlation with swelling as measured by 
either the volume or weight methods. A parallelism between viscos- 
ity and swelling has previously been observed in the case of several 
other colloids (33, 43) . 
VOLUME OF FLOC 
Different sols have often been characterized in a general way by 
the conditions or reagents producing flocculation. Comparative sen- 
sitiveness to electrolytes, for instance, is one of the more important 
criteria recognized as distinguishing lyophobe from lyophile col- 
