Oct. 6 , 1933 
Auxotaxic Curve as a Means of Classifying Soils 
13 
METHOD OF DETERMINING THE RATE OF SWELLING 
In addition to the technique given in the writers’ previous paper on 
the swelling coefficient, it is essential to determine the rate of swelling 
in distilled water at a constant, convenient temperature. A temperature 
of 2 7 0 C. has been selected as both convenient and easily maintained. 
Lowering the temperature retards the rate; increasing the temperature 
accelerates it. The curves in figure 3 show the rates of expansion of a 
calcareous clay at o° C., 27 0 C., and 70° C. This soil from Utah, which 
is discussed in another connection below, is well adapted to illustrate 
temperature effects, since it has a rather long period of slow expansion 
followed by a shorter period of rapid expansion. In this case changes 
in temperature have the effect especially of shortening or lengthening the 
period of slow preliminary swelling. 
While the auxotaxic curve of any soil in distilled water under standard 
conditions is fixed, it may be modified by the addition of electrolytes, 
molecules such as sugar, or colloids. Electrolytes in the water accelerate 
the rate of swelling and cause the curve to approach in form that obtained 
in distilled water at 70° C. Figure 3 illustrates the action of 1 per cent 
sodium chlorid in accelerating the rate of swelling of the calcareous clay 
from Utah. Figure 4 shows how 1 per cent of sodium chlorid caused a 
slowly expanding soil to swell as much in a few moments as it had in 
more than an hour in distilled water. The curve representing the rapid 
swelling, moreover, takes on the two-period form so noticeable in most 
of the other curves. Unmistakable acceleration is produced by 0.05 per 
cent sodium chlorid. Twenty per cent of cane sugar retarded the 
swelling of this clay, giving a curve almost identical with that of water 
at o° C. (curve not reproduced here). Small amounts of gelatin accel¬ 
erate the rate of swelling and large amounts retard it. It is possible that 
the retardation of the swelling by strong sugar or colloidal solutions may 
be due to increased viscosity. This subject is being studied. The fact 
that the same concentration of colloid has a markedly different effect 
on the rate of expansion of different soils suggests that here also may be 
found another means of studying the colloidal properties of different 
soils. Small amounts of electrolytes accompanying strong solutions of 
gelatin also greatly modify its effect. 
LATERAL CRUSHING OF TILE EXPLAINED BY THE AUXOTAXIC 
CURVE 
The calcareous clay from Utah mentioned above was submitted to 
the writers by R. A. Hart, senior drainage engineer of the Bureau of 
Public Roads, Salt Lake City, with the statement that tiles laid through 
it were always broken by lateral pressure. With the aid of the auxo¬ 
taxic curve (fig. 3) the cause of the failure of the pipe line seems evident. 
The soil on the ditch bank dries out to a considerable degree and when 
the back fill is made the slow preliminary swelling serves to compact the 
soil firmly in the ditch and about the pipe. The second period of more 
rapid swelling, acting against the ditch wall and confined by the firmly 
packed soil above would serve to crush the tile laterally. The coefficient 
of expansion of this soil, moreover, is 187.5, which is among the highest 
coefficients obtained for any soils which the writers have so far examined. 
