The Surface Tension of Water. — Ladd. 269 
tube. We see further that the effect of this pull may result 
in the displacement of material, when the conditions are ful- 
filled in clay or rock dust. In this case the movement of in- 
dividual particles is accomplished as the mass enters the wet 
condition, and a primary shrinkage* ensues, which is followed, 
if evaporation is in excess of the water supply, by a secondary 
shrinkage, and at least partial consolidation. 
Looking upon the average clay as a type of the fine- 
grained incoherent rocks, we find it to consist of minute min- 
eral particles, most of which are thin, angular scales. 
The interstices among these may be regarded as a laby- 
rinth of capillary tubes. The particles have a strong attraction 
for water, which is absorbed with varying capacities. Such a 
substance tends also to elevate water from depths below the 
surface, and to replace that which may be evaporated by the 
sun's heat. But when the evaporating process is the dominant 
force at work, drying takes place and shrinkage of the ))iass 
occurs, f 
Our every-day conception of the drying of clays is so inti- 
mately associated with the idea of their consolidation and 
shrinkage, that we are accustomed to think of these as one 
and the same phenomenon. They are, however, distinct phe- 
nomena, the former directly following the application of heat; 
the latter indirectly consequent upon it through the influence 
of surface tension. 
The nature of this process can readily be seen by a com- 
parative study of the accompanying diagrams. 
Fig. I shows the positions of the surfaces of water in a 
series of capillary tubes, the walls of which are comparatively 
rigid and attract the liquid. The dotted lines indicate the posi- 
tions of the surface of the water at successive stages of evapo- 
ration. 
Fig. 2 illustrates conditions similar to those in Fig. i, ex- 
cept that the walls (a) instead of being rigid are collapsible 
*The terms primary, secondary and tertiary shrinkage are used by 
the writer in discussing a series of experiments on clay. They refer 
respectively to the shrinkage following the application of water to dry 
or slightly moist clay, (when in the incoherent state) ; that following 
drying; and that due to the loss of combined water on ignition or 
"burning". 
fThis has a lineal value, in extreme cases, of over 35 per cent. 
