Grout further calculated that if this same volume of clay were con sidered as a square plate one-fifth as thick as wide, instead of a sphere, over 54 per cent of water would be held to the clay particles by this' molecular attraction. Supposing it to be fair, inasmuch as the kaolin ite crystal is "plate-like," to consider that in a clay half of the grains are approximately spherical and the remainder plate-like. Grout fig ures that a clay having all its particles the size here assumed would take by virtue of the molecular attraction of the clay particles, 40 per cent of water.
In a personal interview the writer suggested to him that he was taking the maximum limit of the distance through which this molecular attraction can be said to operate. His defense was that when the spheres were devoid of a water film they touched one another, but as they gathered to themselves this water film, they need not necessarily be separated•.00005 mm., for the film crowded from the points of closest proximity could be considered as filling up the space that would other wise have to be considered as void.
It must be admitted by the supporters of Grout's molecular attraction theory for plasticity, that he used but a portion of a very fine-grained clay on which to calculate his demonstrating example. If he had taken into consideration the data for the sample of clay as published by him instead of only those for the finer portions, quite different results would have been obtained as is shown in Table XVI.
The calculations by which the data in the following table were ob tained are, In Table XVI there is but one instance that of the West Virginia stoneware clay, in which the amount of water molecularly attracted even approached that required to develop plasticity. In many instances it does not greatly exceed the hygroscopic water that the clay would re tain when dried in open rack dryers. In fact the maximum amount of water which Grout admits could be so molecularly attracted, agrees quite closely with the water which in Table XI is shown to be in excess of that required to fill• the pores. While Grout's statement of the facts in this case has been proved incorrect, further investigation may find a relation between the molecularly attracted water and "excess water." As yet, however, such a relation cannot be established.
That a clay particle does possess a molecular attraction peculiar to itself is not denied. That this molecular attraction alone is sufficient to cause a plasticity that is peculiar and belongs to no other substance must be discredited until evidence is brought forward that will bear an analysis such as is given in Table XIV.
It would be most difficult for supporters of the molecular attraction theory to prove that the kaolin grains in primary clays do not possess every physical property that is attributed to the grains of the clay sub stance in the secondary clays, save that of plasticity. Chemically alike, and differing physically only in this one respect, yet to the one, accord ing to this theory, must be accredited no, or very little, molecular at traction for water, and to the other a strong molecular attraction.
Grout' may be quoted as follows: "The attraction of two grains may vary with the nature of the grains. The greater the attraction the farther they can be separated without losing coher ence. — — — —. Another way in which the films become viscous is the result of molecular attraction, which binds a film over the surface of the grain and renders it viscous. The friction between this film and the solid grain of clay is said to be infinite, compared with water outside of the film. But when forced to move, the resistance would depend on the strength of the attraction of clay and liquid. — — — —. The change in viscosity or in thicknes of the film, seems to be beyond the region of experiment. The quantity is too small to admit the determination of slight changes, but such are constantly assumed in physical problems. W. J. A. Bliss speaks of clay particles and the surrounding adherent liquid as follows: `The thickness of this shell depends on the intensity of the attraction between the solid and the liquid.' J. E. Mills says: `Molecular attraction depends primarily on the chemical constitution of the molecule. — — — —. Certain rare organic colloids increase the plasticity by rendering the water viscous. — — — —. The tendency for tensile strength to vary with plasticity is also easily explained in this way. Molecular attraction between two kaolin grains may be high. If the attraction for water is high, some water will be drawn between the grains and rendered viscous by the attraction; this makes plasticity high. But when the water dries out from such a mass, the kaolin grains still attract each other, and the chances are for greater strength than when wet, because the water has acted as a lubricant, allow ing a readjustment of grains to fill the space left as the water moved out. The result is a high degree of consolidation." Mr. Grout's arguments may be summed up as follows: 1. Attraction varies with the nature of grain, i. e., their chemical con stitution, or in other words, molecular structure.