All water particles will press outward the solid particles along the resultant lines of force as shown in Fig. 16. In this case instead of tension we would have a pressure. This pressure is known as surface pressure.
If, on the other hand, the water had a potentiality that was greater than that of the solid par tides, the resultant forces of at traction would be toward the cen ter of the liquid mass as shown in Fig. 17. This would result in surface tension.
The practical conclusion from the above discussion of greatest interest in connection with plas ticity, is that when the surround ing fluid has the greater poten tiality, flocculation, or drawing together of the solid particles will result. When the solid .par ticles have the greatest poten tiality, deflocculation or separa tion of the solid particles will re sult. Citations by the score could be presented showing that clays can be flocculated, or de flocculated, depending upon the material carried in solution by the water used in tempering. It is of interest, therefore, to consider the various solutions and their ef fect on clays.
Solutions causing in "How crops feed" cites a great many instances where solutions of organic compounds have caused deflocculation of soils. Ackison' has shown that tannin will de flocculate clay so thoroughly that when a thin slip of clay suspended in a solution of tamin is poured onto a filter paper the water passing through will be very turbid. Ammonia is used in the water when a clay is being disintegrated preparatory to mechanical analyis. Pe troleum is greedily absorbed by clay because of its low surface tension or potentiality, being held between the minute grains of clay by virtue of the higher potentiality of the clay grains. Whitney' has shown that cotton seed, meal, tankage, etc., have similar effects.
It will be important to note that the surface tension of solutions which cause deflocculation of grains of pure clay substance is without exception lower than the surface tension of water. It will also be impor tant to note that physical differences in conditions such as degree of concentration of the solution, temperature, etc., that tend to decrease
surface tension affect deflocculation. For example, it is a common ex perience of chemists that boiling for the purpose of extracting soluble salts often so thoroughly deflocculates the clay that even filtering through a Gooch crucible will not clearify the filtrate.
In the following tables the surface tension of water and of various solutions is given.
From the above the following conclusions regarding deflocculation ap pears: 1. It has been shown that solutions of organic compounds cause defloc culation. It is needless to go into further discussion of this point, for the facts that have been stated are well understood by practical potters and agri cultural chemists.
2. Increased temperature assists in producing deflocculation. Potters who use hot water in their blungers and brick manufacturers who use hot water in their pug-mills have learned that clays slake and develop plasticity more easily with hot than -with cold water. These cases find their parallel in the laboratory when clay slip is boiled in the process of soluble salt de termination. Deflocculation is increased by the use of hot water.
3. Increased density of a deflocculating solution does not increase its efficiency. "Ammonia (1) has a very marked action in breaking up soils containing particles less than 0.005 mm. in diameter. . . . One drop of ammonia (added to 5 grams or sample in 50 cc. of water) does not seem to be sufficient to break up the flocculations completely, but no great change is produced by the addition of more than 5 drops to 50 cc. of water." The preceding facts given by our foremost agronomists, when consid ered in the light of the fact that increased concentration of a solution increases its surface tension, are proof of the deduction that when the potential of the solid particles is greater than that of the surrounding fluid, deflocculation ensues. In the case of the ammonia solution, in creased concentration by the addition of more than 5 drops of ammonia would so increase the surface tension and consequently the potentiality of the solution as the equalize the potentials of the soil particles and the solution.