Previous application of surface‐tension rules to dilution of coal tars is criticized and it is considered that the surface tension of the mixture of diluent with the oils of the tar must be considered rather than the surface tension of the diluent alone. Maximum insolubles in a tar‐diluent system occur at lower ratios of diluent to tar for both horizontal‐ and vertical‐retort tars. The relation of maximum insolubles to the surface tension of the diluents has been examined and generally it is shown that diluents with surface tensions above that of the tar behave quite differently from those of lower surface tension. The results are interpreted to mean that flocculation and precipitation, followed by solution or redis‐persion, occur when diluent of surface tension lower than the tar is added in increasing amounts.
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