Zevahim 79: Two Liquids and Impurity
Even a mixture of liquid may depend on majority, though the appearance of the mixture may make the difference -- for example, if the color of the liquid is lighter than it would have been without being mixed with another liquid. With different treatment of spit as compared to urine. But the substance itself can't be nullified by the same kind of substance. Plus, an impure person's urine that is nullified by several mixings with (pure) water (how many times is a matter of dispute). Also, once the impure liquid comes in contact with flax, the impurity remains.
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