From this substance, an oxybenzoic acid (meta-), C 6 H 4.
OH COOH, may be prepared; and the two other known oxybenzoic acids (ortho- and para-) may be converted into benzoic acid.
These three acids yield on heating phenol, identical with the substance started with, and since in the three oxybenzoic acids the hydroxyl groups must occupy positions other than I, it follows that four hydrogen atoms are equal in value.
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