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Phenol is another of the multitudes of substances obtainable from coal-tar; it can be prepared from aniline by the action of nitrous acid, and can be shown to be benzene with one hydrogen atom replaced by hydroxyl. If one of the adjacent hydrogen atoms of phenol is replaced by carboxyl, salicylic acid is produced; and in the presence of a suitable dehydrating agent salicylic acid reacts with phenol and phenyl salicylate, known as salol, is formed.

Many of the synthetic chemicals are much more complex than those so far described. They are built up on similar lines, but the processes involve a greater number of stages. Antipyrin (phenazone, or phenyl-*dimethylisopyrazolone) may be added to the examples selected for this notice. Antipyrin is represented by the annexed formula, which is said to be heterocyclic, because its molecules, like those of pyridine, consist of rings not made up exclusively of carbon atoms.

It must be understood that in this sketch only a very few notable instances of modern chemical research have been given, these being some of the more familiar products which have been introduced into medicine.