Many of the popular synthetic remedies belong to the benzene series. Benzene is obtained from coal-tar, but, as shown by Berthelot, it is possible to prepare it by heating the gaseous hydrocarbon, acetylene, C_{2}H_{2}, in a closed vessel. By this means three molecules of acetylene are condensed into one, C_{6}H_{6}, which is benzene. Benzene acted upon by nitric acid yields nitrobenzene, and this by the action of nascent hydrogen is changed into aniline. Aniline may be regarded as ammonia, NH_{3}, in which one hydrogen atom has been replaced by the phenyl group, C_{6}H_{5}, and, like ammonia, it combines with acids to form salts. Aniline acetate being formed, the elements of water being eliminated in the process, the product is acetanilide, or antifebrin. Acetanilide was first prepared by Gerhardt, in 1853, but its physiological action was only discovered by Cahn and Hepp in the ’eighties. By the substitution of an ethoxy-group for one of the hydrogen atoms of acetanilide, para-ethoxy-acetanilide, commonly called “phenacetin,” is produced. SALOL. 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. ANTIPYRIN. 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, H{3}CC-----CH | | H{3}CN CO \ / \ / N | C{6}H{5} because its molecules, like those of pyridine, consist of rings not made up exclusively of carbon atoms. * * * * *