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A STUDY OF PARADICHLOROBENZENE. Historical* The formation of a dichlorobenzene was first observed in 1862 by Muller (Soe.,xv,419 1862.), who obtained it along with other products, by passing chlorine into a benzene solution of iodine; but there is nothing in the papers cited to indicate that he gained, through this work, any insight into the structure of the compound in question beyond the fact that it was a substitution product. In 1865, Jungfleisch (Bull.,ii,241,1865,) confirmed Muller's obser vations on the dichlorobenzene melting at 53°, and added a statement indicating the analysis, the crystal line form and several important properties of the com pound. Xekule' had meanwhile formulated his theory of the benzene ring in several papers (Bull.,i,99,1865; ii,40,1866; Ann.,137,129,1866.), and in one of these (Bull.,i,99,1865.)he predicted^existence of three iso meric modifications of dichlorobenzene. llext, he ob tained evidence (Bull.,ii,200,1867.) which led to the view that three isomeric dihydroxy-derivatives of benzene might be obtained from the disulfonic acids, which tended to support the prediction mentioned aabove. These relationships, however, were still more clearly indicated by work done by Barbaglia and Kekule* (Ber.,5,876,1872.) who found that paraphenolsulfonic acid reacted with phosphorus penetachloride to give,among