It is already known that rubber and gutta-percha behave as if they were cis and trans isomers. Judged by the roentgenographic observations of Meyer and Mark, of Mark and von Susich, and of Lotmar and Meyer, rubber has the cis form and gutta-percha, the trans form. Numerous attempts have been made to transform rubber into gutta-percha. As is well known, isomerization from the cis to the trans form is often induced by light, and for this reason Meyer and Ferri studied the action of ultra-violet radiation on rubber. They were unable to prove that isomerization took place, but rather found a ‘reticulation’ resulting from polymerization. This latter phenomenon was manifest by the irreversible formation of a gel. Sievers obtained the same result by exposing to visible light, with rigorous exclusion of air, a benzene solution of rubber in the presence of a little iodine. Under these conditions likewise, gelatinization took place, without appreciable polymerization. On the other hand, the technical and patent literature describes a series of reaction products of rubber having physical properties similar to those of gutta-percha; at ordinary temperature they are solids, and at elevated temperatures they become plastic. These substances are formed by treatment of rubber with zinc chloride, toluenesulfonyl chloride, titanium tetrachloride, etc.