By far the most valuable work on _Rhus toxicodendron_ is that of Pfaff. From a clinical study of Rhus poisoning, Pfaff came to the conclusion that the poison must be a non-volatile skin irritant. The more volatile the irritant, the quicker is its action on the skin. Formic acid acts very quickly; acetic acid, less volatile than formic, acts more slowly, but still much more quickly than poison ivy, the latent period of which is usually from two to five days. Pfaff thought that the volatile acid obtained by Maisch might have contained some of the poisonous principle as an impurity, but that it would not produce the dermatitis if prepared in a pure state. He therefore prepared a quantity of the acid by distilling the finely divided fresh plant with steam. The yield was increased by acidulating the mixture with sulphuric acid before the distillation. The acid distillate so obtained was freed from a non-poisonous oily substance by shaking the solution with ether. Barium and sodium salts were made by neutralizing the acid, and were purified by crystallization. Analysis showed them to be salts of acetic acid, and they gave the characteristic tests for this acid. The toxicodendric acid of Maisch was thus shown to be acetic acid, and was therefore not the poisonous principle of the plant. Pfaff obtained the active principle by the following process: The plant was extracted with alcohol, the alcohol was distilled off, and the residue was taken up in ether. The ether solution was washed with water and dilute sodium carbonate solution, and the ether was evaporated. An oily, black, poisonous substance partly soluble in alcohol was obtained. To get the active principle in a pure state, this residue was extracted with alcohol and filtered and the filtrate was precipitated fractionally by lead acetate. The final precipitates consisted of the lead compound of the poison in a pure state. On decomposing the lead compounds with ammonium sulphide, shaking out with ether, and letting the ether evaporate spontaneously, a non-volatile oil was obtained which gave the characteristic skin eruptions. The pure lead compounds made in different preparations were analyzed and assigned the formula C_{21}H_{30}O_{4}Pb. The oil itself was not analyzed. Pfaff proposed the name _Toxicodendrol_ for the oil. He found that it was not volatile, was decomposed by heat, was soluble in alcohol, ether, chloroform, benzene, etc., but insoluble in water. Its effects upon the human skin were studied in many experiments upon himself and others. It was shown that an exceedingly minute quantity of the poison will produce the dermatitis, even 1/1000 milligram applied in olive oil being active. The oil was given internally to rabbits, its effects being most marked on the kidneys. The oil obtained by Pfaff from _Rhus venenata_ seemed to be identical with that from _Rhus toxicodendron_. EXPERIMENTAL.