The irritating action of poison ivy has been attributed at different times to the "exhalation," to a volatile alkaloid, to a volatile acid, and to a non-volatile oil. Pfaff,[13] who made the most recent investigation of this poison, obtained from the plant a non-volatile oil having the same action on the skin as the plant itself. He found this oil in all parts of the plant and concluded that it was the active principle, and that one could be poisoned only by actual contact with some part of the plant. He assumed minute quantities of pollen dust to be in the air to account for the cases of "action at a distance" so frequently quoted. Pfaff says: "In my opinion, it is more than doubtful if ever a case of ivy poisoning has occurred without direct contact with the plant or with some article that has been in contact with the plant. The long latent period of the eruption in some cases may obviously render mistakes extremely easy as to the occasion when contact with the plant really occurred." Granting, however, that the active principle is practically non-volatile when isolated from the plant, we cannot say positively that it is not volatile in the juices of the plant, or under the influence of vital forces. It is quite conceivable that the water transpired by the leaves of the plant may carry with it a quantity of the poison sufficient to produce the dermatitis on a person very susceptible to its action. It is also conceivable that a volatile poison manufactured by a living plant could become non-volatile by changes in it consequent upon the death of the plant. Up to the present time, only three important chemical investigations of the active principle of _Rhus toxicodendron_ have appeared in medical and chemical literature, these being the researches of Dr. J. Khittel, J. M. Maisch, a pharmacist, and Dr. Franz Pfaff, of the Harvard University Medical School, to whose work reference has been frequently made. The chemical work of these investigators and their conclusions are given here in some detail for the sake of completeness. FOOTNOTES: [1] Chesnut. Bull. No. 20, U. S. Dept. of Agr., Div. of Botany. [2] Man. of Bot., p. 119. [3] Man. of Bot., p. 119. [4] Brook. Med. Jour., June, 1897. [5] Rein, The Ind. of Jap., p. 338, et seq. [6] H. Yoshida on Urushi Lacquer, Jour. Chem. Soc., 1883, p. 472. [7] Ann. de Chem. et de Phys., Series VII, Vol. 12, p. 125, 1897. [8] Amer. Jour. Pharm. 78, p. 53, Feb., 1906. [9] An account of Pfaff's work will be found in another part of this paper. [10] Amer. Jour. Med. Sci. 51 (1866), p. 560. [11] Med. and Surg. Rep. 17, Nov., 1867. [12] Jour. Exp. Med. 2 (1897), p. 181. [13] Ibid. KHITTEL'S INVESTIGATION.